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diff --git a/manual/conf.texi b/manual/conf.texi new file mode 100644 index 0000000000..86afeca597 --- /dev/null +++ b/manual/conf.texi @@ -0,0 +1,1091 @@ +@node System Configuration, Language Features, System Information, Top +@chapter System Configuration Parameters + +The functions and macros listed in this chapter give information about +configuration parameters of the operating system---for example, capacity +limits, presence of optional POSIX features, and the default path for +executable files (@pxref{String Parameters}). + +@menu +* General Limits:: Constants and functions that describe + various process-related limits that have + one uniform value for any given machine. +* System Options:: Optional POSIX features. +* Version Supported:: Version numbers of POSIX.1 and POSIX.2. +* Sysconf:: Getting specific configuration values + of general limits and system options. +* Minimums:: Minimum values for general limits. + +* Limits for Files:: Size limitations that pertain to individual files. + These can vary between file systems + or even from file to file. +* Options for Files:: Optional features that some files may support. +* File Minimums:: Minimum values for file limits. +* Pathconf:: Getting the limit values for a particular file. + +* Utility Limits:: Capacity limits of some POSIX.2 utility programs. +* Utility Minimums:: Minimum allowable values of those limits. + +* String Parameters:: Getting the default search path. +@end menu + +@node General Limits +@section General Capacity Limits +@cindex POSIX capacity limits +@cindex limits, POSIX +@cindex capacity limits, POSIX + +The POSIX.1 and POSIX.2 standards specify a number of parameters that +describe capacity limitations of the system. These limits can be fixed +constants for a given operating system, or they can vary from machine to +machine. For example, some limit values may be configurable by the +system administrator, either at run time or by rebuilding the kernel, +and this should not require recompiling application programs. + +@pindex limits.h +Each of the following limit parameters has a macro that is defined in +@file{limits.h} only if the system has a fixed, uniform limit for the +parameter in question. If the system allows different file systems or +files to have different limits, then the macro is undefined; use +@code{sysconf} to find out the limit that applies at a particular time +on a particular machine. @xref{Sysconf}. + +Each of these parameters also has another macro, with a name starting +with @samp{_POSIX}, which gives the lowest value that the limit is +allowed to have on @emph{any} POSIX system. @xref{Minimums}. + +@cindex limits, program argument size +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int ARG_MAX +If defined, the unvarying maximum combined length of the @var{argv} and +@var{environ} arguments that can be passed to the @code{exec} functions. +@end deftypevr + +@cindex limits, number of processes +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int CHILD_MAX +If defined, the unvarying maximum number of processes that can exist +with the same real user ID at any one time. In BSD and GNU, this is +controlled by the @code{RLIMIT_NPROC} resource limit; @pxref{Limits on +Resources}. +@end deftypevr + +@cindex limits, number of open files +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int OPEN_MAX +If defined, the unvarying maximum number of files that a single process +can have open simultaneously. In BSD and GNU, this is controlled +by the @code{RLIMIT_NOFILE} resource limit; @pxref{Limits on Resources}. +@end deftypevr + +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int STREAM_MAX +If defined, the unvarying maximum number of streams that a single +process can have open simultaneously. @xref{Opening Streams}. +@end deftypevr + +@cindex limits, time zone name length +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int TZNAME_MAX +If defined, the unvarying maximum length of a time zone name. +@xref{Time Zone Functions}. +@end deftypevr + +These limit macros are always defined in @file{limits.h}. + +@cindex limits, number of supplementary group IDs +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int NGROUPS_MAX +The maximum number of supplementary group IDs that one process can have. + +The value of this macro is actually a lower bound for the maximum. That +is, you can count on being able to have that many supplementary group +IDs, but a particular machine might let you have even more. You can use +@code{sysconf} to see whether a particular machine will let you have +more (@pxref{Sysconf}). +@end deftypevr + +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int SSIZE_MAX +The largest value that can fit in an object of type @code{ssize_t}. +Effectively, this is the limit on the number of bytes that can be read +or written in a single operation. + +This macro is defined in all POSIX systems because this limit is never +configurable. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int RE_DUP_MAX +The largest number of repetitions you are guaranteed is allowed in the +construct @samp{\@{@var{min},@var{max}\@}} in a regular expression. + +The value of this macro is actually a lower bound for the maximum. That +is, you can count on being able to have that many repetitions, but a +particular machine might let you have even more. You can use +@code{sysconf} to see whether a particular machine will let you have +more (@pxref{Sysconf}). And even the value that @code{sysconf} tells +you is just a lower bound---larger values might work. + +This macro is defined in all POSIX.2 systems, because POSIX.2 says it +should always be defined even if there is no specific imposed limit. +@end deftypevr + +@node System Options +@section Overall System Options +@cindex POSIX optional features +@cindex optional POSIX features + +POSIX defines certain system-specific options that not all POSIX systems +support. Since these options are provided in the kernel, not in the +library, simply using the GNU C library does not guarantee any of these +features is supported; it depends on the system you are using. + +@pindex unistd.h +You can test for the availability of a given option using the macros in +this section, together with the function @code{sysconf}. The macros are +defined only if you include @file{unistd.h}. + +For the following macros, if the macro is defined in @file{unistd.h}, +then the option is supported. Otherwise, the option may or may not be +supported; use @code{sysconf} to find out. @xref{Sysconf}. + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int _POSIX_JOB_CONTROL +If this symbol is defined, it indicates that the system supports job +control. Otherwise, the implementation behaves as if all processes +within a session belong to a single process group. @xref{Job Control}. +@end deftypevr + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int _POSIX_SAVED_IDS +If this symbol is defined, it indicates that the system remembers the +effective user and group IDs of a process before it executes an +executable file with the set-user-ID or set-group-ID bits set, and that +explicitly changing the effective user or group IDs back to these values +is permitted. If this option is not defined, then if a nonprivileged +process changes its effective user or group ID to the real user or group +ID of the process, it can't change it back again. @xref{Enable/Disable +Setuid}. +@end deftypevr + +For the following macros, if the macro is defined in @file{unistd.h}, +then its value indicates whether the option is supported. A value of +@code{-1} means no, and any other value means yes. If the macro is not +defined, then the option may or may not be supported; use @code{sysconf} +to find out. @xref{Sysconf}. + +@comment unistd.h +@comment POSIX.2 +@deftypevr Macro int _POSIX2_C_DEV +If this symbol is defined, it indicates that the system has the POSIX.2 +C compiler command, @code{c89}. The GNU C library always defines this +as @code{1}, on the assumption that you would not have installed it if +you didn't have a C compiler. +@end deftypevr + +@comment unistd.h +@comment POSIX.2 +@deftypevr Macro int _POSIX2_FORT_DEV +If this symbol is defined, it indicates that the system has the POSIX.2 +Fortran compiler command, @code{fort77}. The GNU C library never +defines this, because we don't know what the system has. +@end deftypevr + +@comment unistd.h +@comment POSIX.2 +@deftypevr Macro int _POSIX2_FORT_RUN +If this symbol is defined, it indicates that the system has the POSIX.2 +@code{asa} command to interpret Fortran carriage control. The GNU C +library never defines this, because we don't know what the system has. +@end deftypevr + +@comment unistd.h +@comment POSIX.2 +@deftypevr Macro int _POSIX2_LOCALEDEF +If this symbol is defined, it indicates that the system has the POSIX.2 +@code{localedef} command. The GNU C library never defines this, because +we don't know what the system has. +@end deftypevr + +@comment unistd.h +@comment POSIX.2 +@deftypevr Macro int _POSIX2_SW_DEV +If this symbol is defined, it indicates that the system has the POSIX.2 +commands @code{ar}, @code{make}, and @code{strip}. The GNU C library +always defines this as @code{1}, on the assumption that you had to have +@code{ar} and @code{make} to install the library, and it's unlikely that +@code{strip} would be absent when those are present. +@end deftypevr + +@node Version Supported +@section Which Version of POSIX is Supported + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro {long int} _POSIX_VERSION +This constant represents the version of the POSIX.1 standard to which +the implementation conforms. For an implementation conforming to the +1990 POSIX.1 standard, the value is the integer @code{199009L}. + +@code{_POSIX_VERSION} is always defined (in @file{unistd.h}) in any +POSIX system. + +@strong{Usage Note:} Don't try to test whether the system supports POSIX +by including @file{unistd.h} and then checking whether +@code{_POSIX_VERSION} is defined. On a non-POSIX system, this will +probably fail because there is no @file{unistd.h}. We do not know of +@emph{any} way you can reliably test at compilation time whether your +target system supports POSIX or whether @file{unistd.h} exists. + +The GNU C compiler predefines the symbol @code{__POSIX__} if the target +system is a POSIX system. Provided you do not use any other compilers +on POSIX systems, testing @code{defined (__POSIX__)} will reliably +detect such systems. +@end deftypevr + +@comment unistd.h +@comment POSIX.2 +@deftypevr Macro {long int} _POSIX2_C_VERSION +This constant represents the version of the POSIX.2 standard which the +library and system kernel support. We don't know what value this will +be for the first version of the POSIX.2 standard, because the value is +based on the year and month in which the standard is officially adopted. + +The value of this symbol says nothing about the utilities installed on +the system. + +@strong{Usage Note:} You can use this macro to tell whether a POSIX.1 +system library supports POSIX.2 as well. Any POSIX.1 system contains +@file{unistd.h}, so include that file and then test @code{defined +(_POSIX2_C_VERSION)}. +@end deftypevr + +@node Sysconf +@section Using @code{sysconf} + +When your system has configurable system limits, you can use the +@code{sysconf} function to find out the value that applies to any +particular machine. The function and the associated @var{parameter} +constants are declared in the header file @file{unistd.h}. + +@menu +* Sysconf Definition:: Detailed specifications of @code{sysconf}. +* Constants for Sysconf:: The list of parameters @code{sysconf} can read. +* Examples of Sysconf:: How to use @code{sysconf} and the parameter + macros properly together. +@end menu + +@node Sysconf Definition +@subsection Definition of @code{sysconf} + +@comment unistd.h +@comment POSIX.1 +@deftypefun {long int} sysconf (int @var{parameter}) +This function is used to inquire about runtime system parameters. The +@var{parameter} argument should be one of the @samp{_SC_} symbols listed +below. + +The normal return value from @code{sysconf} is the value you requested. +A value of @code{-1} is returned both if the implementation does not +impose a limit, and in case of an error. + +The following @code{errno} error conditions are defined for this function: + +@table @code +@item EINVAL +The value of the @var{parameter} is invalid. +@end table +@end deftypefun + +@node Constants for Sysconf +@subsection Constants for @code{sysconf} Parameters + +Here are the symbolic constants for use as the @var{parameter} argument +to @code{sysconf}. The values are all integer constants (more +specifically, enumeration type values). + +@table @code +@comment unistd.h +@comment POSIX.1 +@item _SC_ARG_MAX +Inquire about the parameter corresponding to @code{ARG_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_CHILD_MAX +Inquire about the parameter corresponding to @code{CHILD_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_OPEN_MAX +Inquire about the parameter corresponding to @code{OPEN_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_STREAM_MAX +Inquire about the parameter corresponding to @code{STREAM_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_TZNAME_MAX +Inquire about the parameter corresponding to @code{TZNAME_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_NGROUPS_MAX +Inquire about the parameter corresponding to @code{NGROUPS_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_JOB_CONTROL +Inquire about the parameter corresponding to @code{_POSIX_JOB_CONTROL}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_SAVED_IDS +Inquire about the parameter corresponding to @code{_POSIX_SAVED_IDS}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_VERSION +Inquire about the parameter corresponding to @code{_POSIX_VERSION}. + +@comment unistd.h +@comment POSIX.1 +@item _SC_CLK_TCK +Inquire about the parameter corresponding to @code{CLOCKS_PER_SEC}; +@pxref{Basic CPU Time}. + +@comment unistd.h +@comment POSIX.2 +@item _SC_2_C_DEV +Inquire about whether the system has the POSIX.2 C compiler command, +@code{c89}. + +@comment unistd.h +@comment POSIX.2 +@item _SC_2_FORT_DEV +Inquire about whether the system has the POSIX.2 Fortran compiler +command, @code{fort77}. + +@comment unistd.h +@comment POSIX.2 +@item _SC_2_FORT_RUN +Inquire about whether the system has the POSIX.2 @code{asa} command to +interpret Fortran carriage control. + +@comment unistd.h +@comment POSIX.2 +@item _SC_2_LOCALEDEF +Inquire about whether the system has the POSIX.2 @code{localedef} +command. + +@comment unistd.h +@comment POSIX.2 +@item _SC_2_SW_DEV +Inquire about whether the system has the POSIX.2 commands @code{ar}, +@code{make}, and @code{strip}. + +@comment unistd.h +@comment POSIX.2 +@item _SC_BC_BASE_MAX +Inquire about the maximum value of @code{obase} in the @code{bc} +utility. + +@comment unistd.h +@comment POSIX.2 +@item _SC_BC_DIM_MAX +Inquire about the maximum size of an array in the @code{bc} +utility. + +@comment unistd.h +@comment POSIX.2 +@item _SC_BC_SCALE_MAX +Inquire about the maximum value of @code{scale} in the @code{bc} +utility. + +@comment unistd.h +@comment POSIX.2 +@item _SC_BC_STRING_MAX +Inquire about the maximum size of a string constant in the +@code{bc} utility. + +@comment unistd.h +@comment POSIX.2 +@item _SC_COLL_WEIGHTS_MAX +Inquire about the maximum number of weights that can necessarily +be used in defining the collating sequence for a locale. + +@comment unistd.h +@comment POSIX.2 +@item _SC_EXPR_NEST_MAX +Inquire about the maximum number of expressions nested within +parentheses when using the @code{expr} utility. + +@comment unistd.h +@comment POSIX.2 +@item _SC_LINE_MAX +Inquire about the maximum size of a text line that the POSIX.2 text +utilities can handle. + +@comment unistd.h +@comment POSIX.2 +@item _SC_EQUIV_CLASS_MAX +Inquire about the maximum number of weights that can be assigned to an +entry of the @code{LC_COLLATE} category @samp{order} keyword in a locale +definition. The GNU C library does not presently support locale +definitions. + +@comment unistd.h +@comment POSIX.2 +@item _SC_VERSION +Inquire about the version number of POSIX.1 that the library and kernel +support. + +@comment unistd.h +@comment POSIX.2 +@item _SC_2_VERSION +Inquire about the version number of POSIX.2 that the system utilities +support. + +@comment unistd.h +@comment GNU +@item _SC_PAGESIZE +Inquire about the virtual memory page size of the machine. +@code{getpagesize} returns the same value. +@c @xref{XXX getpagesize}. !!! ??? +@end table + +@node Examples of Sysconf +@subsection Examples of @code{sysconf} + +We recommend that you first test for a macro definition for the +parameter you are interested in, and call @code{sysconf} only if the +macro is not defined. For example, here is how to test whether job +control is supported: + +@smallexample +@group +int +have_job_control (void) +@{ +#ifdef _POSIX_JOB_CONTROL + return 1; +#else + int value = sysconf (_SC_JOB_CONTROL); + if (value < 0) + /* @r{If the system is that badly wedged,} + @r{there's no use trying to go on.} */ + fatal (strerror (errno)); + return value; +#endif +@} +@end group +@end smallexample + +Here is how to get the value of a numeric limit: + +@smallexample +int +get_child_max () +@{ +#ifdef CHILD_MAX + return CHILD_MAX; +#else + int value = sysconf (_SC_CHILD_MAX); + if (value < 0) + fatal (strerror (errno)); + return value; +#endif +@} +@end smallexample + +@node Minimums +@section Minimum Values for General Capacity Limits + +Here are the names for the POSIX minimum upper bounds for the system +limit parameters. The significance of these values is that you can +safely push to these limits without checking whether the particular +system you are using can go that far. + +@table @code +@comment limits.h +@comment POSIX.1 +@item _POSIX_ARG_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum combined length of the @var{argv} and @var{environ} +arguments that can be passed to the @code{exec} functions. +Its value is @code{4096}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_CHILD_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum number of simultaneous processes per real user ID. Its +value is @code{6}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_NGROUPS_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum number of supplementary group IDs per process. Its +value is @code{0}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_OPEN_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum number of files that a single process can have open +simultaneously. Its value is @code{16}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_SSIZE_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum value that can be stored in an object of type +@code{ssize_t}. Its value is @code{32767}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_STREAM_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum number of streams that a single process can have open +simultaneously. Its value is @code{8}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_TZNAME_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the maximum length of a time zone name. Its value is @code{3}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_RE_DUP_MAX +The value of this macro is the most restrictive limit permitted by POSIX +for the numbers used in the @samp{\@{@var{min},@var{max}\@}} construct +in a regular expression. Its value is @code{255}. +@end table + +@node Limits for Files +@section Limits on File System Capacity + +The POSIX.1 standard specifies a number of parameters that describe the +limitations of the file system. It's possible for the system to have a +fixed, uniform limit for a parameter, but this isn't the usual case. On +most systems, it's possible for different file systems (and, for some +parameters, even different files) to have different maximum limits. For +example, this is very likely if you use NFS to mount some of the file +systems from other machines. + +@pindex limits.h +Each of the following macros is defined in @file{limits.h} only if the +system has a fixed, uniform limit for the parameter in question. If the +system allows different file systems or files to have different limits, +then the macro is undefined; use @code{pathconf} or @code{fpathconf} to +find out the limit that applies to a particular file. @xref{Pathconf}. + +Each parameter also has another macro, with a name starting with +@samp{_POSIX}, which gives the lowest value that the limit is allowed to +have on @emph{any} POSIX system. @xref{File Minimums}. + +@cindex limits, link count of files +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int LINK_MAX +The uniform system limit (if any) for the number of names for a given +file. @xref{Hard Links}. +@end deftypevr + +@cindex limits, terminal input queue +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int MAX_CANON +The uniform system limit (if any) for the amount of text in a line of +input when input editing is enabled. @xref{Canonical or Not}. +@end deftypevr + +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int MAX_INPUT +The uniform system limit (if any) for the total number of characters +typed ahead as input. @xref{I/O Queues}. +@end deftypevr + +@cindex limits, file name length +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int NAME_MAX +The uniform system limit (if any) for the length of a file name component. +@end deftypevr + +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int PATH_MAX +The uniform system limit (if any) for the length of an entire file name (that +is, the argument given to system calls such as @code{open}). +@end deftypevr + +@cindex limits, pipe buffer size +@comment limits.h +@comment POSIX.1 +@deftypevr Macro int PIPE_BUF +The uniform system limit (if any) for the number of bytes that can be +written atomically to a pipe. If multiple processes are writing to the +same pipe simultaneously, output from different processes might be +interleaved in chunks of this size. @xref{Pipes and FIFOs}. +@end deftypevr + +These are alternative macro names for some of the same information. + +@comment dirent.h +@comment BSD +@deftypevr Macro int MAXNAMLEN +This is the BSD name for @code{NAME_MAX}. It is defined in +@file{dirent.h}. +@end deftypevr + +@comment stdio.h +@comment ANSI +@deftypevr Macro int FILENAME_MAX +The value of this macro is an integer constant expression that +represents the maximum length of a file name string. It is defined in +@file{stdio.h}. + +Unlike @code{PATH_MAX}, this macro is defined even if there is no actual +limit imposed. In such a case, its value is typically a very large +number. @strong{This is always the case on the GNU system.} + +@strong{Usage Note:} Don't use @code{FILENAME_MAX} as the size of an +array in which to store a file name! You can't possibly make an array +that big! Use dynamic allocation (@pxref{Memory Allocation}) instead. +@end deftypevr + +@node Options for Files +@section Optional Features in File Support + +POSIX defines certain system-specific options in the system calls for +operating on files. Some systems support these options and others do +not. Since these options are provided in the kernel, not in the +library, simply using the GNU C library does not guarantee any of these +features is supported; it depends on the system you are using. They can +also vary between file systems on a single machine. + +@pindex unistd.h +This section describes the macros you can test to determine whether a +particular option is supported on your machine. If a given macro is +defined in @file{unistd.h}, then its value says whether the +corresponding feature is supported. (A value of @code{-1} indicates no; +any other value indicates yes.) If the macro is undefined, it means +particular files may or may not support the feature. + +Since all the machines that support the GNU C library also support NFS, +one can never make a general statement about whether all file systems +support the @code{_POSIX_CHOWN_RESTRICTED} and @code{_POSIX_NO_TRUNC} +features. So these names are never defined as macros in the GNU C +library. + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int _POSIX_CHOWN_RESTRICTED +If this option is in effect, the @code{chown} function is restricted so +that the only changes permitted to nonprivileged processes is to change +the group owner of a file to either be the effective group ID of the +process, or one of its supplementary group IDs. @xref{File Owner}. +@end deftypevr + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int _POSIX_NO_TRUNC +If this option is in effect, file name components longer than +@code{NAME_MAX} generate an @code{ENAMETOOLONG} error. Otherwise, file +name components that are too long are silently truncated. +@end deftypevr + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro {unsigned char} _POSIX_VDISABLE +This option is only meaningful for files that are terminal devices. +If it is enabled, then handling for special control characters can +be disabled individually. @xref{Special Characters}. +@end deftypevr + +@pindex unistd.h +If one of these macros is undefined, that means that the option might be +in effect for some files and not for others. To inquire about a +particular file, call @code{pathconf} or @code{fpathconf}. +@xref{Pathconf}. + +@node File Minimums +@section Minimum Values for File System Limits + +Here are the names for the POSIX minimum upper bounds for some of the +above parameters. The significance of these values is that you can +safely push to these limits without checking whether the particular +system you are using can go that far. + +@table @code +@comment limits.h +@comment POSIX.1 +@item _POSIX_LINK_MAX +The most restrictive limit permitted by POSIX for the maximum value of a +file's link count. The value of this constant is @code{8}; thus, you +can always make up to eight names for a file without running into a +system limit. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_MAX_CANON +The most restrictive limit permitted by POSIX for the maximum number of +bytes in a canonical input line from a terminal device. The value of +this constant is @code{255}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_MAX_INPUT +The most restrictive limit permitted by POSIX for the maximum number of +bytes in a terminal device input queue (or typeahead buffer). +@xref{Input Modes}. The value of this constant is @code{255}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_NAME_MAX +The most restrictive limit permitted by POSIX for the maximum number of +bytes in a file name component. The value of this constant is +@code{14}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_PATH_MAX +The most restrictive limit permitted by POSIX for the maximum number of +bytes in a file name. The value of this constant is @code{255}. + +@comment limits.h +@comment POSIX.1 +@item _POSIX_PIPE_BUF +The most restrictive limit permitted by POSIX for the maximum number of +bytes that can be written atomically to a pipe. The value of this +constant is @code{512}. +@end table + +@node Pathconf +@section Using @code{pathconf} + +When your machine allows different files to have different values for a +file system parameter, you can use the functions in this section to find +out the value that applies to any particular file. + +These functions and the associated constants for the @var{parameter} +argument are declared in the header file @file{unistd.h}. + +@comment unistd.h +@comment POSIX.1 +@deftypefun {long int} pathconf (const char *@var{filename}, int @var{parameter}) +This function is used to inquire about the limits that apply to +the file named @var{filename}. + +The @var{parameter} argument should be one of the @samp{_PC_} constants +listed below. + +The normal return value from @code{pathconf} is the value you requested. +A value of @code{-1} is returned both if the implementation does not +impose a limit, and in case of an error. In the former case, +@code{errno} is not set, while in the latter case, @code{errno} is set +to indicate the cause of the problem. So the only way to use this +function robustly is to store @code{0} into @code{errno} just before +calling it. + +Besides the usual file name errors (@pxref{File Name Errors}), +the following error condition is defined for this function: + +@table @code +@item EINVAL +The value of @var{parameter} is invalid, or the implementation doesn't +support the @var{parameter} for the specific file. +@end table +@end deftypefun + +@comment unistd.h +@comment POSIX.1 +@deftypefun {long int} fpathconf (int @var{filedes}, int @var{parameter}) +This is just like @code{pathconf} except that an open file descriptor +is used to specify the file for which information is requested, instead +of a file name. + +The following @code{errno} error conditions are defined for this function: + +@table @code +@item EBADF +The @var{filedes} argument is not a valid file descriptor. + +@item EINVAL +The value of @var{parameter} is invalid, or the implementation doesn't +support the @var{parameter} for the specific file. +@end table +@end deftypefun + +Here are the symbolic constants that you can use as the @var{parameter} +argument to @code{pathconf} and @code{fpathconf}. The values are all +integer constants. + +@table @code +@comment unistd.h +@comment POSIX.1 +@item _PC_LINK_MAX +Inquire about the value of @code{LINK_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_MAX_CANON +Inquire about the value of @code{MAX_CANON}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_MAX_INPUT +Inquire about the value of @code{MAX_INPUT}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_NAME_MAX +Inquire about the value of @code{NAME_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_PATH_MAX +Inquire about the value of @code{PATH_MAX}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_PIPE_BUF +Inquire about the value of @code{PIPE_BUF}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_CHOWN_RESTRICTED +Inquire about the value of @code{_POSIX_CHOWN_RESTRICTED}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_NO_TRUNC +Inquire about the value of @code{_POSIX_NO_TRUNC}. + +@comment unistd.h +@comment POSIX.1 +@item _PC_VDISABLE +Inquire about the value of @code{_POSIX_VDISABLE}. +@end table + +@node Utility Limits +@section Utility Program Capacity Limits + +The POSIX.2 standard specifies certain system limits that you can access +through @code{sysconf} that apply to utility behavior rather than the +behavior of the library or the operating system. + +The GNU C library defines macros for these limits, and @code{sysconf} +returns values for them if you ask; but these values convey no +meaningful information. They are simply the smallest values that +POSIX.2 permits. + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int BC_BASE_MAX +The largest value of @code{obase} that the @code{bc} utility is +guaranteed to support. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int BC_SCALE_MAX +The largest value of @code{scale} that the @code{bc} utility is +guaranteed to support. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int BC_DIM_MAX +The largest number of elements in one array that the @code{bc} utility +is guaranteed to support. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int BC_STRING_MAX +The largest number of characters in one string constant that the +@code{bc} utility is guaranteed to support. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int BC_DIM_MAX +The largest number of elements in one array that the @code{bc} utility +is guaranteed to support. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int COLL_WEIGHTS_MAX +The largest number of weights that can necessarily be used in defining +the collating sequence for a locale. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int EXPR_NEST_MAX +The maximum number of expressions that can be nested within parenthesis +by the @code{expr} utility. +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int LINE_MAX +The largest text line that the text-oriented POSIX.2 utilities can +support. (If you are using the GNU versions of these utilities, then +there is no actual limit except that imposed by the available virtual +memory, but there is no way that the library can tell you this.) +@end deftypevr + +@comment limits.h +@comment POSIX.2 +@deftypevr Macro int EQUIV_CLASS_MAX +The maximum number of weights that can be assigned to an entry of the +@code{LC_COLLATE} category @samp{order} keyword in a locale definition. +The GNU C library does not presently support locale definitions. +@end deftypevr + +@node Utility Minimums +@section Minimum Values for Utility Limits + +@table @code +@comment limits.h +@comment POSIX.2 +@item _POSIX2_BC_BASE_MAX +The most restrictive limit permitted by POSIX.2 for the maximum value of +@code{obase} in the @code{bc} utility. Its value is @code{99}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_BC_DIM_MAX +The most restrictive limit permitted by POSIX.2 for the maximum size of +an array in the @code{bc} utility. Its value is @code{2048}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_BC_SCALE_MAX +The most restrictive limit permitted by POSIX.2 for the maximum value of +@code{scale} in the @code{bc} utility. Its value is @code{99}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_BC_STRING_MAX +The most restrictive limit permitted by POSIX.2 for the maximum size of +a string constant in the @code{bc} utility. Its value is @code{1000}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_COLL_WEIGHTS_MAX +The most restrictive limit permitted by POSIX.2 for the maximum number +of weights that can necessarily be used in defining the collating +sequence for a locale. Its value is @code{2}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_EXPR_NEST_MAX +The most restrictive limit permitted by POSIX.2 for the maximum number +of expressions nested within parenthesis when using the @code{expr} utility. +Its value is @code{32}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_LINE_MAX +The most restrictive limit permitted by POSIX.2 for the maximum size of +a text line that the text utilities can handle. Its value is +@code{2048}. + +@comment limits.h +@comment POSIX.2 +@item _POSIX2_EQUIV_CLASS_MAX +The most restrictive limit permitted by POSIX.2 for the maximum number +of weights that can be assigned to an entry of the @code{LC_COLLATE} +category @samp{order} keyword in a locale definition. Its value is +@code{2}. The GNU C library does not presently support locale +definitions. +@end table + +@node String Parameters +@section String-Valued Parameters + +POSIX.2 defines a way to get string-valued parameters from the operating +system with the function @code{confstr}: + +@comment unistd.h +@comment POSIX.2 +@deftypefun size_t confstr (int @var{parameter}, char *@var{buf}, size_t @var{len}) +This function reads the value of a string-valued system parameter, +storing the string into @var{len} bytes of memory space starting at +@var{buf}. The @var{parameter} argument should be one of the +@samp{_CS_} symbols listed below. + +The normal return value from @code{confstr} is the length of the string +value that you asked for. If you supply a null pointer for @var{buf}, +then @code{confstr} does not try to store the string; it just returns +its length. A value of @code{0} indicates an error. + +If the string you asked for is too long for the buffer (that is, longer +than @code{@var{len} - 1}), then @code{confstr} stores just that much +(leaving room for the terminating null character). You can tell that +this has happened because @code{confstr} returns a value greater than or +equal to @var{len}. + +The following @code{errno} error conditions are defined for this function: + +@table @code +@item EINVAL +The value of the @var{parameter} is invalid. +@end table +@end deftypefun + +Currently there is just one parameter you can read with @code{confstr}: + +@table @code +@comment unistd.h +@comment POSIX.2 +@item _CS_PATH +This parameter's value is the recommended default path for searching for +executable files. This is the path that a user has by default just +after logging in. +@end table + +The way to use @code{confstr} without any arbitrary limit on string size +is to call it twice: first call it to get the length, allocate the +buffer accordingly, and then call @code{confstr} again to fill the +buffer, like this: + +@smallexample +@group +char * +get_default_path (void) +@{ + size_t len = confstr (_CS_PATH, NULL, 0); + char *buffer = (char *) xmalloc (len); + + if (confstr (_CS_PATH, buf, len + 1) == 0) + @{ + free (buffer); + return NULL; + @} + + return buffer; +@} +@end group +@end smallexample |