@node Signal Handling @chapter Signal Handling @cindex signal A @dfn{signal} is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line. The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default. If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives. Finally, one process can send a signal to another process; this allows parent process to abort a child, or two related processes to communicate and synchronize. @menu * Concepts of Signals:: Introduction to the signal facilities. * Standard Signals:: Particular kinds of signals with standard names and meanings. * Signal Actions:: Specifying what happens when a particular signal is delivered. * Defining Handlers:: How to write a signal handler function. * Generating Signals:: How to send a signal to a process. * Blocking Signals:: Making the system hold signals temporarily. * Waiting for a Signal:: Suspending your program until a signal arrives. * BSD Signal Handling:: Additional functions for backward compatibility with BSD. @end menu @node Concepts of Signals @section Basic Concepts of Signals This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals. @menu * Kinds of Signals:: Some examples of what can cause a signal. * Signal Generation:: Concepts of why and how signals occur. * Delivery of Signal:: Concepts of what a signal does to the process. @end menu @node Kinds of Signals @subsection Some Kinds of Signals A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or @dfn{generate}, or @dfn{raise}) a signal: @itemize @bullet @item A program error such as dividing by zero or issuing an address outside the valid range. @item A user request to interrupt or terminate the program. Most environments are set up to let a user suspend the program by typing @kbd{C-z}, or terminate it with @kbd{C-c}. Whatever key sequence is used, the operating system sends the proper signal to interrupt the process. @item The termination of a child process. @item Expiration of a timer or alarm. @item A call to @code{kill} or @code{raise} by the same process. @item A call to @code{kill} from another process. Signals are a limited but useful form of interprocess communication. @end itemize Each of these kinds of events (excepting explicit calls to @code{kill} and @code{raise}) generates its own particular kind of signal. The various kinds of signals are listed and described in detail in @ref{Standard Signals}. @node Signal Generation @subsection Concepts of Signal Generation @cindex generation of signals In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests. An error means that a program has done something invalid and cannot continue execution. But not all kinds of errors generate signals---in fact, most do not. For example, opening a nonexistant file is an error, but it does not raise a signal; instead, @code{open} returns @code{-1}. In general, errors that are necessarily associated with system calls are reported by returning a value that indicates an error. The errors which raise signals are those which can happen anywhere in the program, not just in system calls. There include division by zero and invalid memory addresses. An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process. An explicit request means the use of a library function such as @code{kill} whose purpose is specifically to generate a signal. Signals may be generated @dfn{synchronously} or @dfn{asynchronously}. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process. A given type of signal is either typically synchrous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request. @node Delivery of Signal @subsection How Signals Are Delivered @cindex delivery of signals @cindex pending signals @cindex blocked signals When a signal is generated, it becomes @dfn{pending}. Normally it remains pending for just a short period of time and then is @dfn{delivered} to the process that was signaled. However, if that kind of signal is currently @dfn{blocked}, it may remain pending indefinitely---until signals of that kind are @dfn{unblocked}. Once unblocked, it will be delivered immediately. @xref{Blocking Signals}. @cindex specified action (for a signal) @cindex default action (for a signal) @cindex signal action @cindex catching signals When the signal is delivered, whether right away or after a long delay, the @dfn{specified action} for that signal is taken. For certain signals, such as @code{SIGKILL} and @code{SIGSTOP}, the action is fixed, but for most signals, the program has a choice: ignore the signal, specify a @dfn{handler function}, or accept the @dfn{default action} for that kind of signal. The program specifies its choice using functions such as @code{signal} or @code{sigaction} (@pxref{Signal Actions}). We sometimes say that a handler @dfn{catches} the signal. While the handler is running, that particular signal is normally blocked. If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it. If a signal arrives which the program has neither handled nor ignored, its @dfn{default action} takes place. Each kind of signal has its own default action, documented below (@pxref{Standard Signals}). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent ``harmless'' events, the default action is to do nothing. When a signal terminates a process, its parent process can determine the cause of termination by examining the termination status code reported by the @code{wait} or @code{waitpid} functions. (This is discussed in more detail in @ref{Process Completion}.) The information it can get includes the fact that termination was due to a signal, and the kind of signal involved. If a program you run from a shell is terminated by a signal, the shell typically prints some kind of error message. The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a @dfn{core dump file} which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error. If you raise a ``program error'' signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error. @node Standard Signals @section Standard Signals @cindex signal names @cindex names of signals @pindex signal.h @cindex signal number This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer---the @dfn{signal number} for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform. The signal names are defined in the header file @file{signal.h}. @comment signal.h @comment GNU @deftypevr Macro int NSIG The value of this symbolic constant is the total number of signals defined. Since the signal numbers are allocated consecutively, @code{NSIG} is also one greater than the largest defined signal number. @end deftypevr @menu * Program Error Signals:: Used to report serious program errors. * Termination Signals:: Used to interrupt and/or terminate the program. * Alarm Signals:: Used to indicate expiration of timers. * Asynchronous I/O Signals:: Used to indicate input is available. * Miscellanous Signals:: Used to report various random conditions. * Job Control Signals:: Signals used to support job control. * Other Signals:: Implementations can support other signals. * Signal Messages:: Printing a message describing a signal. @end menu @node Program Error Signals @subsection Program Error Signals @cindex program error signals The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error. Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (@xref{Termination in Handler}.) Termination is the sensible ultimate outcome from a program error in most programs. However, programming systems such as Lisp that can load compiled user programs might need to keep executing even if a user program incurs an error. These programs have handlers which use @code{longjmp} to return control to the command level. The default action for all of these signals is to cause the process to terminate. If you block or ignore these signals or establish handlers for them that return normally, your program will probably break horribly when such signals happen, unless they are generated by @code{raise} or @code{kill} instead of a real error. When one of these program error signals terminates a process, it also writes a @dfn{core dump file} which records the state of the process at the time of termination. The core dump file is named @file{core} and is written in whichever directory is current in the process at the time. The purpose of core dump files is so that you can examine them with a debugger to investigate what caused the error. @comment signal.h @comment ANSI @deftypevr Macro int SIGFPE The @code{SIGFPE} signal reports a fatal arithmetic error. Although the name is derived from ``floating-point exception'', this signal actually covers all arithmetic errors, including division by zero and overflow. If a program stores integer data in a location which is then used in a floating-point operation, this often causes an ``invalid operation'' exception, because the processor cannot recognize the data as a floating-point number. @cindex exception @cindex floating-point exception Actual floating-point exceptions are a complicated subject because there are many types of exceptions with subtly different meanings, and the @code{SIGFPE} signal doesn't distinguish between them. The @cite{IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985)} defines various floating-point exceptions and requires conforming computer systems to report their occurrences. However, this standard does not specify how the exceptions are reported, or what kinds of handling and control the operating system can offer to the programmer. If you're writing a library of numeric routines that has to be able to trap and deal with the different kinds of exceptions intelligently, you'll have to look at the documentation for your specific computer and operating system to find out how. @end deftypevr @comment signal.h @comment ANSI @deftypevr Macro int SIGILL The name of this signal is derived from ``illegal instruction''; it means your program is trying to execute garbage or a privileged instruction. Since the C compiler generates only valid instructions, @code{SIGILL} typically indicates that the executable file is corrupted, or that you are trying to execute data. Some common ways of getting into the latter situation are by passing an invalid object where a pointer to a function was expected, or by writing past the end of an automatic array (or similar problems with pointers to automatic variables) and corrupting other data on the stack such as the return address of a stack frame. @end deftypevr @cindex illegal instruction @comment signal.h @comment ANSI @deftypevr Macro int SIGSEGV @cindex segmentation violation This signal is generated when a program tries to read or write outside the memory that is allocated for it. (Actually, the signals only occur when the program goes far enough outside to be detected by the system's memory protection mechanism.) The name is an abbreviation for ``segmentation violation''. The most common way of getting a @code{SIGSEGV} condition is by dereferencing a null or uninitialized pointer. A null pointer refers to the address 0, and most operating systems make sure this address is always invalid precisely so that dereferencing a null pointer will cause @code{SIGSEGV}. (Some operating systems place valid memory at address 0, and dereferencing a null pointer does not cause a signal on these systems.) As for uninitialized pointer variables, they contain random addresses which may or may not be valid. Another common way of getting into a @code{SIGSEGV} situation is when you use a pointer to step through an array, but fail to check for the end of the array. @end deftypevr @comment signal.h @comment BSD @deftypevr Macro int SIGBUS This signal is generated when an invalid pointer is dereferenced. Like @code{SIGSEGV}, this signal is typically the result of dereferencing an uninitialized pointer. The difference between the two is that @code{SIGSEGV} indicates an invalid access to valid memory, while @code{SIGBUS} indicates an access to an invalid address. In particular, @code{SIGBUS} signals often result from dereferencing a misaligned pointer, such as referring to a four-word integer at an address not divisible by four. (Each kind of computer has its own requirements for address alignment.) The name of this signal is an abbreviation for ``bus error''. @end deftypevr @cindex bus error @comment signal.h @comment ANSI @deftypevr Macro int SIGABRT @cindex abort signal This signal indicates an error detected by the program itself and reported by calling @code{abort}. @xref{Aborting a Program}. @end deftypevr @node Termination Signals @subsection Termination Signals @cindex program termination signals These signals are all used to tell a process to terminate, in one way or another. They have different names because they're used for slightly different purposes, and programs might want to handle them differently. The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (@xref{Termination in Handler}.) The (obvious) default action for all of these signals is to cause the process to terminate. @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGHUP @cindex hangup signal The @code{SIGHUP} (``hang-up'') signal is used to report that the user's terminal is disconnected, perhaps because a network or telephone connection was broken. For more information about this, see @ref{Control Modes}. This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see @ref{Process Termination Details}. @end deftypevr @comment signal.h @comment ANSI @deftypevr Macro int SIGINT @cindex interrupt signal The @code{SIGINT} (``program interrupt'') signal is sent when the user types the INTR character (normally @kbd{C-c}). @xref{Special Characters}, for information about terminal driver support for @kbd{C-c}. @end deftypevr @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGQUIT @cindex quit signal @cindex quit signal The @code{SIGQUIT} signal is similar to @code{SIGINT}, except that it's controlled by a different key---the QUIT character, usually @kbd{C-\}---and produces a core dump when it terminates the process, just like a program error signal. You can think of this as a program error condition ``detected'' by the user. @xref{Program Error Signals}, for information about core dumps. @xref{Special Characters}, for information about terminal driver support. Certain kinds of cleanups are best omitted in handling @code{SIGQUIT}. For example, if the program creates temporary files, it should handle the other termination requests by deleting the temporary files. But it is better for @code{SIGQUIT} not to delete them, so that the user can examine them in conjunction with the core dump. @end deftypevr @comment signal.h @comment ANSI @deftypevr Macro int SIGTERM @cindex termination signal The @code{SIGTERM} signal is a generic signal used to cause program termination. Unlike @code{SIGKILL}, this signal can be blocked, handled, and ignored. The shell command @code{kill} generates @code{SIGTERM} by default. @pindex kill @end deftypevr @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGKILL The @code{SIGKILL} signal is used to cause immediate program termination. It cannot be handled or ignored, and is therefore always fatal. It is also not possible to block this signal. This signal is generated only by explicit request. Since it cannot be handled, you should generate it only as a last resort, after first trying a less drastic method such as @kbd{C-c} or @code{SIGTERM}. If a process does not respond to any other termination signals, sending it a @code{SIGKILL} signal will almost always cause it to go away. In fact, if @code{SIGKILL} fails to terminate a process, that by itself constitutes an operating system bug which you should report. @end deftypevr @cindex kill signal @node Alarm Signals @subsection Alarm Signals These signals are used to indicate the expiration of timers. @xref{Setting an Alarm}, for information about functions that cause these signals to be sent. The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case. @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGALRM This signal typically indicates expiration of a timer that measures real or clock time. It is used by the @code{alarm} function, for example. @end deftypevr @cindex alarm signal @comment signal.h @comment BSD @deftypevr Macro int SIGVTALRM This signal typically indicates expiration of a timer that measures CPU time used by the current process. The name is an abbreviation for ``virtual time alarm''. @end deftypevr @cindex virtual time alarm signal @comment signal.h @comment BSD @deftypevr Macro int SIGPROF This signal is typically indicates expiration of a timer that measures both CPU time used by the current process, and CPU time expended on behalf of the process by the system. Such a timer is used to implement code profiling facilities, hence the name of this signal. @end deftypevr @cindex profiling alarm signal @node Asynchronous I/O Signals @subsection Asynchronous I/O Signals The signals listed in this section are used in conjunction with asynchronous I/O facilities. You have to take explicit action by calling @code{fcntl} to enable a particular file descriptior to generate these signals (@pxref{Interrupt Input}). The default action for these signals is to ignore them. @comment signal.h @comment BSD @deftypevr Macro int SIGIO @cindex input available signal @cindex output possible signal This signal is sent when a file descriptor is ready to perform input or output. On most operating systems, terminals and sockets are the only kinds of files that can generate @code{SIGIO}; other kinds, including ordinary files, never generate @code{SIGIO} even if you ask them to. @end deftypevr @comment signal.h @comment BSD @deftypevr Macro int SIGURG @cindex urgent data signal This signal is sent when ``urgent'' or out-of-band data arrives on a socket. @xref{Out-of-Band Data}. @end deftypevr @node Job Control Signals @subsection Job Control Signals @cindex job control signals These signals are used to support job control. If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled. You should generally leave these signals alone unless you really understand how job control works. @xref{Job Control}. @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGCHLD @cindex child process signal This signal is sent to a parent process whenever one of its child processes terminates or stops. The default action for this signal is to ignore it. If you establish a handler for this signal while there are child processes that have terminated but not reported their status via @code{wait} or @code{waitpid} (@pxref{Process Completion}), whether your new handler applies to those processes or not depends on the particular operating system. @end deftypevr @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGCONT @cindex continue signal You can send a @code{SIGCONT} signal to a process to make it continue. The default behavior for this signal is to make the process continue if it is stopped, and to ignore it otherwise. Most programs have no reason to handle @code{SIGCONT}; they simply resume execution without realizing they were ever stopped. You can use a handler for @code{SIGCONT} to make a program do something special when it is stopped and continued---for example, to reprint a prompt when it is suspended while waiting for input. @end deftypevr @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGSTOP The @code{SIGSTOP} signal stops the process. It cannot be handled or ignored. @end deftypevr @cindex stop signal @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGTSTP The @code{SIGTSTP} signal is an interactive stop signal. Unlike @code{SIGSTOP}, this signal can be handled and ignored. Your program should handle this signal if you have a special need to leave files or system tables in a secure state when a process is stopped. For example, programs that turn off echoing should handle @code{SIGTSTP} so they can turn echoing back on before stopping. This signal is generated when the user types the SUSP character (normally @kbd{C-z}). For more information about terminal driver support, see @ref{Special Characters}. @end deftypevr @cindex interactive stop signal @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGTTIN A process cannot read from the the user's terminal while it is running as a background job. When any process in a background job tries to read from the terminal, all of the processes in the job are sent a @code{SIGTTIN} signal. The default action for this signal is to stop the process. For more information about how this interacts with the terminal driver, see @ref{Access to the Controlling Terminal}. @end deftypevr @cindex terminal input signal @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGTTOU This is similar to @code{SIGTTIN}, but is generated when a process in a background job attempts to write to the terminal. Again, the default action is to stop the process. @end deftypevr @cindex terminal output signal While a process is stopped, no more signals can be delivered to it until it is continued, except @code{SIGKILL} signals and (obviously) @code{SIGCONT} signals. The @code{SIGKILL} signal always causes termination of the process and can't be blocked or ignored. You can block or ignore @code{SIGCONT}, but it always causes the process to be continued anyway if it is stopped. Sending a @code{SIGCONT} signal to a process causes any pending stop signals for that process to be discarded. Likewise, any pending @code{SIGCONT} signals for a process are discarded when it receives a stop signal. The default action for a process that is a member of an orphaned process group (@pxref{Process Termination Details}) that receives a @code{SIGTSTP}, @code{SIGTTIN}, or @code{SIGTTOU} signal is not to stop the process. This makes sense; if a process is stopped by one of these signals, it must be continued by the shell, and if a process group has been orphaned, that means there's no shell around to continue the processes. Instead, the signal might be ignored, or the operating system might turn the stop signal into another signal like @code{SIGHUP}. @strong{Incomplete:} What does the GNU system do? @node Miscellaneous Signals @subsection Miscellaneous Signals These signals are used to report various other conditions. The default action for all of them is to cause the process to terminate. @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGPIPE If you use pipes or FIFO special files, you have to design your application so that one process opens the pipe for reading before another starts writing. If the reading process never starts, or terminates unexpectedly, a write to the pipe or FIFO causes the writing process to receive a @code{SIGPIPE} signal. Pipes and FIFO special files are discussed in more detail in @ref{Pipes and FIFOs}. @end deftypevr @cindex pipe signal @cindex broken pipe signal @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGUSR1 @end deftypevr @comment signal.h @comment POSIX.1 @deftypevr Macro int SIGUSR2 The @code{SIGUSR1} and @code{SIGUSR2} signals are set aside for you to use any way you want. They're useful for interprocess communication. Since these signals are normally fatal, you should write a signal handler for them in the program that receives the signal. There is an example showing the use of @code{SIGUSR1} and @code{SIGUSR2} in @ref{Signaling Another Process}. @end deftypevr @cindex user signals @node Other Signals @subsection Other Signals Particular operating systems support additional signals not listed above. The ANSI C standard reserves all identifiers beginning with @samp{SIG} followed by an uppercase letter for the names of signals. You should consult the documentation or header files for your particular operating system and processor type to find out about the specific signals it supports. For example, some systems support extra signals which correspond to hardware traps. Some other kinds of signals commonly supported are used to implement limits on CPU time or file system usage, asynchronous changes to terminal configuration, and the like. Systems may also define signal names that are aliases for standard signal names. You can generally assume that the default action (or the action set up by the shell) for implementation-defined signals is reasonable, and not worry about them yourself. In fact, it's usually a bad idea to ignore or block signals you don't know anything about, or try to establish a handler for signals whose meanings you don't know. Here are some of the other signals found on commonly used operating systems: @table @code @item SIGCLD Obsolete name for @code{SIGCHLD}. @item SIGTRAP Generated by the machine's breakpoint instruction. Used by debuggers. Default action is to dump core. @item SIGIOT Generated by the PDP-11 ``iot'' instruction; equivalent to @code{SIGABRT}. Default action is to dump core. @item SIGEMT Emulator trap; this results from certain unimplemented instructions. It is a program error signal. @item SIGSYS Bad system call; that is to say, the instruction to trap to the operating system was executed, but the code number for the system call to perform was invalid. This is a program error signal. @item SIGPOLL This is a System V signal name, more or less similar to @code{SIGIO}. @item SIGXCPU CPU time limit exceeded. This is used for batch processing. Default action is program termination. @item SIGXFSZ File size limit exceeded. This is used for batch processing. Default action is program termination. @item SIGWINCH Window size change. This is generated on certain systems when the size of the current window on the screen is changed. Default action is to ignore it. @end table @node Signal Messages @subsection Signal Messages @cindex signal messages We mentioned above that the shell prints a message describing the signal that terminated a child process. The clean and portable way to print a message describing a signal is to use the functions @code{strsignal} and @code{psignal}. These functions use a signal number to specify which kind of signal to describe. The signal number may come from the termination status of a child process (@pxref{Process Completion}) or it may come from a signal handler in the same process. @comment string.h @comment GNU @deftypefun {char *} strsignal (int @var{signum}) This function returns a pointer to a statically-allocated string containing a message describing the signal @var{signum}. You should not modify the contents of this string; and, since it can be rewritten on subsequent calls, you should save a copy of it if you need to reference it later. @pindex string.h This function is declared in the header file @file{string.h}. @end deftypefun @comment stdio.h @comment GNU @deftypefun void psignal (int @var{signum}, const char *@var{message}) This function prints a message describing the signal @var{signum} to the standard error output stream @code{stderr}; see @ref{Standard Streams}. If you call @code{psignal} with a @var{message} that is either a null pointer or an empty string, @code{psignal} just prints the message corresponding to @var{signum}, adding a trailing newline. If you supply a non-null @var{message} argument, then @code{psignal} prefixes its output with this string. It adds a colon and a space character to separate the @var{message} from the string corresponding to @var{signum}. @pindex stdio.h This function is declared in the header file @file{stdio.h}. @end deftypefun @node Signal Actions @section Specifying Signal Actions @cindex signal actions @cindex establishing a handler The simplest way to change the action for a signal is to use the @code{signal} function. You can specify a built-in action (such as to ignore the signal), or you can @dfn{establish a handler}. The GNU library also implements the more versatile @code{sigaction} facility. This section describes both facilities and gives suggestions on which to use when. @menu * Basic Signal Handling:: The simple @code{signal} function. * Advanced Signal Handling:: The more powerful @code{sigaction} function. * Signal and Sigaction:: How those two functions interact. * Sigaction Function Example:: An example of using the sigaction function. * Flags for Sigaction:: Specifying options for signal handling. * Initial Signal Actions:: How programs inherit signal actions. @end menu @node Basic Signal Handling @subsection Basic Signal Handling @cindex @code{signal} function The @code{signal} function provides a simple interface for establishing an action for a particular signal. The function and associated macros are declared in the header file @file{signal.h}. @pindex signal.h @comment signal.h @comment GNU @deftp {Data Type} sighandler_t This is the type of signal handler functions. Signal handlers take one integer argument specifying the signal number, and have return type @code{void}. So, you should define handler functions like this: @example void @var{handler} (int @code{signum}) @{ @dots{} @} @end example The name @code{sighandler_t} for this data type is a GNU extension. @end deftp @comment signal.h @comment ANSI @deftypefun sighandler_t signal (int @var{signum}, sighandler_t @var{action}) The @code{signal} function establishes @var{action} as the action for the signal @var{signum}. The first argument, @var{signum}, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names described in @ref{Standard Signals}---don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system. The second argument, @var{action}, specifies the action to use for the signal @var{signum}. This can be one of the following: @table @code @item SIG_DFL @vindex SIG_DFL @cindex default action for a signal @code{SIG_DFL} specifies the default action for the particular signal. The default actions for various kinds of signals are stated in @ref{Standard Signals}. @item SIG_IGN @vindex SIG_IGN @cindex ignore action for a signal @code{SIG_IGN} specifies that the signal should be ignored. Your program generally should not ignore signals that represent serious events or that are normally used to request termination. You cannot ignore the @code{SIGKILL} or @code{SIGSTOP} signals at all. You can ignore program error signals like @code{SIGSEGV}, but ignoring the error won't enable the program to continue executing meaningfully. Ignoring user requests such as @code{SIGINT}, @code{SIGQUIT}, and @code{SIGTSTP} is unfriendly. When you do not wish signals to be delivered during a certain part of the program, the thing to do is to block them, not ignore them. @xref{Blocking Signals}. @item @var{handler} Supply the address of a handler function in your program, to specify running this handler as the way to deliver the signal. For more information about defining signal handler functions, see @ref{Defining Handlers}. @end table If you set the action for a signal to @code{SIG_IGN}, or if you set it to @code{SIG_DFL} and the default action is to ignore that signal, then any pending signals of that type are discarded (even if they are blocked). Discarding the pending signals means that they will never be delivered, not even if you subsequently specify another action and unblock this kind of signal. The @code{signal} function returns the action that was previously in effect for the specified @var{signum}. You can save this value and restore it later by calling @code{signal} again. If @code{signal} can't honor the request, it returns @code{SIG_ERR} instead. The following @code{errno} error conditions are defined for this function: @table @code @item EINVAL You specified an invalid @var{signum}; or you tried to ignore or provide a handler for @code{SIGKILL} or @code{SIGSTOP}. @end table @end deftypefun Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen: @example #include void termination_handler (int signum) @{ struct temp_file *p; for (p = temp_file_list; p; p = p->next) unlink (p->name); @} main () @{ @dots{} if (signal (SIGINT, termination_handler) == SIG_IGN) signal (SIGINT, SIG_IGN); if (signal (SIGHUP, termination_handler) == SIG_IGN) signal (SIGHUP, SIG_IGN); if (signal (SIGTERM, termination_handler) == SIG_IGN) signal (SIGTERM, SIG_IGN); @dots{} @} @end example @noindent Note how if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this. We do not handle @code{SIGQUIT} or the program error signals in this example because these are designed to provide information for debugging (a core dump), and the temporary files may give useful information. @comment signal.h @comment SVID @deftypefun sighandler_t ssignal (int @var{signum}, sighandler_t @var{action}) The @code{ssignal} function does the same thing as @code{signal}; it is provided only for compatibility with SVID. @end deftypefun @comment signal.h @comment ANSI @deftypevr Macro sighandler_t SIG_ERR The value of this macro is used as the return value from @code{signal} to indicate an error. @end deftypevr @ignore @comment RMS says that ``we don't do this''. Implementations might define additional macros for built-in signal actions that are suitable as a @var{action} argument to @code{signal}, besides @code{SIG_IGN} and @code{SIG_DFL}. Identifiers whose names begin with @samp{SIG_} followed by an uppercase letter are reserved for this purpose. @end ignore @node Advanced Signal Handling @subsection Advanced Signal Handling @cindex @code{sigaction} function The @code{sigaction} function has the same basic effect as @code{signal}: to specify how a signal should be handled by the process. However, @code{sigaction} offers more control, at the expense of more complexity. In particular, @code{sigaction} allows you to specify additional flags to control when the signal is generated and how the handler is invoked. The @code{sigaction} function is declared in @file{signal.h}. @pindex signal.h @comment signal.h @comment POSIX.1 @deftp {Data Type} {struct sigaction} Structures of type @code{struct sigaction} are used in the @code{sigaction} function to specify all the information about how to handle a particular signal. This structure contains at least the following members: @table @code @item sighandler_t sa_handler This is used in the same way as the @var{action} argument to the @code{signal} function. The value can be @code{SIG_DFL}, @code{SIG_IGN}, or a function pointer. @xref{Basic Signal Handling}. @item sigset_t sa_mask This specifies a set of signals to be blocked while the handler runs. Blocking is explained in @ref{Blocking for Handler}. Note that the signal that was delivered is automatically blocked by default before its handler is started; this is true regardless of the value in @code{sa_mask}. If you want that signal not to be blocked within its handler, you must write code in the handler to unblock it. @item int sa_flags This specifies various flags which can affect the behavior of the signal. These are described in more detail in @ref{Flags for Sigaction}. @end table @end deftp @comment signal.h @comment POSIX.1 @deftypefun int sigaction (int @var{signum}, const struct sigaction *@var{action}, struct sigaction *@var{old_action}) The @var{action} argument is used to set up a new action for the signal @var{signum}, while the @var{old_action} argument is used to return information about the action previously associated with this symbol. (In other words, @var{old_action} has the same purpose as the @code{signal} function's return value---you can check to see what the old action in effect for the signal was, and restore it later if you want.) Either @var{action} or @var{old_action} can be a null pointer. If @var{old_action} is a null pointer, this simply suppresses the return of information about the old action. If @var{action} is a null pointer, the action associated with the signal @var{signum} is unchanged; this allows you to inquire about how a signal is being handled without changing that handling. The return value from @code{sigaction} is zero if it succeeds, and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EINVAL The @var{signum} argument is not valid, or you are trying to trap or ignore @code{SIGKILL} or @code{SIGSTOP}. @end table @end deftypefun @node Signal and Sigaction @subsection Interaction of @code{signal} and @code{sigaction} It's possible to use both the @code{signal} and @code{sigaction} functions within a single program, but you have to be careful because they can interact in slightly strange ways. The @code{sigaction} function specifies more information than the @code{signal} function, so the return value from @code{signal} cannot express the full range of @code{sigaction} possibilities. Therefore, if you use @code{signal} to save and later reestablish an action, it may not be able to reestablish properly a handler that was established with @code{sigaction}. To avoid having problems as a result, always use @code{sigaction} to save and restore a handler if your program uses @code{sigaction} at all. Since @code{sigaction} is more general, it can properly save and reestablish any action, regardless of whether it was established originally with @code{signal} or @code{sigaction}. If you establish an action with @code{signal} and then examine it with @code{sigaction}, the handler address that you get may not be the same as what you specified with @code{signal}. It may not even be suitable for use as an action argument with @code{signal}. But you can rely on using it as an argument to @code{sigaction}. So, you're better off using one or the other of the mechanisms consistently within a single program. @strong{Portability Note:} The basic @code{signal} function is a feature of ANSI C, while @code{sigaction} is part of the POSIX.1 standard. If you are concerned about portability to non-POSIX systems, then you should use the @code{signal} function instead. @node Sigaction Function Example @subsection @code{sigaction} Function Example In @ref{Basic Signal Handling}, we gave an example of establishing a simple handler for termination signals using @code{signal}. Here is an equivalent example using @code{sigaction}: @example #include void termination_handler (int signum) @{ struct temp_file *p; for (p = temp_file_list; p; p = p->next) unlink (p->name); @} main () @{ @dots{} struct sigaction new_action, old_action; /* @r{Set up the structure to specify the new action.} */ new_action.sa_handler = termination_handler; sigemptyset (&new_action.sa_mask); new_action.sa_flags = 0; sigaction (SIGINT, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) sigaction (SIGINT, &new_action, NULL); sigaction (SIGHUP, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) sigaction (SIGHUP, &new_action, NULL); sigaction (SIGTERM, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) sigaction (SIGTERM, &new_action, NULL); @dots{} @} @end example The program just loads the @code{new_action} structure with the desired parameters and passes it in the @code{sigaction} call. The usage of @code{sigemptyset} is described later; see @ref{Blocking Signals}. As in the example using @code{signal}, we avoid handling signals previously set to be ignored. Here we can avoid altering the signal handler even momentarily, by using the feature of @code{sigaction} that lets us examine the current action without specifying a new one. Here is another example. It retrieves information about the current action for @code{SIGINT} without changing that action. @example struct sigaction query_action; if (sigaction (SIGINT, NULL, &query_action) < 0) /* @r{@code{sigaction} returns -1 in case of error.} */ else if (query_action.sa_handler == SIG_DFL) /* @r{@code{SIGINT} is handled in the default, fatal manner.} */ else if (query_action.sa_handler == SIG_IGN) /* @r{@code{SIGINT} is ignored.} */ else /* @r{A programmer-defined signal handler is in effect.} */ @end example @node Flags for Sigaction @subsection Flags for @code{sigaction} @cindex signal flags @cindex flags for @code{sigaction} @cindex @code{sigaction} flags This @code{sa_flags} member of the @code{sigaction} structure is a catch-all for special features. Most of the time, you can simply use @code{0} for this field. The value of @code{sa_flags} is interpreted as a bit mask. Thus, you should choose the flags you want to set, @sc{or} those flags together, and store the result in the @code{sa_flags} member of your @code{sigaction} structure. Each signal number has its own set of flags. Each call to @code{sigaction} affects one particular signal number, and the flags that you specify apply only to that particular signal. @c ??? This should be checked in posix.1.1. All of the flag values are zero by default if you establish the handler with @code{signal} rather than @code{sigaction}. @pindex signal.h These macros are defined in the header file @file{signal.h}. @comment signal.h @comment POSIX.1 @deftypevr Macro int SA_NOCLDSTOP This flag is meaningful only for the @code{SIGCHLD} signal. When the flag is set, the system delivers the signal for a terminated child process but not for one that is stopped. By default, @code{SIGCHLD} is delivered for both terminated children and stopped children. Setting this flag for a signal other than @code{SIGCHLD} has no effect. @end deftypevr @comment signal.h @comment BSD @deftypevr Macro int SA_ONSTACK If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. @xref{BSD Signal Handling}. @end deftypevr @comment signal.h @comment BSD @deftypevr Macro int SA_RESTART This flag controls what happens when a signal is delivered during a system call (such as @code{open}, @code{read} or @code{write}), and the signal handler returns normally. There are two alternatives: the system call can resume, or it can return failure with error code @code{EINTR}. The choice is controlled by the @code{SA_RESTART} flag for the particular kind of signal that was delivered. If the flag is set, the system call will resume. If the flag is clear, the system call will return failure. @end deftypevr @node Initial Signal Actions @subsection Initial Signal Actions @cindex initial signal actions When a new process is created (@pxref{Creating a Process}), it inherits handling of signals from its parent process. However, when you load a new process image using the @code{exec} function (@pxref{Executing a File}), any signals that you've defined your own handlers for revert to their @code{SIG_DFL} handling. (If you think about it a little, this makes sense; the handler functions from the old program are specific to that program, and aren't even present in the address space of the new program image.) Of course, the new program can establish its own handlers. When a program is run by a shell, the shell normally sets the initial actions for the child process to @code{SIG_DFL} or @code{SIG_IGN}, as appropriate. It's a good idea to check to make sure that the shell has not set up an initial action of @code{SIG_IGN} before you establish your own signal handlers. Here is an example of how to establish a handler for @code{SIGHUP}, but not if @code{SIGHUP} is currently ignored: @example @dots{} struct sigaction temp; sigaction (SIGHUP, NULL, &temp); if (temp.sa_handler != SIG_IGN) @{ temp.sa_handler = handle_sighup; sigemptyset (&temp.sa_mask); sigaction (SIGHUP, &temp, NULL); @} @end example @node Defining Handlers @section Defining Signal Handlers @cindex signal handler function This section describes how to write a signal handler function that can be established with the @code{signal} or @code{sigaction} functions. A signal handler is just a function that you compile together with the rest of the program. Instead of directly invoking the function, you use @code{signal} or @code{sigaction} to tell the operating system to call it when a signal arrives. This is known as @dfn{establishing} the handler. @xref{Signal Actions}. There are two basic strategies you can use in signal handler functions: @itemize @bullet @item You can have the handler function note that the signal arrived by tweaking some global data structures, and then return normally. @item You can have the handler function terminate the program or transfer control to a point where it can recover from the situation that caused the signal. @end itemize You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, the operating system might even restart the handler while it is handling the first signal! This section describes what your handler should do, and what you should avoid. @menu * Handler Returns:: * Termination in Handler:: * Longjmp in Handler:: * Signals in Succession:: * Nonreentrancy:: * Atomic Data Access:: @end menu @node Handler Returns @subsection Signal Handlers That Return Handlers which return normally are usually used for signals such as @code{SIGALRM} and the I/O and interprocess communication signals. But a handler for @code{SIGINT} might also return normally after setting a flag that tells the program to exit at a convenient time. It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. @xref{Program Error Signals}. Handlers that return normally must modify some global variable in order to have any effect. Typically, the variable is one that is examined periodically by the program during normal operation. Its data type should be @code{sig_atomic_t} for reasons described in @ref{Atomic Data Access}. Here is a simple example of such a program. It executes the body of the loop until it has noticed that a @code{SIGALRM} signal has arrived. This technique is useful because it allows the iteration in progress when the signal arrives to complete before the loop exits. @example #include #include /* @r{This flag controls termination of the main loop.} */ volatile sig_atomic_t keep_going = 1; /* @r{The signal handler just clears the flag and re-enables itself.} @r{Actually, for this program re-enabling the handler isn't necessary,} @r{since only one alarm signal has to be caught in order to cause it} @r{to terminate.} */ void catch_alarm (int sig) @{ keep_going = 0; signal (sig, catch_alarm); @} main (void) @{ /* @r{Establish a handler for @code{SIGALRM} signals.} */ signal (SIGALRM, catch_alarm); /* @r{Set an alarm to go off in a little while.} */ alarm (2); /* @r{Check the flag once in a while to see when to quit.} */ while (keep_going) do_stuff (); exit (EXIT_SUCCESS); @} @end example @node Termination in Handler @subsection Handlers That Terminate the Process Handler functions terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts. The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place---but disestablish the handler, so it does not run again. For example, GNU Emacs sets up a handler for most fatal signals that looks something like: @example volatile sig_atomic_t fatal_error_in_progress = 0; void fatal_error_signal (int sig) @{ /* @r{Immediately set the action for this signal back to the default.} @r{This will prevent the handler from being invoked recursively if} @r{another fatal signal happens while the handler is executing.} */ signal (sig, SIG_DFL); /* @r{Since this handler is established for more than one kind of signal, } @r{it might still get invoked recursively by delivery of some other kind} @r{of signal. Use a static variable to keep track of that.} */ if (fatal_error_in_progress) raise (sig); fatal_error_in_progress = 1; /* @r{Now do the clean up actions:} @r{- reset terminal modes} @r{- kill child processes} @r{- auto save buffers being edited} @r{- remove lock files} */ @dots{} /* @r{Now resend the signal. Since we set the handling for it back to} @r{its default, this will cause the program to terminate. We could} @r{just call @code{exit} or @code{abort} here, but resending the signal} @r{will set the return status from the process correctly.} */ raise (sig); @} @end example @node Longjmp in Handler @subsection Nonlocal Control Transfer in Handlers @cindex non-local exit, from signal handler You can do a nonlocal transfer of control out of a signal handler using the @code{setjmp} and @code{longjmp} facilities (@pxref{Non-Local Exits}). When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on. There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. @xref{Blocking Signals}. The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent. Here is a rather schematic example showing the reinitialization of one global variable. @example #include #include jmp_buf return_to_top_level; volatile sig_atomic_t waiting_for_input; void handle_sigint (int signum) @{ /* @r{We may have been waiting for input when the signal arrived,} @r{but we are no longer waiting once we transfer control.} */ waiting_for_input = 0; longjmp (return_to_top_level, 1); @} main () @{ @dots{} signal (SIGINT, sigint_handler); @dots{} while (1) @{ prepare_for_command (); if (setjmp (return_to_top_level) == 0) read_and_execute_command (); @} @} /* @r{Imagine this is a subroutine used by various commands.} */ char * read_data () @{ if (input_from_terminal) @{ waiting_for_input = 1; @dots{} waiting_for_input = 0; @} else @{ @dots{} @} @} @end example @node Signals in Succession @subsection Signals Arriving in Quick Succession @cindex race conditions, relating to signals @cindex handling multiple signals @cindex successive signals What happens if another signal arrives when your signal handler function is running? When the handler for a particular signal is invoked, that signal is normally blocked until the handler returns. That means that if two signals of the same kind arrive close together, the second one will be held until the first has been handled. (The handler can explicitly unblock the signal using @code{sigprocmask}, if you want to allow more signals of this type to arrive; see @ref{Process Signal Mask}.) However, your handler can still be interrupted by delivery of another kind of signal. To avoid this, you can use the @code{sa_mask} member of the action structure passed to @code{sigaction} to explicitly specify which signals should be blocked while the signal handler runs. These signals are in addition to the signal for which the handler was invoked, and any other signals that are normally blocked by the process. @xref{Blocking for Handler}. If more than one signal of the same type is delivered to your process before your signal handler has a chance to be invoked at all, your handler may only be invoked once, as if only a single signal had arrived. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. @strong{Incomplete:} RMS suggests putting an example here showing a handler for @code{SIGCHLD}; the handler needs to have a loop since there may be more than one child process with status information available. @strong{Portability Note:} In System V Unix, handlers established with @code{signal} behave somewhat differently. Before calling the handler function, the action for the signal is set back to @code{SIG_DFL}. This means that the handler must re-enable itself before exiting and do any signal blocking explicitly. It also means that there is still a possibility that another signal could arrive in the moment before you've had a chance to alter the handling of the signal. The ANSI C standard permits this behavior, but you do not need to worry about this if you are using the GNU system. @node Nonreentrancy @subsection Signal Handling and Nonreentrant Functions @cindex restrictions on signal handler functions Handler functions usually don't do very much. The recommended behavior is to just have the handler set an external variable that the program checks regularly, and leave all serious work to the program. This is because the handler can be called at asynchronously, at unpredictable times---perhaps in the middle of a system call, or even between the beginning and the end of a floating-point operation that requires multiple instructions. The data structures being manipulated might therefore be in an inconsistent state when the handler function is invoked. This means you have to be very careful about what you do in a signal handler. @itemize @bullet @item Do not call any functions, including those from the standard libraries, unless you are sure they are @dfn{reentrant} with respect to signals. @cindex reentrant functions The most usual reason why a function fails to be reentrant is the use of static or global data structures. As an example of what can go wrong, suppose that the sample program prints a message through a @code{printf} call in the signal handler. Suppose also that @code{printf} and @code{fprintf} share some common internal data structures, and that the main program was in the middle of an @code{fprintf} call when the signal was delivered. Both the signal handler's message and the program's data could end up getting corrupted. @strong{Incomplete:} There should be a table here listing functions that are guaranteed to be reentrant, and/or something in each individual function description to indicate whether it's reentrant. @item If your handler calls any library functions that can modify the value of @code{errno}, it should save and restore the original value of @code{errno} before returning normally. This prevents errors that occur within the signal handler from being confused with errors from system calls at the point the program is interrupted to run the handler. @item If your handler needs to access any global variables from your program, those variables must be declared @code{volatile}. This tells the compiler that the value of the variable might change asynchronously, and keeps it from making some kinds of optimizations that would be invalidated by such modifications. @cindex @code{volatile} declarations @end itemize @node Atomic Data Access @subsection Atomic Data Access and Signal Handling Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessary @dfn{atomic}. This means that a signal handler can run in the middle of reading or writing a single object. It's very hard to predict what a program will do when that happens. To avoid the problem, you can use a particular data type for which access is always atomic: @code{sig_atomic_t}. The way you can get around this problem is by blocking all signals that have handlers that might access the variables around all the parts of the program that manipulate the variables, including in the handlers themselves. @xref{Blocking Signals}, for information on how to do this. @comment signal.h @comment ANSI @deftp {Data Type} sig_atomic_t This is an integer data type. It identifies a type of object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts. @end deftp @example #include #include struct two_words @{int a,b;@} struct two_words memory; void handler(int signum) @{ printf ("%d,%d\n", memory.a, memory.b); alarm (1); @} void main () @{ static struct two_words zeros=@{0,0@}, ones=@{1,1@}; signal (SIGALRM, handler); memory = zeros; alarm (1); while (1) @{ memory = zeros; memory = ones; @} @} @end example This program fills @code{memory} with zeros, ones, zeros, ones, alternating forever; meanwhile, once persecond, the alarm signal handler prints the current contents. (Calling @code{printf} in the handler is safe in this program because it is certainly not being called outside the handler when the signal happens.) Clearly, this program can print a pair of zeros or a pair of ones. But that's not all it can do! On most machines, it takes several instructions to store a new value in @code{memory}, and the value is stored one word at a time. If the signal is delivered in between these instructions, the handler might find that @code{memory.a} is zero and @code{memory.b} is one (or vice versa). On some machines it may be possible to store a new value in @code{memory} with just one instruction that cannot be interrupted. Then we say the value is stored @dfn{atomically}. On these machines, the handler will always print two zeros or two ones. @node Generating Signals @section Generating Signals @cindex sending signals @cindex raising signals @cindex signals, generating Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process. @menu * Raising a Signal:: Send yourself any desired signal. * Signaling Another Process:: Send a signal to another process. @end menu @node Signaling Yourself @subsection Signaling Yourself A process can send itself a signal with the @code{raise} function. This function is declared in @file{signal.h}. @pindex signal.h @comment signal.h @comment ANSI @deftypefun int raise (int @var{signum}) The @code{raise} function sends the signal @var{signum} to the calling process. It returns zero if successful and a nonzero value if it fails. About the only reason for failure would be if the value of @var{signum} is invalid. @end deftypefun @comment signal.h @comment SVID @deftypefun int gsignal (int @var{signum}) The @code{gsignal} function does the same thing as @code{raise}; it is provided only for compatibility with SVID. @end deftypefun One convenient use for @code{raise} is to reproduce the default behavior of a signal that you have trapped. For instance, suppose a user of your program types the SUSP character (usually @kbd{C-z}; @pxref{Special Characters}) to send it an interactive stop stop signal (@code{SIGTSTP}), and you want to clean up some internal data buffers before stopping. You might set this up like this: @comment RMS suggested getting rid of the handler for SIGCONT in this function. @comment But that would require that the handler for SIGTSTP unblock the @comment signal before doing the call to raise. We haven't covered that @comment topic yet, and I don't want to distract from the main point of @comment the example with a digression to explain what is going on. As @comment the example is written, the signal that is raise'd will be delivered @comment as soon as the SIGTSTP handler returns, which is fine. @example #include /* @r{When a stop signal arrives, set the action back to the default} * @r{and then resend the signal after doing cleanup actions.} */ void tstp_handler (int sig) @{ signal (SIGTSTP, SIG_DFL); /* @r{Do cleanup actions here.} */ @dots{} raise (SIGTSTP); @} /* @r{When the process is continued again, restore the signal handler.} */ void cont_handler (int sig) @{ signal (SIGCONT, cont_handler); signal (SIGTSTP, tstp_handler); @} /* @r{Enable both handlers during program initialization.} */ main (void) @{ signal (SIGCONT, cont_handler); signal (SIGTSTP, tstp_handler); @dots{} @} @end example @strong{Portability note:} @code{raise} was invented by the ANSI C committee. Older systems may not support it, so using @code{kill} may be more portable. @xref{Signaling Another Process}. @node Signaling Another Process @subsection Signaling Another Process @cindex killing a process The @code{kill} function can be used to send a signal to another process. In spite of its name, it can be used for a lot of things other than causing a process to terminate. Some examples of situations where you might want to send signals between processes are: @itemize @bullet @item A parent process starts a child to perform a task---perhaps having the child running an infinite loop---and then terminates the child when the task is no longer needed. @item A process executes as part of a group, and needs to terminate or notify the other processes in the group when an error or other event occurs. @item Two processes need to synchronize while working together. @end itemize This section assumes that you know a little bit about how processes work. For more information on this subject, see @ref{Processes}. The @code{kill} function is declared in @file{signal.h}. @pindex signal.h @comment signal.h @comment POSIX.1 @deftypefun int kill (pid_t @var{pid}, int @var{signum}) The @code{kill} function sends the signal @var{signum} to the process or process group specified by @var{pid}. Besides the signals listed in @ref{Standard Signals}, @var{signum} can also have a value of zero to check the validity of the @var{pid}. The @var{pid} specifies the process or process group to receive the signal: @table @code @item @var{pid} > 0 The process whose identifier is @var{pid}. @item @var{pid} == 0 All processes in the same process group as the sender. The sender itself does not receive the signal. @item @var{pid} < -1 The process group whose identifier is @minus{} of @var{pid}. @item @var{pid} == -1 If the process is privileged, send the signal to all processes except for some special system processes. Otherwise, send the signal to all processes with the same effective user ID. @end table A process can send a signal to itself with @code{kill (getpid(), @var{signum};}. If @code{kill} is used by a process to send a signal to itself, and the signal is not blocked, then @code{kill} delivers at least one signal (which might be some other pending unblocked signal instead of the signal @var{signum}) to that process before it returns. The return value from @code{kill} is zero if the signal can be sent successfully. Otherwise, no signal is sent, and a value of @code{-1} is returned. If @var{pid} specifies sending a signal to several processes, @code{kill} succeeds if it can send the signal to at least one of them. There's no way you can tell which of the processes got the signal or whether all of them did. The following @code{errno} error conditions are defined for this function: @table @code @item EINVAL The @var{signum} argument is an invalid or unsupported number. @item EPERM You do not have the privilege to send a signal to the process or any of the processes in the process group named by @var{pid}. @item ESCRH The @var{pid} argument does not refer to an existing process or group. @end table @end deftypefun @comment signal.h @comment BSD @deftypefun int killpg (int @var{pgid}, int @var{signum}) This is similar to @code{kill}, but sends signal @var{signum} to the process group @var{pgid}. This function is provided for compatibility with BSD; using @code{kill} to do this is more portable. @end deftypefun As a simple example of @code{kill}, the call: @example kill (getpid (), @var{sig}) @end example @noindent has the same effect as: @example raise (@var{sig}) @end example @node Permission for kill @subsection Permission for using @code{kill} There are restrictions that prevent you from using @code{kill} to send signals to any random process. These are intended to prevent antisocial behavior such as arbitrarily killing off processes belonging to another user. In typical use, @code{kill} is used to pass signals between parent, child, and sibling processes, and in these situations you normally do have permission to send signals. The only common execption is when you run a setuid program in a child process; if the program chanes its real uid as well as its effective uid, you may not have permission to send a signal. The @code{su} program does this. Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in @ref{User/Group IDs of a Process}. Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like @samp{root}), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions. The @code{SIGCONT} signal is a special case. It can be sent if the sender is part of the same session as the receiver, regardless of user IDs. @node Kill Example @subsection Using @code{kill} for Communication @cindex interprocess communication, with signals Here is a longer example showing how signals can be used for interprocess communication. This is what the @code{SIGUSR1} and @code{SIGUSR2} signals are provided for. Since these signals are fatal by default, the process that is supposed to receive them must trap them through @code{signal} or @code{sigaction}. In this example, a parent process forks a child process and then waits for the child to complete its initialization. The child process tells the parent when it is ready by sending it a @code{SIGUSR1} signal, using the @code{kill} function. @example #include #include #include #include /* @r{When a @code{SIGUSR1} signal arrives, set this variable.} */ volatile sig_atomic_t usr_interrupt = 0; void synch_signal (int sig) @{ usr_interrupt = 1; @} /* @r{The child process executes this function.} */ void child_function () @{ /* @r{Perform initialization.} */ printf ("I'm here!!! My pid is %d.\n", getpid ()); /* @r{Let parent know you're done.} */ kill (getppid (), SIGUSR1); /* @r{Continue with execution.} */ printf ("Bye, now....\n"); exit (0); @} void main () @{ struct sigaction usr_action; sigset_t block_mask; pid_t child_id; /* @r{Establish the signal handler.} */ sigfillset (&block_mask); usr_action.sa_handler = synch_signal; usr_action.sa_mask = block_mask; usr_action.sa_flags = 0; sigaction (SIGUSR1, &usr_action, NULL); /* @r{Create the child process.} */ if ((child_id = fork ()) == 0) child_function (); /* @r{Does not return} */ /* @r{Busy wait for child to send a signal.} */ while (!usr_interrupt) sleep(1); /* @r{Now continue execution.} */ printf ("That's all, folks!\n"); exit (0); @} @end example Most of the time, the signal is delivered during a @code{sleep}, and @code{sleep} returns immediately as a result. Once in a while, by luck, the signal arrives just before @code{sleep} is called. Then the program waits one extra second--an imperfection, but not a serious problem. There is an example in @ref{Waiting for a Signal} that shows you how you can make a program block until a signal arrives. @node Blocking Signals @section Blocking Signals @cindex blocking signals Blocking a signal means telling the operating system to hold it and deliver it later. Generally, a program does not block signals indefinitely---it might as well ignore them by setting their actions to @code{SIG_IGN}. But it is useful to block signals briefly, to prevent them from interrupting sensitive operations. For instance: @itemize @bullet @item You can use the @code{sigprocmask} function to block signals while you modify global variables that are also modified by the handlers for these signals. @item You can set @code{sa_mask} in your @code{sigaction} call to block certain signals while a particular signal handler runs. This way, the signal handler can run without being interrupted itself by signals. @end itemize @menu * Why Block:: The purpose of blocking signals. * Signal Sets:: How to specify which signals to block. * Process Signal Mask:: Blocking delivery of signals to your process during normal execution. * Blocking for Handler:: Blocking additional signals while a handler is being run. * Pending Signals:: How to tell if there are signals waiting to be delivered. * Remembering a Signal:: How you can get almost the same effect as blocking a signal, by handling it and setting a flag to be tested later. @end menu @node Why Block @subsection Why Blocking Signals is Useful Temporary blocking of signals with @code{sigprocmask} gives you a way to prevent interrupts during critical parts of your code. If signals arrive in that part of the program, they are delivered later, after you unblock them. One example where this is useful is for sharing data between a signal handler and the rest of the program. If the type of the data is not @code{sig_atomic_t} (@pxref{Atomic Data Access}), then the signal handler could run when the rest of the program has only half finished reading or writing the data. This would lead to confusing consequences. To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data---by blocking the appropriate signal around the parts of the program that touch the data. Blocking signals is also necessary when you want to perform a certain action only if a signal has not arrived. Suppose that the handler for the signal sets a flag of type @code{sig_atomic_t}; you would like to test the flag and perform the action if the flag is not set. This is unreliable. Suppose the signal is delivered immediately after you test the flag, but before the consequent action: then the program will perform the action even though the signal has arrived. The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked. @node Signal Sets @subsection Signal Sets All of the signal blocking functions use a data structure called a @dfn{signal set} to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function. @cindex signal set These facilities are declared in the header file @file{signal.h}. @pindex signal.h @comment signal.h @comment POSIX.1 @deftp {Data Type} sigset_t The @code{sigset_t} data type is used to represent a signal set. Internally, it may be implemented as either an integer or structure type. For portability, use only the functions described in this section to initialize, change, and retrieve information from @code{sigset_t} objects---don't try to manipulate them directly. @end deftp There are two ways to initialize a signal set. You can initially specify it to be empty with @code{sigemptyset} and then add specified signals individually. Or you can specify it to be full with @code{sigfillset} and then delete specified signals individually. You must always initialize the signal set with one of these two functions before using it in any other way. Don't try to set all the signals explicitly because the @code{sigset_t} object might include some other information (like a version field) that needs to be initialized as well. (In addition, it's not wise to put into your program an assumption that the system has no signals aside from the ones you know about.) @comment signal.h @comment POSIX.1 @deftypefun int sigemptyset (sigset_t *@var{set}) This function initializes the signal set @var{set} to exclude all of the defined signals. It always returns @code{0}. @end deftypefun @comment signal.h @comment POSIX.1 @deftypefun int sigfillset (sigset_t *@var{set}) This function initializes the signal set @var{set} to include all of the defined signals. Again, the return value is @code{0}. @end deftypefun @comment signal.h @comment POSIX.1 @deftypefun int sigaddset (sigset_t *@var{set}, int @var{signum}) This function adds the signal @var{signum} to the signal set @var{set}. All @code{sigaddset} does is modify @var{set}; it does not block or unblock any signals. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error condition is defined for this function: @table @code @item EINVAL The @var{signum} argument doesn't specify a valid signal. @end table @end deftypefun @comment signal.h @comment POSIX.1 @deftypefun int sigdelset (sigset_t *@var{set}, int @var{signum}) This function removes the signal @var{signum} from the signal set @var{set}. All @code{sigdelset} does is modify @var{set}; it does not block or unblock any signals. The return value and error conditions are the same as for @code{sigaddset}. @end deftypefun Finally, there is a function to test what signals are in a signal set: @comment signal.h @comment POSIX.1 @deftypefun int sigismember (const sigset_t *@var{set}, int @var{signum}) The @code{sigismember} function tests whether the signal @var{signum} is a member of the signal set @var{set}. It returns @code{1} if the signal is in the set, @code{0} if not, and @code{-1} if there is an error. The following @code{errno} error condition is defined for this function: @table @code @item EINVAL The @var{signum} argument doesn't specify a valid signal. @end table @end deftypefun @node Process Signal Mask @subsection Process Signal Mask @cindex signal mask @cindex process signal mask The collection of signals that are currently blocked is called the @dfn{signal mask}. Each process has its own signal mask. When you create a new process (@pxref{Creating New Processes}), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask. The prototype for the @code{sigprocmask} function is in @file{signal.h}. @pindex signal.h @comment signal.h @comment POSIX.1 @deftypefun int sigprocmask (int @var{how}, const sigset_t *@var{set}, sigset_t *@var{oldset}) The @code{sigprocmask} function is used to examine or change the calling process's signal mask. The @var{how} argument determines how the signal mask is changed, and must be one of the following values: @table @code @comment signal.h @comment POSIX.1 @vindex SIG_BLOCK @item SIG_BLOCK Block the signals in @code{set}---add them to the existing mask. In other words, the new mask is the union of the existing mask and @var{set}. @comment signal.h @comment POSIX.1 @vindex SIG_UNBLOCK @item SIG_UNBLOCK Unblock the signals in @var{set}---remove them from the existing mask. @comment signal.h @comment POSIX.1 @vindex SIG_SETMASK @item SIG_SETMASK Use @var{set} for the mask; ignore the previous value of the mask. @end table The last argument, @var{oldset}, is used to return information about the old process signal mask. If you just want to change the mask without looking at it, pass a null pointer as the @var{oldset} argument. Similarly, if you want to know what's in the mask without changing it, pass a null pointer for @var{set} (in this case the @var{how} argument is not significant). The @var{oldset} argument is often used to remember the previous signal mask in order to restore it later. (Since the signal mask is inherited over @code{fork} and @code{exec} calls, you can't predict what its contents are when your program starts running.) If invoking @code{sigprocmask} causes any pending signals to be unblocked, at least one of those signals is delivered to the process before @code{sigprocmask} returns. The order in which pending signals are delivered is not specified, but you can control the order explicitly by making multiple @code{sigprockmask} calls to unblock various signals one at a time. The @code{sigprocmask} function returns @code{0} if successful, and @code{-1} to indicate an error. The following @code{errno} error conditions are defined for this function: @table @code @item EINVAL The @var{how} argument is invalid. @end table You can't block the @code{SIGKILL} and @code{SIGSTOP} signals, but if the signal set includes these, @code{sigprocmask} just ignores them instead of returning an error status. Remember, too, that blocking program error signals such as @code{SIGFPE} leads to undesirable results for signals generated by an actual program error (as opposed to signals sent with @code{raise} or @code{kill}). This is because your program may be too broken to be able to continue executing to a point where the signal is unblocked again. @end deftypefun @node Testing for Delivery @subsection Blocking to Test for Delivery of a Signal Now for a simple example. Suppose you establish a handler for @code{SIGALRM} signals that sets a flag whenever a signal arrives, and your main program checks this flag from time to time and then resets it. You can prevent additional @code{SIGALRM} signals from arriving in the meantime by wrapping the critical part of the code with calls to @code{sigprocmask}, like this: @example /* @r{This variable is set by the SIGALRM signal handler.} */ volatile sig_atomic_t flag = 0; void main () @{ sigset_t block_alarm; @dots{} /* @r{Initialize the signal mask.} */ sigemptyset (&block_alarm); sigaddset (&block_alarm, SIGALRM); while (1) @{ /* @r{Check if a signal has arrived; if so, reset the flag.} */ sigprocmask (SIG_BLOCK, &block_alarm, NULL); if (flag) @{ @var{actions-if-not-arrived} flag = 0; @} sigprocmask (SIG_UNBLOCK, &block_alarm, NULL); @dots{} @} @} @end example @node Blocking for Handler @subsection Blocking Signals for a Handler @cindex blocking signals, in a handler When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data. When a handler function is invoked on a signal, that signal is automatically blocked (in addition to any other signals that are already in the process's signal mask) during the time the handler is running. If you set up a handler for @code{SIGTSTP}, for instance, then the arrival of that signal forces further @code{SIGTSTP} signals to wait during the execution of the handler. However, by default, other kinds of signals are not blocked; they can arrive during handler execution. The reliable way to block other kinds of signals during the execution of the handler is to use the @code{sa_mask} member of the @code{sigaction} structure. Here is an example: @example #include #include void catch_stop (); void install_handler (void) @{ struct sigaction setup_action; sigset_t block_mask; sigemptyset (&block_mask); /* @r{Block other terminal-generated signals while handler runs.} */ sigaddset (&block_mask, SIGINT); sigaddset (&block_mask, SIGQUIT); setup_action.sa_handler = catch_stop; setup_action.sa_mask = block_mask; setup_action.sa_flags = 0; sigaction (SIGTSTP, &setup_action, NULL); @} @end example This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicity in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked. You cannot remove signals from the process's current mask using this mechanism. However, you can make calls to @code{sigprocmask} within your handler to block or unblock signals as you wish. In any case, when the handler returns, the system restores the mask that was in place before the handler was entered. @node Checking for Pending Signals @subsection Checking for Pending Signals @cindex pending signals, checking for @cindex blocked signals, checking for @cindex checking for pending signals You can find out which signals are pending at any time by calling @code{sigpending}. This function is declared in @file{signal.h}. @pindex signal.h @comment signal.h @comment POSIX.1 @deftypefun int sigpending (sigset_t *@var{set}) The @code{sigpending} function stores information about pending signals in @var{set}. If there is a pending signal that is blocked from delivery, then that signal is a member of the returned set. (You can test whether a particular signal is a member of this set using @code{sigismember}; see @ref{Signal Sets}.) The return value is @code{0} if successful, and @code{-1} on failure. @end deftypefun Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design. Here is an example. @example #include #include sigset_t base_mask, waiting_mask; sigemptyset (&base_mask); sigaddset (&base_mask, SIGINT); sigaddset (&base_mask, SIGTSTP); /* @r{Block user interrupts while doing other processing.} */ sigprocmask (SIG_SETMASK, &base_mask, NULL); @dots{} /* @r{After a while, check to see whether any signals are pending.} */ sigpending (&waiting_mask); if (sigismember (&waiting_mask, SIGINT)) @{ /* @r{User has tried to kill the process.} */ @} else if (sigismember (&waiting_mask, SIGTSTP)) @{ /* @r{User has tried to stop the process.} */ @} @end example Remember that if there is a particular signal pending for your process, additional signals of that same type that arrive in the meantime might be discarded. For example, if a @code{SIGINT} signal is pending when another @code{SIGINT} signal arrives, your program will probably only see one of them when you unblock this signal. @node Remembering a Signal @subsection Remembering a Signal to Act On Later Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you ``unblock''. Here is an example: @example /* @r{If this flag is nonzero, don't handle the signal right away.} */ volatile sig_atomic_t signal_pending; /* @r{This is nonzero if a signal arrived and was not handled.} */ volatile sig_atomic_t defer_signal; void handler (int signum) @{ if (defer_signal) signal_pending = signum; else @dots{} /* @r{``Really'' handle the signal.} */ @} @dots{} void update_mumble (int frob) @{ /* @r{Prevent signals from having immediate effect.} */ defer_signal++; /* @r{Now update @code{mumble}, without worrying about interruption.} */ mumble.a = 1; mumble.b = hack (); mumble.c = frob; /* @r{We have updated @code{mumble}. Handle any signal that came in.} */ defer_signal--; if (defer_signal == 0 && signal_pending != 0) raise (signal_pending); @} @end example Note how the particular signal that arrives is stored in @code{signal_pending}. That way, we can handle several types of inconvenient signals with the same mechanism. We increment and decrement @code{defer_signal} so that nested critical sections will work properly; thus, if @code{update_mumble} were called with @code{signal_pending} already nonzero, signals would be deferred not only within @code{update_mumble}, but also within the caller. This is also why we do not check @code{signal_pending} if @code{defer_signal} is still nonzero. It is absolutely vital to decrement @code{defer_signal} before testing @code{signal_pending}, because this avoids a subtle bug. If we did these things in the other order, like this, @example if (defer_signal == 1 && signal_pending != 0) raise (signal_pending); defer_signal--; @end example @noindent then a signal arriving in between the if statement and the decrement would be effetively ``lost'' for an indefinite amount of time. The handler would merely set @code{defer_signal}, but the program having already tested this variable, it would not test the variable again. @cindex timing error in signal handling Bugs like these are called @dfn{timing errors}. They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can't expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them. (You would not be tempted to write the code in this order, given the use of @code{defer_signal} as a counter which must be tested along with @code{signal_pending}. After all, testing for zero is cleaner than testing for one. But if you did not use @code{defer_signal} as a counter, and gave it values of zero and one only, then either order might seem equally simple. This is a further advantage of using a counter for @code{defer_signal}: it will reduce the chance you will write the code in the wrong order and create a subtle bug.) @node Waiting for a Signal @section Waiting for a Signal @cindex waiting for a signal @cindex @code{pause} function If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives. @menu * Using Pause:: The simple way, using @code{pause}. * Pause Problems:: Why the simple way is often not very good. * Sigsuspend:: Reliably waiting for a specific signal. @end menu @node Using Pause @subsection Using @code{pause} The simple way to wait until a signal arrives is to call @code{pause}. Please read about its disadvantages, in the following section, before you use it. @comment unistd.h @comment POSIX.1 @deftypefun int pause (void) The @code{pause} function suspends program execution until a signal arrives whose action is either to execute a handler function, or to terminate the process. If the signal causes a handler function to be executed, then @code{pause} returns. This is considered an unsuccessful return (since ``successful'' behavior would be to suspend the program forever), so the return value is @code{-1}. The following @code{errno} error conditions are defined for this function: @table @code @item EINTR The function was interrupted by delivery of a signal. @end table If the signal causes program termination, @code{pause} doesn't return (obviously). The @code{pause} function is declared in @file{unistd.h}. @end deftypefun @node Pause Problems @subsection Problems with @code{pause} The simplicity of @code{pause} can conceal serious timing errors that can make a program hang mysteriously. It is safe to use @code{pause} if the real work of your program is done by the signal handlers themselves, and the ``main program'' does nothing but call @code{pause}. Each time a signal is delivered, the handler will do the next batch of work that is to be done, and then return, so that the main loop of the program can call @code{pause} again. You can't safely use @code{pause} to wait until one more signal arrives, and then resume real work. Even if you arrange for the signal handler to cooperate by setting a flag, you still can't use @code{pause} reliably. Here is an example of this problem: @example /* @r{@code{usr_interrupt} is set by the signal handler.} */ if (!usr_interrupt) pause (); /* @r{Do work once the signal arrives.} */ @dots{} @end example @noindent This has a bug: the signal could arrive after the variable @code{usr_interrupt} is checked, but before the call to @code{pause}. If no further signals arrive, the process would never wake up again. You can put an upper limit on the excess waiting by using @code{sleep} in a loop, instead of using @code{pause}. (@xref{Sleeping}, for more about @code{sleep}.) Here is what this looks like: @example /* @r{@code{usr_interrupt} is set by the signal handler.} while (!usr_interrupt) sleep (1); /* @r{Do work once the signal arrives.} */ @dots{} @end example For some purposes, that is good enough. But with a little more complexity, you can wait reliably until a particular signal handler is run, using @code{sigsuspend}. @ifinfo @xref{Sigsuspend}. @end ifinfo @node Sigsuspend @subsection Using @code{sigsuspend} The clean and reliable way to wait for a signal to arrive is to block it and then use @code{sigsuspend}. This also enables you to wait for certain kinds of signals, while letting other kinds of signals be handled by their handlers. @comment signal.h @comment POSIX.1 @deftypefun int sigsuspend (const sigset_t *@var{set}) This function replaces the process's signal mask with @var{set} and then suspends the process until a signal is delivered whose action is either to terminate the process or invoke a signal handling function. In other words, the program is effectively suspended until one of the signals that is not a member of @var{set} arrives. If the process is woken up by deliver of a signal that invokes a handler function, and the handler function returns, then @code{sigsuspend} also returns. The mask remains @var{set} only as long as @code{sigsuspend} is waiting. The function @code{sigsuspend} always restores the previous signal mask when it returns. The return value and error conditions are the same as for @code{pause}. @end deftypefun Here is how to replace the @code{pause} or @code{sleep} loop with something completely reliable: @example sigset_t mask, oldmask; @dots{} /* @r{Set up the mask of signals to temporarily block.} */ sigemptyset (&mask); sigaddset (&mask, SIGUSR1); @dots{} /* @r{Wait for a signal to arrive.} */ sigprocmask (SIG_BLOCK, &mask, &oldmask); while (!usr_interrupt) sigsuspend (&oldmask); sigprocmask (SIG_UNBLOCK, &mask, NULL); @end example This last piece of code is a little tricky. The key point to remember here is that when @code{sigsuspend} returns, it resets the process's signal mask to the original value, the value from before the call to @code{sigsuspend}---in this case, the @code{SIGUSR1} signal is once again blocked. The second call to @code{sigprocmask} is necessary to explicitly unblock this signal. One other point: you may be wondering why the @code{while} loop is necessary at all, since the program is apparently only waiting for one @code{SIGUSR1} signal. The answer is that the mask passed to @code{sigsuspend} permits the process to be woken up by the delivery of other kinds of signals, as well---for example, job control signals. If the process is woken up by a signal that doesn't set @code{usr_interrupt}, it just suspends itself again until the ``right'' kind of signal eventually arrives. This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines. @node BSD Signal Handling @section BSD Signal Handling This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix. They do provide one feature that is not available through the POSIX functions: You can specify a separate stack for use in certain signal handlers. Using a signal stack is the only way you can handle a signal caused by stack overflow. @menu * POSIX vs BSD:: Overview comparing BSD and POSIX signal functions. * BSD Handlers:: BSD functions for specifying signal handling. * Blocking in BSD:: BSD functions for blocking, unblocking, and waiting. * Signal Stack:: How to use a separate signal stack. @end menu @node POSIX vs BSD @subsection POSIX and BSD Signal Facilities There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are: @itemize @bullet @item BSD Unix represents signal masks as an @code{int} bit mask, rather than as a @code{sigset_t} object. @item The BSD facilities use a different default for whether an interrupted system call should fail or resume. The POSIX facilities make system calls fail unless you specify that they should resume. With the BSD facility, the default is to make system calls resume unless you say they should fail. @item BSD Unix has a concept of a @dfn{signal stack}. This is an alternate stack that is used during the execution of signal handler functions, instead of its normal execution stack. @cindex signal stack @end itemize The BSD facilities are declared in @file{signal.h}. @pindex signal.h @node BSD Handler @section BSD Function to Establish a Handler @comment signal.h @comment BSD @deftp {Data Type} {struct sigvec} This data type is the BSD equivalent of @code{struct sigaction} (@pxref{Advanced Signal Handling}); it is used to specify signal actions to the @code{sigvec} function. It contains the following members: @table @code @item sighandler_t sv_handler This is the handler function. @item int sv_mask This is the mask of additional signals to be blocked while the handler function is being called. @item int sv_flags This is a bit mask used to specify various flags which affect the behavior of the signal. You can also refer to this field as @code{sv_onstack}. @end table @end deftp These symbolic constants can be used to provide values for the @code{sv_flags} field of a @code{sigvec} structure. This field is a bit mask value, so you bitwise-OR the flags of interest to you together. @comment signal.h @comment BSD @deftypevr Macro int SV_ONSTACK If this bit is set in the @code{sv_flags} field of a @code{sigvec} structure, it means to use the signal stack when delivering the signal. @end deftypevr @comment signal.h @comment BSD @deftypevr Macro int SV_INTERRUPT If this bit is set in the @code{sv_flags} field of a @code{sigvec} structure, it means that system calls interrupted by this kind of signal should not be restarted if the handler returns; instead, the system calls should return with a @code{EINTR} error status. @end deftypevr @comment signal.h @comment BSD @deftypevr Macro int SV_RESETHAND If this bit is set in the @code{sv_flags} field of a @code{sigvec} structure, it means to reset the action for the signal back to @code{SIG_DFL} when the signal is received. @end deftypevr @comment signal.h @comment BSD @deftypefun int sigvec (int @var{signum}, const struct sigvec *@var{action},struct sigvec *@var{old_action}) This function is the equivalent of @code{sigaction} (@pxref{Advanced Signal Handling}); it installs the action @var{action} for the signal @var{signum}, returning information about the previous action in effect for that signal in @var{old_action}. @end deftypefun @comment signal.h @comment BSD @deftypefun int siginterrupt (int @var{signum}, int @var{interrupt}) This function is used to change the system call interrupt behavior. If @var{interrupt} is false, system calls are restarted when interrupted by receipt of the signal @var{signum}. If @var{interrupt} is true, system calls return with a @code{EINTR} error when interrupted. @end deftypefun @node Blocking in BSD @subsection BSD Functions for Blocking Signals @comment signal.h @comment BSD @deftypefn Macro int sigmask (int @var{signum}) This macro returns a signal mask that has the bit for signal @var{signum} set. You can bitwise-OR the results of several calls to @code{sigmask} together to specify more than one signal. For example, @example (sigmask (SIGTSTP) | sigmask (SIGSTOP) | sigmask (SIGTTIN) | sigmask (SIGTTOU)) @end example @noindent specifies a mask that includes all the job-control stop signals. @end deftypefn @comment signal.h @comment BSD @deftypefun int sigblock (int @var{mask}) This function is the equivalent of @code{sigprocmask} (@pxref{Process Signal Mask}) with a @var{how} argument of @code{SIG_BLOCK}: it adds the signals specified by @var{mask} to the calling process's signal mask. The return value is the previous set of blocked signals. @end deftypefun @comment signal.h @comment BSD @deftypefun int sigsetmask (int @var{mask}) This function is the equivalent of @code{sigprocmask} (@pxref{Process Signal Mask}) with a @var{how} argument of @code{SIG_SETMASK}: it sets the calling process's signal mask to @var{mask}. The return value is the previous set of blocked signals. @end deftypefun @comment signal.h @comment BSD @deftypefun int sigpause (int @var{mask}) This function is the equivalent of @code{sigsuspend} (@pxref{Waiting for a Signal}): it sets the calling process's signal mask to @var{mask}, and waits for a signal to arrive. On return the previous set of blocked signals is restored. @end deftypefun @node Signal Stack @subsection Using a Separate Signal Stack A signal stack is a special area of memory to be used as the execution stack during signal handlers. It should be fairly large, to avoid any danger that it will overflow in turn---we recommend at least 16,000 bytes. You can use @code{malloc} to allocate the space for the stack. Then call @code{sigstack} to tell the system to use that space for the signal stack. You don't need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. However, some debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running. @comment signal.h @comment BSD @deftp {Data Type} {struct sigstack} This structure describes a signal stack. It contains the following members: @table @code @item void *ss_sp This is the stack pointer. @item int ss_onstack This field is true if the process is currently using this stack. @end table @end deftp @comment signal.h @comment BSD @deftypefun int sigstack (const struct sigstack *@var{stack}, struct sigstack *@var{oldstack}) The @code{sigstack} function specifies an alternate stack for use during signal handling. When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed. If @var{oldstack} is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If @var{stack} is not a null pointer, then this is installed as the new stack for use by signal handlers. The return value is @code{0} on success and @code{1} on failure. @end deftypefun