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28 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 (see section 28.12 String-Valued Parameters).

28.1 General Capacity Limits

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.

Each of the following limit parameters has a macro that is defined in `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 sysconf to find out the limit that applies at a particular time on a particular machine. See section 28.4 Using sysconf.

Each of these parameters also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See section 28.5 Minimum Values for General Capacity Limits.

Macro: int ARG_MAX
If defined, the unvarying maximum combined length of the argv and environ arguments that can be passed to the exec functions.

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 RLIMIT_NPROC resource limit; see section 17.6 Limiting Resource Usage.

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 RLIMIT_NOFILE resource limit; see section 17.6 Limiting Resource Usage.

Macro: int STREAM_MAX
If defined, the unvarying maximum number of streams that a single process can have open simultaneously. See section 7.3 Opening Streams.

Macro: int TZNAME_MAX
If defined, the unvarying maximum length of a time zone name. See section 17.2.6 Functions and Variables for Time Zones.

These limit macros are always defined in `limits.h'.

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 sysconf to see whether a particular machine will let you have more (see section 28.4 Using sysconf).

Macro: int SSIZE_MAX
The largest value that can fit in an object of type 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.

Macro: int RE_DUP_MAX
The largest number of repetitions you are guaranteed is allowed in the construct `\{min,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 sysconf to see whether a particular machine will let you have more (see section 28.4 Using sysconf). And even the value that 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.

28.2 Overall System Options

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.

You can test for the availability of a given option using the macros in this section, together with the function sysconf. The macros are defined only if you include `unistd.h'.

For the following macros, if the macro is defined in `unistd.h', then the option is supported. Otherwise, the option may or may not be supported; use sysconf to find out. See section 28.4 Using sysconf.

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. See section 24 Job Control.

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. See section 26.8 Enabling and Disabling Setuid Access.

For the following macros, if the macro is defined in `unistd.h', then its value indicates whether the option is supported. A value of -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 sysconf to find out. See section 28.4 Using sysconf.

Macro: int _POSIX2_C_DEV
If this symbol is defined, it indicates that the system has the POSIX.2 C compiler command, c89. The GNU C library always defines this as 1, on the assumption that you would not have installed it if you didn't have a C compiler.

Macro: int _POSIX2_FORT_DEV
If this symbol is defined, it indicates that the system has the POSIX.2 Fortran compiler command, fort77. The GNU C library never defines this, because we don't know what the system has.

Macro: int _POSIX2_FORT_RUN
If this symbol is defined, it indicates that the system has the POSIX.2 asa command to interpret Fortran carriage control. The GNU C library never defines this, because we don't know what the system has.

Macro: int _POSIX2_LOCALEDEF
If this symbol is defined, it indicates that the system has the POSIX.2 localedef command. The GNU C library never defines this, because we don't know what the system has.

Macro: int _POSIX2_SW_DEV
If this symbol is defined, it indicates that the system has the POSIX.2 commands ar, make, and strip. The GNU C library always defines this as 1, on the assumption that you had to have ar and make to install the library, and it's unlikely that strip would be absent when those are present.

28.3 Which Version of POSIX is Supported

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 199009L.

_POSIX_VERSION is always defined (in `unistd.h') in any POSIX system.

Usage Note: Don't try to test whether the system supports POSIX by including `unistd.h' and then checking whether _POSIX_VERSION is defined. On a non-POSIX system, this will probably fail because there is no `unistd.h'. We do not know of any way you can reliably test at compilation time whether your target system supports POSIX or whether `unistd.h' exists.

The GNU C compiler predefines the symbol __POSIX__ if the target system is a POSIX system. Provided you do not use any other compilers on POSIX systems, testing defined (__POSIX__) will reliably detect such systems.

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.

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 `unistd.h', so include that file and then test defined (_POSIX2_C_VERSION).

28.4 Using sysconf

When your system has configurable system limits, you can use the sysconf function to find out the value that applies to any particular machine. The function and the associated parameter constants are declared in the header file `unistd.h'.

28.4.1 Definition of sysconf

Function: long int sysconf (int parameter)
This function is used to inquire about runtime system parameters. The parameter argument should be one of the `_SC_' symbols listed below.

The normal return value from sysconf is the value you requested. A value of -1 is returned both if the implementation does not impose a limit, and in case of an error.

The following errno error conditions are defined for this function:

EINVAL
The value of the parameter is invalid.

28.4.2 Constants for sysconf Parameters

Here are the symbolic constants for use as the parameter argument to sysconf. The values are all integer constants (more specifically, enumeration type values).

_SC_ARG_MAX
Inquire about the parameter corresponding to ARG_MAX.
_SC_CHILD_MAX
Inquire about the parameter corresponding to CHILD_MAX.
_SC_OPEN_MAX
Inquire about the parameter corresponding to OPEN_MAX.
_SC_STREAM_MAX
Inquire about the parameter corresponding to STREAM_MAX.
_SC_TZNAME_MAX
Inquire about the parameter corresponding to TZNAME_MAX.
_SC_NGROUPS_MAX
Inquire about the parameter corresponding to NGROUPS_MAX.
_SC_JOB_CONTROL
Inquire about the parameter corresponding to _POSIX_JOB_CONTROL.
_SC_SAVED_IDS
Inquire about the parameter corresponding to _POSIX_SAVED_IDS.
_SC_VERSION
Inquire about the parameter corresponding to _POSIX_VERSION.
_SC_CLK_TCK
Inquire about the parameter corresponding to CLOCKS_PER_SEC; see section 17.1.1 Basic CPU Time Inquiry.
_SC_2_C_DEV
Inquire about whether the system has the POSIX.2 C compiler command, c89.
_SC_2_FORT_DEV
Inquire about whether the system has the POSIX.2 Fortran compiler command, fort77.
_SC_2_FORT_RUN
Inquire about whether the system has the POSIX.2 asa command to interpret Fortran carriage control.
_SC_2_LOCALEDEF
Inquire about whether the system has the POSIX.2 localedef command.
_SC_2_SW_DEV
Inquire about whether the system has the POSIX.2 commands ar, make, and strip.
_SC_BC_BASE_MAX
Inquire about the maximum value of obase in the bc utility.
_SC_BC_DIM_MAX
Inquire about the maximum size of an array in the bc utility.
_SC_BC_SCALE_MAX
Inquire about the maximum value of scale in the bc utility.
_SC_BC_STRING_MAX
Inquire about the maximum size of a string constant in the bc utility.
_SC_COLL_WEIGHTS_MAX
Inquire about the maximum number of weights that can necessarily be used in defining the collating sequence for a locale.
_SC_EXPR_NEST_MAX
Inquire about the maximum number of expressions nested within parentheses when using the expr utility.
_SC_LINE_MAX
Inquire about the maximum size of a text line that the POSIX.2 text utilities can handle.
_SC_EQUIV_CLASS_MAX
Inquire about the maximum number of weights that can be assigned to an entry of the LC_COLLATE category `order' keyword in a locale definition. The GNU C library does not presently support locale definitions.
_SC_VERSION
Inquire about the version number of POSIX.1 that the library and kernel support.
_SC_2_VERSION
Inquire about the version number of POSIX.2 that the system utilities support.
_SC_PAGESIZE
Inquire about the virtual memory page size of the machine. getpagesize returns the same value.

28.4.3 Examples of sysconf

We recommend that you first test for a macro definition for the parameter you are interested in, and call sysconf only if the macro is not defined. For example, here is how to test whether job control is supported:

int
have_job_control (void)
{
#ifdef _POSIX_JOB_CONTROL
  return 1;
#else
  int value = sysconf (_SC_JOB_CONTROL);
  if (value < 0)
    /* If the system is that badly wedged,
       there's no use trying to go on.  */
    fatal (strerror (errno));
  return value;
#endif
}

Here is how to get the value of a numeric limit:

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
}

28.5 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.

_POSIX_ARG_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum combined length of the argv and environ arguments that can be passed to the exec functions. Its value is 4096.
_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 6.
_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 0.
_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 16.
_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 ssize_t. Its value is 32767.
_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 8.
_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 3.
_POSIX2_RE_DUP_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the numbers used in the `\{min,max\}' construct in a regular expression. Its value is 255.

28.6 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.

Each of the following macros is defined in `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 pathconf or fpathconf to find out the limit that applies to a particular file. See section 28.9 Using pathconf.

Each parameter also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See section 28.8 Minimum Values for File System Limits.

Macro: int LINK_MAX
The uniform system limit (if any) for the number of names for a given file. See section 9.3 Hard Links.

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. See section 12.3 Two Styles of Input: Canonical or Not.

Macro: int MAX_INPUT
The uniform system limit (if any) for the total number of characters typed ahead as input. See section 12.2 I/O Queues.

Macro: int NAME_MAX
The uniform system limit (if any) for the length of a file name component.

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 open).

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. See section 10 Pipes and FIFOs.

These are alternative macro names for some of the same information.

Macro: int MAXNAMLEN
This is the BSD name for NAME_MAX. It is defined in `dirent.h'.

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 `stdio.h'.

Unlike 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. This is always the case on the GNU system.

Usage Note: Don't use 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 (see section 3 Memory Allocation) instead.

28.7 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.

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 `unistd.h', then its value says whether the corresponding feature is supported. (A value of -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 _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC features. So these names are never defined as macros in the GNU C library.

Macro: int _POSIX_CHOWN_RESTRICTED
If this option is in effect, the 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. See section 9.8.4 File Owner.

Macro: int _POSIX_NO_TRUNC
If this option is in effect, file name components longer than NAME_MAX generate an ENAMETOOLONG error. Otherwise, file name components that are too long are silently truncated.

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. See section 12.4.9 Special Characters.

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 pathconf or fpathconf. See section 28.9 Using pathconf.

28.8 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.

_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 8; thus, you can always make up to eight names for a file without running into a system limit.
_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 255.
_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). See section 12.4.4 Input Modes. The value of this constant is 255.
_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 14.
_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 255.
_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 512.

28.9 Using 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 parameter argument are declared in the header file `unistd.h'.

Function: long int pathconf (const char *filename, int parameter)
This function is used to inquire about the limits that apply to the file named filename.

The parameter argument should be one of the `_PC_' constants listed below.

The normal return value from pathconf is the value you requested. A value of -1 is returned both if the implementation does not impose a limit, and in case of an error. In the former case, errno is not set, while in the latter case, errno is set to indicate the cause of the problem. So the only way to use this function robustly is to store 0 into errno just before calling it.

Besides the usual file name errors (see section 6.2.3 File Name Errors), the following error condition is defined for this function:

EINVAL
The value of parameter is invalid, or the implementation doesn't support the parameter for the specific file.

Function: long int fpathconf (int filedes, int parameter)
This is just like 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 errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The value of parameter is invalid, or the implementation doesn't support the parameter for the specific file.

Here are the symbolic constants that you can use as the parameter argument to pathconf and fpathconf. The values are all integer constants.

_PC_LINK_MAX
Inquire about the value of LINK_MAX.
_PC_MAX_CANON
Inquire about the value of MAX_CANON.
_PC_MAX_INPUT
Inquire about the value of MAX_INPUT.
_PC_NAME_MAX
Inquire about the value of NAME_MAX.
_PC_PATH_MAX
Inquire about the value of PATH_MAX.
_PC_PIPE_BUF
Inquire about the value of PIPE_BUF.
_PC_CHOWN_RESTRICTED
Inquire about the value of _POSIX_CHOWN_RESTRICTED.
_PC_NO_TRUNC
Inquire about the value of _POSIX_NO_TRUNC.
_PC_VDISABLE
Inquire about the value of _POSIX_VDISABLE.

28.10 Utility Program Capacity Limits

The POSIX.2 standard specifies certain system limits that you can access through 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 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.

Macro: int BC_BASE_MAX
The largest value of obase that the bc utility is guaranteed to support.

Macro: int BC_SCALE_MAX
The largest value of scale that the bc utility is guaranteed to support.

Macro: int BC_DIM_MAX
The largest number of elements in one array that the bc utility is guaranteed to support.

Macro: int BC_STRING_MAX
The largest number of characters in one string constant that the bc utility is guaranteed to support.

Macro: int BC_DIM_MAX
The largest number of elements in one array that the bc utility is guaranteed to support.

Macro: int COLL_WEIGHTS_MAX
The largest number of weights that can necessarily be used in defining the collating sequence for a locale.

Macro: int EXPR_NEST_MAX
The maximum number of expressions that can be nested within parenthesis by the expr utility.

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.)

Macro: int EQUIV_CLASS_MAX
The maximum number of weights that can be assigned to an entry of the LC_COLLATE category `order' keyword in a locale definition. The GNU C library does not presently support locale definitions.

28.11 Minimum Values for Utility Limits

_POSIX2_BC_BASE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum value of obase in the bc utility. Its value is 99.
_POSIX2_BC_DIM_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of an array in the bc utility. Its value is 2048.
_POSIX2_BC_SCALE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum value of scale in the bc utility. Its value is 99.
_POSIX2_BC_STRING_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of a string constant in the bc utility. Its value is 1000.
_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 2.
_POSIX2_EXPR_NEST_MAX
The most restrictive limit permitted by POSIX.2 for the maximum number of expressions nested within parenthesis when using the expr utility. Its value is 32.
_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 2048.
_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 LC_COLLATE category `order' keyword in a locale definition. Its value is 2. The GNU C library does not presently support locale definitions.

28.12 String-Valued Parameters

POSIX.2 defines a way to get string-valued parameters from the operating system with the function confstr:

Function: size_t confstr (int parameter, char *buf, size_t len)
This function reads the value of a string-valued system parameter, storing the string into len bytes of memory space starting at buf. The parameter argument should be one of the `_CS_' symbols listed below.

The normal return value from confstr is the length of the string value that you asked for. If you supply a null pointer for buf, then confstr does not try to store the string; it just returns its length. A value of 0 indicates an error.

If the string you asked for is too long for the buffer (that is, longer than len - 1), then confstr stores just that much (leaving room for the terminating null character). You can tell that this has happened because confstr returns a value greater than or equal to len.

The following errno error conditions are defined for this function:

EINVAL
The value of the parameter is invalid.

Currently there is just one parameter you can read with confstr:

_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.

The way to use 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 confstr again to fill the buffer, like this:

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;
}


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