@node Processes, Job Control, Signal Handling, Top @chapter Processes @cindex process @dfn{Processes} are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies. Processes are organized hierarchically. Child processes are created by a parent process, and inherit many of their attributes from the parent process. This chapter describes how a program can create, terminate, and control child processes. @menu * Program Arguments:: Parsing the command-line arguments to a program. * Environment Variables:: How to access parameters inherited from a parent process. * Program Termination:: How to cause a process to terminate and return status information to its parent. * Creating New Processes:: Running other programs. @end menu @node Program Arguments, Environment Variables, , Processes @section Program Arguments @cindex program arguments @cindex command line arguments @cindex @code{main} function When your C program starts, it begins by executing the function called @code{main}. You can define @code{main} either to take no arguments, or to take two arguments that represent the command line arguments to the program, like this: @example int main (int @var{argc}, char *@var{argv}[]) @end example @cindex argc (program argument count) @cindex argv (program argument vector) The command line arguments are the whitespace-separated tokens typed by the user to the shell in invoking the program. The value of the @var{argc} argument is the number of command line arguments. The @var{argv} argument is a vector of pointers to @code{char}; sometimes it is also declared as @samp{char **@var{argv}}. The elements of @var{argv} are the individual command line argument strings. By convention, @code{@var{argv}[0]} is the file name of the program being run, and @code{@var{argv}[@var{argc}]} is a null pointer. If the syntax for the command line arguments to your program is simple enough, you can simply pick the arguments off from @var{argv} by hand. But unless your program takes a fixed number of arguments, or all of the arguments are interpreted in the same way (as file names, for example), you are usually better off using @code{getopt} to do the parsing. @menu * Argument Syntax Conventions:: By convention, program options are specified by a leading hyphen. * Parsing Program Arguments:: The @code{getopt} function. * Example Using getopt:: An example of @code{getopt}. @end menu @node Argument Syntax Conventions, Parsing Program Arguments, , Program Arguments @subsection Program Argument Syntax Conventions @cindex program argument syntax @cindex syntax, for program arguments @cindex command argument syntax The @code{getopt} function decodes options following the usual conventions for POSIX utilities: @itemize @bullet @item Arguments are options if they begin with a hyphen delimiter (@samp{-}). @item Multiple options may follow a hyphen delimiter in a single token if the options do not take arguments. Thus, @samp{-abc} is equivalent to @samp{-a -b -c}. @item Option names are single alphanumeric (as for @code{isalnum}; see @ref{Classification of Characters}). @item Certain options require an argument. For example, the @samp{-o} command of the ld command requires an argument---an output file name. @item An option and its argument may or may appear as separate tokens. (In other words, the whitespace separating them is optional.) Thus, @samp{-o foo} and @samp{-ofoo} are equivalent. @item Options typically precede other non-option arguments. The implementation of @code{getopt} in the GNU C library normally makes it appear as if all the option arguments were specified before all the non-option arguments for the purposes of parsing, even if the user of your program intermixed option and non-option arguments. It does this by reordering the elements of the @var{argv} array. This behavior is nonstandard; if you want to suppress it, define the @code{_POSIX_OPTION_ORDER} environment variable. @xref{Standard Environment Variables}. @item The argument @samp{--} terminates all options; any following arguments are treated as non-option arguments, even if they begin with a hyphen. @item A token consisting of a single hyphen character is interpreted as an ordinary non-option argument. By convention, it is used to specify input from or output to the standard input and output streams. @item Options may be supplied in any order, or appear multiple times. The interpretation is left up to the particular application program. @end itemize @node Parsing Program Arguments, Example Using getopt, Argument Syntax Conventions, Program Arguments @subsection Parsing Program Arguments @cindex program arguments, parsing @cindex command arguments, parsing @cindex parsing program arguments Here are the details about how to call the @code{getopt} function. To use this facility, your program must include the header file @file{unistd.h}. @pindex unistd.h @comment unistd.h @comment POSIX.2 @deftypevar int opterr If the value of this variable is nonzero, then @code{getopt} prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument. This is the default behavior. If you set this variable to zero, @code{getopt} does not print any messages, but it still returns @code{?} to indicate an error. @end deftypevar @comment unistd.h @comment POSIX.2 @deftypevar int optopt When @code{getopt} encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable. You can use this for providing your own diagnostic messages. @end deftypevar @comment unistd.h @comment POSIX.2 @deftypevar int optind This variable is set by @code{getopt} to the index of the next element of the @var{argv} array to be processed. Once @code{getopt} has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin. The initial value of this variable is @code{1}. @end deftypevar @comment unistd.h @comment POSIX.2 @deftypevar {char *} optarg This variable is set by @code{getopt} to point at the value of the option argument, for those options that accept arguments. @end deftypevar @comment unistd.h @comment POSIX.2 @deftypefun int getopt (int @var{argc}, char **@var{argv}, const char *@var{options}) The @code{getopt} function gets the next option argument from the argument list specified by the @var{argv} and @var{argc} arguments. Normally these arguments' values come directly from the arguments of @code{main}. The @var{options} argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (@samp{:}) to indicate that it takes a required argument. If the @var{options} argument string begins with a hyphen (@samp{-}), this is treated specially. It permits arguments without an option to be returned as if they were associated with option character @samp{\0}. The @code{getopt} function returns the option character for the next command line option. When no more option arguments are available, it returns @code{-1}. There may still be more non-option arguments; you must compare the external variable @code{optind} against the @var{argv} parameter to check this. If the options has an argument, @code{getopt} returns the argument by storing it in the varables @var{optarg}. You don't ordinarily need to copy the @code{optarg} string, since it is a pointer into the original @var{argv} array, not into a static area that might be overwritten. If @code{getopt} finds an option character in @var{argv} that was not included in @var{options}, or a missing option argument, it returns @samp{?} and sets the external variable @code{optopt} to the actual option character. In addition, if the external variable @code{opterr} is nonzero, @code{getopt} prints an error message. @end deftypefun @node Example Using getopt, , Parsing Program Arguments, Program Arguments @subsection Example of Parsing Program Arguments Here is an example showing how @code{getopt} is typically used. The key points to notice are: @itemize @bullet @item Normally, @code{getopt} is called in a loop. When @code{getopt} returns @code{-1}, indicating no more options are present, the loop terminates. @item A @code{switch} statement is used to dispatch on the return value from @code{getopt}. In typical use, each case just sets a variable that is used later in the program. @item A second loop is used to process the remaining non-option arguments. @end itemize @example @include testopt.c.texi @end example Here are some examples showing what this program prints with different combinations of arguments: @example % testopt aflag = 0, bflag = 0, cvalue = (null) % testopt -a -b aflag = 1, bflag = 1, cvalue = (null) % testopt -ab aflag = 1, bflag = 1, cvalue = (null) % testopt -c foo aflag = 0, bflag = 0, cvalue = foo % testopt -cfoo aflag = 0, bflag = 0, cvalue = foo % testopt arg1 aflag = 0, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -a arg1 aflag = 1, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -c foo arg1 aflag = 0, bflag = 0, cvalue = foo Non-option argument arg1 % testopt -a -- -b aflag = 1, bflag = 0, cvalue = (null) Non-option argument -b % testopt -a - aflag = 1, bflag = 0, cvalue = (null) Non-option argument - @end example @node Environment Variables, Program Termination, Program Arguments, Processes @section Environment Variables @cindex environment variable When a program is executed, it receives information about the context in which it was invoked in two ways. The first mechanism uses the @var{argv} and @var{argc} arguments to its @code{main} function, and is discussed in @ref{Program Arguments}. The second mechanism is uses @dfn{environment variables} and is discussed in this section. The @var{argv} mechanism is typically used to pass command-line arguments specific to the particular program being invoked. The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently accessed. The environment variables discussed in this section are the same environment variables that you set using the assignments and the @code{export} command in the shell. Programs executed from the shell inherit all of the environment variables from the shell. @cindex environment Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes. The set of all environment variables that have values is collectively known as the @dfn{environment}. Names of environment variables are case-sensitive and must not contain the character @samp{=}. System-defined environment variables are invariably uppercase. The values of environment variables can be anything that can be represented as a string. A value must not contain an embedded null character, since this is assumed to terminate the string. @menu * Environment Access:: How to get and set the values of environment variables. * Standard Environment Variables:: These environment variables have standard interpretations. @end menu @node Environment Access, Standard Environment Variables, , Environment Variables @subsection Environment Access @cindex environment access @cindex environment representation The value of an environment variable can be accessed with the @code{getenv} function. This is declared in the header file @file{stdlib.h}. @pindex stdlib.h @comment stdlib.h @comment ANSI @deftypefun {char *} getenv (const char *@var{name}) This function returns a string that is the value of the environment variable @var{name}. You must not modify this string. In some systems not using the GNU library, it might be overwritten by subsequent calls to @code{getenv} (but not by any other library function). If the environment variable @var{name} is not defined, the value is a null pointer. @end deftypefun @comment stdlib.h @comment SVID @deftypefun int putenv (const char *@var{string}) The @code{putenv} function adds or removes definitions from the environment. If the @var{string} is of the form @samp{@var{name}=@var{value}}, the definition is added to the environment. Otherwise, the @var{string} is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed. The GNU library provides this function for compatibility with SVID; it may not be available in other systems. @end deftypefun You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see @ref{Executing a File}). @comment unistd.h @comment POSIX.1 @deftypevar {char **} environ The environment is represented as an array of strings. Each string is of the format @samp{@var{name}=@var{value}}. The order in which strings appear in the environment is not significant, but the same @var{name} must not appear more than once. The last element of the array is a null pointer. This variable is not declared in any header file, but if you declare it in your own program as @code{extern}, the right thing will happen. If you just want to get the value of an environment variable, use @code{getenv}. @end deftypevar @node Standard Environment Variables, , Environment Access, Environment Variables @subsection Standard Environment Variables @cindex standard environment variables These environment variables have standard meanings. This doesn't mean that they are always present in the environment, though; it just means that if these variables @emph{are} present, they have these meanings, and that you shouldn't try to use these environment variable names for some other purpose. @table @code @item HOME @cindex HOME environment variable @cindex home directory This is a string representing the user's @dfn{home directory}, or initial default working directory. @xref{User Database}, for a more secure way of determining this information. @comment RMS says to explay why HOME is better, but I don't know why. @item LOGNAME @cindex LOGNAME environment variable This is the name that the user used to log in. Since the value in the environment can be tweaked arbitrarily, this is not a reliable way to identify the user who is running a process; a function like @code{getlogin} (@pxref{User Identification Functions}) is better for that purpose. @comment RMS says to explay why LOGNAME is better, but I don't know why. @item PATH @cindex PATH environment variable A @dfn{path} is a sequence of directory names which is used for searching for a file. The variable @var{PATH} holds a path The @code{execlp} and @code{execvp} functions (@pxref{Executing a File}) uses this environment variable, as do many shells and other utilities which are implemented in terms of those functions. The syntax of a path is a sequence of directory names separated by colons. An empty string instead of a directory name stands for the current directory. (@xref{Working Directory}.) A typical value for this environment variable might be a string like: @example .:/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local:/usr/local/bin @end example This means that if the user tries to execute a program named @code{foo}, the system will look for files named @file{./foo}, @file{/bin/foo}, @file{/etc/foo}, and so on. The first of these files that exists is the one that is executed. @item TERM @cindex TERM environment variable This specifies the kind of terminal that is receiving program output. Some programs can make use of this information to take advantage of special escape sequences or terminal modes supported by particular kinds of terminals. Many programs which use the termcap library (@pxref{Finding a Terminal Description,Find,,termcap,The Termcap Library Manual}) use the @code{TERM} environment variable, for example. @item TZ @cindex TZ environment variable This specifies the time zone. @xref{Time Zone}, for information about the format of this string and how it is used. @item LANG @cindex LANG environment variable This specifies the default locale to use for attribute categories where neither @code{LC_ALL} nor the specific environment variable for that category is set. @xref{Locales}, for more information about locales. @item LC_ALL @cindex LC_ALL environment variable This is similar to the @code{LANG} environment variable. However, its value takes precedence over any values provided for the individual attribute category environment variables, or for the @code{LANG} environment variable. @item LC_COLLATE @cindex LC_COLLATE environment variable This specifies what locale to use for string sorting. @item LC_CTYPE @cindex LC_CTYPE environment variable This specifies what locale to use for character sets and character classification. @item LC_MONETARY @cindex LC_MONETARY environment variable This specifies what locale to use for formatting monetary values. @item LC_NUMERIC @cindex LC_NUMERIC environment variable This specifies what locale to use for formatting numbers. @item LC_TIME @cindex LC_TIME environment variable This specifies what locale to use for formatting date/time values. @item _POSIX_OPTION_ORDER @cindex _POSIX_OPTION_ORDER environment variable. If this environment variable is defined, it suppresses the usual reordering of command line arguments by @code{getopt}. @xref{Program Argument Syntax Conventions}. @end table @node Program Termination, Creating New Processes, Environment Variables, Processes @section Program Termination @cindex program termination @cindex process termination @cindex exit status value The usual way for a program to terminate is simply for its @code{main} function to return. The @dfn{exit status value} returned from the @code{main} function is used to report information back to the process's parent process or shell. A program can also terminate normally calling the @code{exit} function In addition, programs can be terminated by signals; this is discussed in more detail in @ref{Signal Handling}. The @code{abort} function causes a terminal that kills the program. @menu * Normal Program Termination:: * Exit Status:: Exit Status * Cleanups on Exit:: Cleanups on Exit * Aborting a Program:: * Termination Internals:: Termination Internals @end menu @node Normal Program Termination, Exit Status, , Program Termination @subsection Normal Program Termination @comment stdlib.h @comment ANSI @deftypefun void exit (int @var{status}) The @code{exit} function causes normal program termination with status @var{status}. This function does not return. @end deftypefun When a program terminates normally by returning from its @code{main} function or by calling @code{exit}, the following actions occur in sequence: @enumerate @item Functions that were registered with the @code{atexit} or @code{on_exit} functions are called in the reverse order of their registration. This mechanism allows your application to specify its own ``cleanup'' actions to be performed at program termination. Typically, this is used to do things like saving program state information in a file, or unlock locks in shared data bases. @item All open streams are closed; writing out any buffered output data. See @ref{Opening and Closing Streams}. In addition, temporary files opened with the @code{tmpfile} function are removed; see @ref{Temporary Files}. @item @code{_exit} is called. @xref{Termination Internals} @end enumerate @node Exit Status, Cleanups on Exit, Normal Program Termination, Program Termination @subsection Exit Status @cindex exit status When a program exits, it can return to the parent process a small amount of information about the cause of termination, using the @dfn{exit status}. This is a value between 0 and 255 that the exiting process passes as an argument to @code{exit}. Normally you should use the exit status to report very broad information about success or failure. You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway. There are conventions for what sorts of status values certain programs should return. The most common convention is simply 0 for success and 1 for failure. Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare. Your program should follow an existing convention if an existing convention makes sense for it. A general convention reserves status values 128 and up for special purposes. In particular, the value 128 is used to indicate failure to execute another program in a subprocess. This convention is not universally obeyed, but it is a good idea to follow it in your programs. @strong{Warning:} Don't try to use the number of errors as the exit status. This is actually not very useful; a parent process would generally not care how many errors occurred. Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors---that is, success. For the same reason, it does not work to use the value of @code{errno} as the exit status---these can exceed 255. @strong{Portability note:} Some non-POSIX systems use different conventions for exit status values. For greater portability, you can use the macros @code{EXIT_SUCCESS} and @code{EXIT_FAILURE} for the conventional status value for success and failure, respectively. They are declared in the file @file{stdlib.h}. @pindex stdlib.h @comment stdlib.h @comment ANSI @deftypevr Macro int EXIT_SUCCESS This macro can be used with the @code{exit} function to indicate successful program completion. On POSIX systems, the value of this macro is @code{0}. On other systems, the value might be some other (possibly non-constant) integer expression. @end deftypevr @comment stdlib.h @comment ANSI @deftypevr Macro int EXIT_FAILURE This macro can be used with the @code{exit} function to indicate unsuccessful program completion in a general sense. On POSIX systems, the value of this macro is @code{1}. On other systems, the value might be some other (possibly non-constant) integer expression. Other nonzero status values also indicate future. Certain programs use different nonzero status values to indicate particular kinds of "non-success". For example, @code{diff} uses status value @code{1} to mean that the files are different, and @code{2} or more to mean that there was difficulty in opening the files. @end deftypevr @node Cleanups on Exit, Aborting a Program, Exit Status, Program Termination @subsection Cleanups on Exit @comment stdlib.h @comment ANSI @deftypefun int atexit (void (*@var{function})) The @code{atexit} function registers the function @var{function} to be called at normal program termination. The @var{function} is called with no arguments. The return value from @code{atexit} is zero on success and nonzero if the function cannot be registered. @end deftypefun @comment stdlib.h @comment GNU @deftypefun int on_exit (void (*@var{function})(int @var{status}, void *@var{arg}), void *@var{arg}) This function is a somewhat more powerful variant of @code{atexit}. It accepts two arguments, a function @var{function} and an arbitrary pointer @var{arg}. At normal program termination, the @var{function} is called with two arguments: the @var{status} value passed to @code{exit}, and the @var{arg}. This function is a GNU extension, and may not be supported by other implementations. @end deftypefun Here's a trivial program that illustrates the use of @code{exit} and @code{atexit}: @example #include <stdio.h> #include <stdlib.h> void bye (void) @{ printf ("Goodbye, cruel world....\n"); @} void main (void) @{ atexit (bye); exit (EXIT_SUCCESS); @} @end example @noindent When this program is executed, it just prints the message and exits. @node Aborting a Program, Termination Internals, Cleanups on Exit, Program Termination @subsection Aborting a Program @cindex aborting a program You can abort your program using the @code{abort} function. The prototype for this function is in @file{stdlib.h}. @pindex stdlib.h @comment stdlib.h @comment ANSI @deftypefun void abort () The @code{abort} function causes abnormal program termination, without executing functions registered with @code{atexit} or @code{on_exit}. This function actually terminates the process by raising a @code{SIGABRT} signal, and your program can include a handler to intercept this signal; see @ref{Signal Handling}. @strong{Incomplete:} Why would you want to define such a handler? @end deftypefun @node Termination Internals, , Aborting a Program, Program Termination @subsection Termination Internals The @code{_exit} function is the primitive used for process termination by @code{exit}. It is declared in the header file @file{unistd.h}. @pindex unistd.h @comment unistd.h @comment POSIX.1 @deftypefun void _exit (int @var{status}) The @code{_exit} function is the primitive for causing a process to terminate with status @var{status}. Calling this function does not execute cleanup functions registered with @code{atexit} or @code{on_exit}. @end deftypefun When a process terminates for any reason---either by an explicit termination call, or termination as a result of a signal---the following things happen: @itemize @bullet @item All open file descriptors in the process are closed. @xref{Low-Level Input/Output}. @item The low-order 8 bits of the return status code are saved to be reported back to the parent process via @code{wait} or @code{waitpid}; see @ref{Process Completion}. @item Any child processes of the process being terminated are assigned a new parent process. (This is the @code{init} process, with process ID 1.) @item A @code{SIGCHLD} signal is sent to the parent process. @item If the process is a session leader that has a controlling terminal, then a @code{SIGHUP} signal is sent to each process in the foreground job, and the controlling terminal is disassociated from that session. @xref{Job Control}. @item If termination of a process causes a process group to become orphaned, and any member of that process group is stopped, then a @code{SIGHUP} signal and a @code{SIGCONT} signal are sent to each process in the group. @xref{Job Control}. @end itemize @node Creating New Processes, , Program Termination, Processes @section Creating New Processes This section describes how your program can cause other programs to be executed. Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program. The @code{system} function provides a simple, portable mechanism for running another program; it does all three steps automatically. If you need more control over the details of how this is done, you can use the primitive functions to do each step individually instead. @menu * Running a Command:: The easy way to run another program. * Process Creation Concepts:: An overview of the hard way to do it. * Process Identification:: How to get the process ID of a process. * Creating a Process:: How to fork a child process. * Executing a File:: How to get a process to execute another program. * Process Completion:: How to tell when a child process has completed. * Process Completion Status:: How to interpret the status value returned from a child process. * BSD wait Functions:: More functions, for backward compatibility. * Process Creation Example:: A complete example program. @end menu @node Running a Command, Process Creation Concepts, , Creating New Processes @subsection Running a Command @cindex running a command The easy way to run another program is to use the @code{system} function. This function does all the work of running a subprogram, but it doesn't give you much control over the details: you have to wait until the subprogram terminates before you can do anything else. @pindex stdlib.h @comment stdlib.h @comment ANSI @deftypefun int system (const char *@var{command}) This function executes @var{command} as a shell command. In the GNU C library, it always uses the default shell @code{sh} to run the command. In particular, it searching the directories in @code{PATH} to find programs to execute. The return value is @code{-1} if it wasn't possible to create the shell process, and otherwise is the status of the shell process. @xref{Process Completion}, for details on how this status code can be interpreted. @pindex sh @end deftypefun The @code{system} function is declared in the header file @file{stdlib.h}. @strong{Portability Note:} Some C implementations may not have any notion of a command processor that can execute other programs. You can determine whether a command processor exists by executing @code{system (o)}; in this case the return value is nonzero if and only if such a processor is available. The @code{popen} and @code{pclose} functions (@pxref{Pipe to a Subprocess}) are closely related to the @code{system} function. They allow the parent process to communicate with the standard input and output channels of the command being executed. @node Process Creation Concepts, Process Identification, Running a Command, Creating New Processes @subsection Process Creation Concepts This section gives an overview of processes and of the steps involved in creating a process and making it run another program. @cindex process ID @cindex process lifetime Each process is named by a @dfn{process ID} number. A unique process ID is allocated to each process when it is created. The @dfn{lifetime} of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed. @cindex creating a process @cindex forking a process @cindex child process @cindex parent process Processes are created with the @code{fork} system call (so the operation of creating a new process is sometimes called @dfn{forking} a process). The @dfn{child process} created by @code{fork} is an exact clone of the original @dfn{parent process}, except that it has its own process ID. After forking a child process, both the parent and child processes continue to execute normally. If you want your program to wait for a child process to finish executing before continuing, you must do this explicitly after the fork operation. This is done with the @code{wait} or @code{waitpid} functions (@pxref{Process Completion}). These functions give the parent information about why the child terminated---for example, its exit status code. A newly forked child process continues to execute the same program as its parent process, at the point where the @code{fork} call returns. You can use the return value from @code{fork} to tell whether the program is running in the parent process or the child. @cindex process image Having all processes run the same program is usually not very useful. But the child can execute another program using one of the @code{exec} functions; see @ref{Executing a File}. The program that the process is executing is called its @dfn{process image}. Starting execution of a new program causes the process to forget all about its current process image; when the new program exits, the process exits too, instead of returning to the previous process image. @node Process Identification, Creating a Process, Process Creation Concepts, Creating New Processes @subsection Process Identification The @code{pid_t} data type represents process IDs. You can get the process ID of a process by calling @code{getpid}. The function @code{getppid} returns the process ID of the parent of the parent of the current process (this is also known as the @dfn{parent process ID}). Your program should include the header files @file{unistd.h} and @file{sys/types.h} to use these functions. @pindex sys/types.h @pindex unistd.h @comment sys/types.h @comment POSIX.1 @deftp {Data Type} pid_t The @code{pid_t} data type is a signed integer type which is capable of representing a process ID. In the GNU library, this is an @code{int}. @end deftp @comment unistd.h @comment POSIX.1 @deftypefun pid_t getpid () The @code{getpid} function returns the process ID of the current process. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun pid_t getppid () The @code{getppid} function returns the process ID of the parent of the current process. @end deftypefun @node Creating a Process, Executing a File, Process Identification, Creating New Processes @subsection Creating a Process The @code{fork} function is the primitive for creating a process. It is declared in the header file @file{unistd.h}. @pindex unistd.h @comment unistd.h @comment POSIX.1 @deftypefun pid_t fork () The @code{fork} function creates a new process. If the operation is successful, there are then both parent and child processes and both see @code{fork} return, but with different values: it returns a value of @code{0} in the child process and returns the child's process ID in the parent process. If the child process could not be created, a value of @code{-1} is returned in the parent process. The following @code{errno} error conditions are defined for this function: @table @code @item EAGAIN There aren't enough system resources to create another process, or the user already has too many processes running. @item ENOMEM The process requires more space than the system can supply. @end table @end deftypefun The specific attributes of the child process that differ from the parent process are: @itemize @bullet @item The child process has its own unique process ID. @item The parent process ID of the child process is the process ID of its parent process. @item The child process gets its own copies of the parent process's open file descriptors. Subsequently changing attributes of the file descriptors in the parent process won't affect the file descriptors in the child, and vice versa. @xref{Control Operations}. @item The elapsed processor times for the child process are set to zero; see @ref{Processor Time}. @item The child doesn't inherit file locks set by the parent process. @xref{Control Operations}. @item The child doesn't inherit alarms set by the parent process. @xref{Setting an Alarm}. @item The set of pending signals (@pxref{Delivery of Signal}) for the child process is cleared. (The child process inherits its mask of blocked signals and signal actions from the parent process.) @end itemize @comment unistd.h @comment BSD @deftypefun pid_t vfork (void) The @code{vfork} function is similar to @code{fork} but more efficient; however, there are restrictions you must follow to use it safely. While @code{fork} makes a complete copy of the calling process's address space and allows both the parent and child to execute independently, @code{vfork} does not make this copy. Instead, the child process created with @code{vfork} shares its parent's address space until it calls one of the @code{exec} functions. In the meantime, the parent process suspends execution. You must be very careful not to allow the child process created with @code{vfork} to modify any global data or even local variables shared with the parent. Furthermore, the child process cannot return from (or do a long jump out of) the function that called @code{vfork}! This would leave the parent process's control information very confused. If in doubt, use @code{fork} instead. Some operating systems don't really implement @code{vfork}. The GNU C library permits you to use @code{vfork} on all systems, but actually executes @code{fork} if @code{vfork} isn't available. @end deftypefun @node Executing a File, Process Completion, Creating a Process, Creating New Processes @subsection Executing a File @cindex executing a file @cindex @code{exec} functions This section describes the @code{exec} family of functions, for executing a file as a process image. You can use these functions to make a child process execute a new program after it has been forked. The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file @file{unistd.h}. @pindex unistd.h @comment unistd.h @comment POSIX.1 @deftypefun int execv (const char *@var{filename}, char *const @var{argv}@t{[]}) The @code{execv} function executes the file named by @var{filename} as a new process image. The @var{argv} argument is an array of null-terminated strings that is used to provide a value for the @code{argv} argument to the @code{main} function of the program to be executed. The last element of this array must be a null pointer. @xref{Program Arguments}, for information on how programs can access these arguments. The environment for the new process image is taken from the @code{environ} variable of the current process image; see @ref{Environment Variables}, for information about environments. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int execl (const char *@var{filename}, const char *@var{arg0}, @dots{}) This is similar to @code{execv}, but the @var{argv} strings are specified individually instead of as an array. A null pointer must be passed as the last such argument. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int execve (const char *@var{filename}, char *const @var{argv}@t{[]}, char *const @var{env}@t{[]}) This is similar to @code{execv}, but permits you to specify the environment for the new program explicitly as the @var{env} argument. This should be an array of strings in the same format as for the @code{environ} variable; see @ref{Environment Access}. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int execle (const char *@var{filename}, const char *@var{arg0}, char *const @var{env}@t{[]}, @dots{}) This is similar to @code{execl}, but permits you to specify the environment for the new program explicitly. The environment argument is passed following the null pointer that marks the last @var{argv} argument, and should be an array of strings in the same format as for the @code{environ} variable. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int execvp (const char *@var{filename}, char *const @var{argv}@t{[]}) The @code{execvp} function is similar to @code{execv}, except that it searches the directories listed in the @code{PATH} environment variable (@pxref{Standard Environment Variables}) to find the full file name of a file from @var{filename} if @var{filename} does not contain a slash. This function is useful for executing installed system utility programs, so that the user can control where to look for them. It is also useful in shells, for executing commands typed by the user. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int execlp (const char *@var{filename}, const char *@var{arg0}, @dots{}) This function is like @code{execl}, except that it performs the same file name searching as the @code{execvp} function. @end deftypefun The size of the argument list and environment list taken together must not be greater than @code{ARG_MAX} bytes. @xref{System Parameters}. @strong{Incomplete:} The POSIX.1 standard requires some statement here about how null terminators, null pointers, and alignment requirements affect the total size of the argument and environment lists. These functions normally don't return, since execution of a new program causes the currently executing program to go away completely. A value of @code{-1} is returned in the event of a failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following @code{errno} error conditions are defined for these functions: @table @code @item E2BIG The combined size of the new program's argument list and environment list is larger than @code{ARG_MAX} bytes. @item ENOEXEC The specified file can't be executed because it isn't in the right format. @item ENOMEM Executing the specified file requires more storage than is available. @end table If execution of the new file is successful, the access time field of the file is updated as if the file had been opened. @xref{File Times}, for more details about access times of files. The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed. Executing a new process image completely changes the contents of memory, except for the arguments and the environment, but many other attributes of the process are unchanged: @itemize @bullet @item The process ID and the parent process ID. @xref{Process Creation Concepts}. @item Session and process group membership. @xref{Job Control Concepts}. @item Real user ID and group ID, and supplementary group IDs. @xref{User/Group IDs of a Process}. @item Pending alarms. @xref{Setting an Alarm}. @item Current working directory and root directory. @xref{Working Directory}. @item File mode creation mask. @xref{Setting Permissions}. @item Process signal mask; see @ref{Process Signal Mask}. @item Pending signals; see @ref{Blocking Signals}. @item Elapsed processor time associated with the process; see @ref{Processor Time}. @end itemize If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in @ref{User/Group IDs of a Process}. Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see @ref{Signal Handling}. File descriptors open in the existing process image remain open in the new process image, unless they have the @code{FD_CLOEXEC} (close-on-exec) flag set. The files that remain open inherit all attributes of the open file description from the existing process image, including file locks. File descriptors are discussed in @ref{Low-Level Input/Output}. Streams, by contrast, cannot survive through @code{exec} functions, because they are located in the memory of the process itself. The new process image has no streams except those it creates afresh. Each of the streams in the pre-@code{exec} process image has a descriptor inside it, and these descriptors do survive through @code{exec} (provided that they do not have @code{FD_CLOEXEC} set. The new process image can reconnect these to new streams using @code{fdopen}. @node Process Completion, Process Completion Status, Executing a File, Creating New Processes @subsection Process Completion @cindex process completion @cindex waiting for completion of child process @cindex testing exit status of child process The functions described in this section are used to wait for a child process to terminate or stop, and determine its status. These functions are declared in the header file @file{sys/wait.h}. @pindex sys/wait.h @comment sys/wait.h @comment POSIX.1 @deftypefun pid_t waitpid (pid_t @var{pid}, int *@var{status_ptr}, int @var{options}) The @code{waitpid} function is used to request status information from a child process whose process ID is @var{pid}. Normally, the calling process is suspended until the child process makes status information available by terminating. Other values for the @var{pid} argument have special interpretations. A value of @code{-1} or @code{WAIT_ANY} requests status information for any child process; a value of @code{0} or @code{WAIT_MYPGRP} requests information for any child process in the same process group as the calling process; and any other negative value @minus{} @var{pgid} requests information for any child process whose process group ID is @var{pgid}. If status information for a child process is available immediately, this function returns immediately without waiting. If more than one eligible child process has status information available, one of them is chosen randomly, and its status is returned immediately. To get the status from the other programs, you need to call @code{waitpid} again. The @var{options} argument is a bit mask. Its value should be the bitwise OR (that is, the @samp{|} operator) of zero or more of the @code{WNOHANG} and @code{WUNTRACED} flags. You can use the @code{WNOHANG} flag to indicate that the parent process shouldn't wait; and the @code{WUNTRACED} flag to request status information from stopped processes as well as processes that have terminated. The status information from the child process is stored in the object that @var{status_ptr} points to, unless @var{status_ptr} is a null pointer. The return value is normally the process ID of the child process whose status is reported. If the @code{WNOHANG} option was specified and no child process is waiting to be noticed, a value of zero is returned. A value of @code{-1} is returned in case of error. The following @code{errno} error conditions are defined for this function: @table @code @item EINTR The function was interrupted by delivery of a signal to the calling process. @item ECHILD There are no child processes to wait for, or the specified @var{pid} is not a child of the calling process. @item EINVAL An invalid value was provided for the @var{options} argument. @end table @end deftypefun These symbolic constants are defined as values for the @var{pid} argument to the @code{waitpid} function. @table @code @item WAIT_ANY This constant macro (whose value is @code{-1}) specifies that @code{waitpid} should return status information about any child process. @item WAIT_MYPGRP This constant (with value @code{0}) specifies that @code{waitpid} should return status information about any child process in the same process group as the calling process. These symbolic constants are defined as flags for the @var{options} argument to the @code{waitpid} function. You can bitwise-OR the flags together to obtain a value to use as the argument. @item WNOHANG This flag specifies that @code{waitpid} should return immediately instead of waiting if there is no child process ready to be noticed. @item WUNTRACED This macro is used to specify that @code{waitpid} should also report the status of any child processes that have been stopped as well as those that have terminated. @end table @deftypefun pid_t wait (int *@var{status_ptr}) This is a simplified version of @code{waitpid}, and is used to wait until any one child process terminates. @example wait (&status) @end example @noindent is equivalent to: @example waitpid (-1, &status, 0) @end example Here's an example of how to use @code{waitpid} to get the status from all child processes that have terminated, without ever waiting. This function is designed to be used as a handler for @code{SIGCHLD}, the signal that indicates that at least one child process has terminated. @example void sigchld_handler (int signum) @{ int pid; int status; while (1) @{ pid = waitpid (WAIT_ANY, Estatus, WNOHANG); if (pid < 0) @{ perror ("waitpid"); break; @} if (pid == 0) break; notice_termination (pid, status); @} @} @end example @end deftypefun @node Process Completion Status, BSD wait Functions, Process Completion, Creating New Processes @subsection Process Completion Status If the exit status value (@pxref{Program Termination}) of the child process is zero, then the status value reported by @code{waitpid} or @code{wait} is also zero. You can test for other kinds of information encoded in the returned status value using the following macros. These macros are defined in the header file @file{sys/wait.h}. @pindex sys/wait.h @comment sys/wait.h @comment POSIX.1 @deftypefn Macro int WIFEXITED (int @var{status}) This macro returns a non-zero value if the child process terminated normally with @code{exit} or @code{_exit}. @end deftypefn @comment sys/wait.h @comment POSIX.1 @deftypefn Macro int WEXITSTATUS (int @var{status}) If @code{WIFEXITED} is true of @var{status}, this macro returns the low-order 8 bits of the exit status value from the child process. @end deftypefn @comment sys/wait.h @comment POSIX.1 @deftypefn Macro int WIFSIGNALED (int @var{status}) This macro returns a non-zero value if the child process terminated by receiving a signal that was not handled. @end deftypefn @comment sys/wait.h @comment POSIX.1 @deftypefn Macro int WTERMSIG (int @var{status}) If @code{WIFSIGNALED} is true of @var{status}, this macro returns the number of the signal that terminated the child process. @end deftypefn @comment sys/wait.h @comment BSD @deftypefn Macro int WCOREDUMP (int @var{status}) This macro returns a non-zero value if the child process terminated and produced a core dump. @end deftypefn @comment sys/wait.h @comment POSIX.1 @deftypefn Macro int WIFSTOPPED (int @var{status}) This macro returns a non-zero value if the child process is stopped. @end deftypefn @comment sys/wait.h @comment POSIX.1 @deftypefn Macro int WSTOPSIG (int @var{status}) If @code{WIFSTOPPED} is true of @var{status}, this macro returns the number of the signal that caused the child process to stop. @end deftypefn @node BSD wait Functions, Process Creation Example, Process Completion Status, Creating New Processes @subsection BSD Process Completion Functions The GNU library also provides these related facilities for compatibility with BSD Unix. BSD uses the @code{union wait} data type to represent status values rather than an @code{int}. The two representations are actually interchangeable; they describe the same bit patterns. The macros such as @code{WEXITSTATUS} are defined so that they will work on either kind of object, and the @code{wait} function is defined to accept either type of pointer as its @var{status_ptr} argument. These functions are declared in @file{sys/wait.h}. @pindex sys/wait.h @comment sys/wait.h @comment BSD @deftp {union Type} wait This data type represents program termination status values. It has the following members: @table @code @item int w_termsig This member is equivalent to the @code{WTERMSIG} macro. @item int w_coredump This member is equivalent to the @code{WCOREDUMP} macro. @item int w_retcode This member is equivalent to the @code{WEXISTATUS} macro. @item int w_stopsig This member is equivalent to the @code{WSTOPSIG} macro. @end table Instead of accessing these members directly, you should use the equivalent macros. @end deftp @comment sys/wait.h @comment BSD @deftypefun pid_t wait3 (union wait *@var{status_ptr}, int @var{options}, void * @var{usage}) If @var{usage} is a null pointer, this function is equivalent to @code{waitpid (-1, @var{status_ptr}, @var{options})}. The @var{usage} argument may also be a pointer to a @code{struct rusage} object. Information about system resources used by terminated processes (but not stopped processes) is returned in this structure. @strong{Incomplete:} The description of the @code{struct rusage} structure hasn't been written yet. Put in a cross-reference here. @end deftypefun @comment sys/wait.h @comment BSD @deftypefun pid_t wait4 (pid_t @var{pid}, union wait *@var{status_ptr}, int @var{options}, void *@var{usage}) If @var{usage} is a null pointer, this function is equivalent to @code{waitpid (@var{pid}, @var{status_ptr}, @var{options})}. The @var{usage} argument may also be a pointer to a @code{struct rusage} object. Information about system resources used by terminated processes (but not stopped processes) is returned in this structure. @strong{Incomplete:} The description of the @code{struct rusage} structure hasn't been written yet. Put in a cross-reference here. @end deftypefun @node Process Creation Example, , BSD wait Functions, Creating New Processes @subsection Process Creation Example Here is an example program showing how you might write a function similar to the built-in @code{system}. It executes its @var{command} argument using the equivalent of @samp{sh -c @var{command}}. @example #include <stddef.h> #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <sys/wait.h> /* @r{Execute the command using this shell program.} */ #define SHELL "/bin/sh" int my_system (char *command) @{ int status; pid_t pid; pid = fork (); if (pid == 0) @{ /* @r{This is the child process. Execute the shell command.} */ execl (SHELL, SHELL, "-c", command, NULL); exit (EXIT_FAILURE); @} else if (pid < 0) /* @r{The fork failed. Report failure.} */ status = -1; else @{ /* @r{This is the parent process. Wait for the child to complete.} */ if (waitpid (pid, &status, 0) != pid) status = -1; @} return status; @} @end example @comment Yes, this example has been tested. There are a couple of things you should pay attention to in this example. Remember that the first @code{argv} argument supplied to the program represents the name of the program being executed. That is why, in the call to @code{execl}, @code{SHELL} is supplied once to name the program to execute and a second time to supply a value for @code{argv[0]}. The @code{execl} call in the child process doesn't return if it is successful. If it fails, you must do something to make the child process terminate. Just returning a bad status code with @code{return} would leave two processes running the original program. Instead, the right behavior is for the child process to report failure to its parent process. To do this, @code{exit} is called with a failure status.