SYSCONF(3)                  Linux Programmer's Manual                 SYSCONF(3)

       sysconf - get configuration information at run time

       #include <unistd.h>

       long sysconf(int name);

       POSIX allows an application to test at compile or run time whether
       certain options are supported, or what the value is of certain
       configurable constants or limits.

       At compile time this is done by including <unistd.h> and/or <limits.h>
       and testing the value of certain macros.

       At run time, one can ask for numerical values using the present function
       sysconf().  One can ask for numerical values that may depend on the
       filesystem in which a file resides using fpathconf(3) and pathconf(3).
       One can ask for string values using confstr(3).

       The values obtained from these functions are system configuration
       constants.  They do not change during the lifetime of a process.

       For options, typically, there is a constant _POSIX_FOO that may be
       defined in <unistd.h>.  If it is undefined, one should ask at run time.
       If it is defined to -1, then the option is not supported.  If it is
       defined to 0, then relevant functions and headers exist, but one has to
       ask at run time what degree of support is available.  If it is defined to
       a value other than -1 or 0, then the option is supported.  Usually the
       value (such as 200112L) indicates the year and month of the POSIX
       revision describing the option.  Glibc uses the value 1 to indicate
       support as long as the POSIX revision has not been published yet.  The
       sysconf() argument will be _SC_FOO.  For a list of options, see

       For variables or limits, typically, there is a constant _FOO, maybe
       defined in <limits.h>, or _POSIX_FOO, maybe defined in <unistd.h>.  The
       constant will not be defined if the limit is unspecified.  If the
       constant is defined, it gives a guaranteed value, and a greater value
       might actually be supported.  If an application wants to take advantage
       of values which may change between systems, a call to sysconf() can be
       made.  The sysconf() argument will be _SC_FOO.

   POSIX.1 variables
       We give the name of the variable, the name of the sysconf() argument used
       to inquire about its value, and a short description.

       First, the POSIX.1 compatible values.

       ARG_MAX - _SC_ARG_MAX
              The maximum length of the arguments to the exec(3) family of
              functions.  Must not be less than _POSIX_ARG_MAX (4096).

              The maximum number of simultaneous processes per user ID.  Must
              not be less than _POSIX_CHILD_MAX (25).

              Maximum length of a hostname, not including the terminating null
              byte, as returned by gethostname(2).  Must not be less than
              _POSIX_HOST_NAME_MAX (255).

              Maximum length of a login name, including the terminating null
              byte.  Must not be less than _POSIX_LOGIN_NAME_MAX (9).

              Maximum number of supplementary group IDs.

       clock ticks - _SC_CLK_TCK
              The number of clock ticks per second.  The corresponding variable
              is obsolete.  It was of course called CLK_TCK.  (Note: the macro
              CLOCKS_PER_SEC does not give information: it must equal 1000000.)

              The maximum number of files that a process can have open at any
              time.  Must not be less than _POSIX_OPEN_MAX (20).

              Size of a page in bytes.  Must not be less than 1.

              A synonym for PAGESIZE/_SC_PAGESIZE.  (Both PAGESIZE and PAGE_SIZE
              are specified in POSIX.)

              The number of repeated occurrences of a BRE permitted by
              regexec(3) and regcomp(3).  Must not be less than
              _POSIX2_RE_DUP_MAX (255).

              The maximum number of streams that a process can have open at any
              time.  If defined, it has the same value as the standard C macro
              FOPEN_MAX.  Must not be less than _POSIX_STREAM_MAX (8).

              The maximum number of symbolic links seen in a pathname before
              resolution returns ELOOP.  Must not be less than
              _POSIX_SYMLOOP_MAX (8).

              The maximum length of terminal device name, including the
              terminating null byte.  Must not be less than _POSIX_TTY_NAME_MAX

              The maximum number of bytes in a timezone name.  Must not be less
              than _POSIX_TZNAME_MAX (6).

              indicates the year and month the POSIX.1 standard was approved in
              the format YYYYMML; the value 199009L indicates the Sept. 1990

   POSIX.2 variables
       Next, the POSIX.2 values, giving limits for utilities.

              indicates the maximum obase value accepted by the bc(1) utility.

              indicates the maximum value of elements permitted in an array by

              indicates the maximum scale value allowed by bc(1).

              indicates the maximum length of a string accepted by bc(1).

              indicates the maximum numbers of weights that can be assigned to
              an entry of the LC_COLLATE order keyword in the locale definition

              is the maximum number of expressions which can be nested within
              parentheses by expr(1).

              The maximum length of a utility's input line, either from standard
              input or from a file.  This includes space for a trailing newline.

              The maximum number of repeated occurrences of a regular expression
              when the interval notation \{m,n\} is used.

              indicates the version of the POSIX.2 standard in the format of

       POSIX2_C_DEV - _SC_2_C_DEV
              indicates whether the POSIX.2 C language development facilities
              are supported.

              indicates whether the POSIX.2 FORTRAN development utilities are

              indicates whether the POSIX.2 FORTRAN run-time utilities are

              indicates whether the POSIX.2 creation of locales via localedef(1)
              is supported.

       POSIX2_SW_DEV - _SC_2_SW_DEV
              indicates whether the POSIX.2 software development utilities
              option is supported.

       These values also exist, but may not be standard.

        - _SC_PHYS_PAGES
              The number of pages of physical memory.  Note that it is possible
              for the product of this value and the value of _SC_PAGESIZE to

        - _SC_AVPHYS_PAGES
              The number of currently available pages of physical memory.

              The number of processors configured.  See also get_nprocs_conf(3).

              The number of processors currently online (available).  See also

       The return value of sysconf() is one of the following:

       *  On error, -1 is returned and errno is set to indicate the error (for
          example, EINVAL, indicating that name is invalid).

       *  If name corresponds to a maximum or minimum limit, and that limit is
          indeterminate, -1 is returned and errno is not changed.  (To
          distinguish an indeterminate limit from an error, set errno to zero
          before the call, and then check whether errno is nonzero when -1 is

       *  If name corresponds to an option, a positive value is returned if the
          option is supported, and -1 is returned if the option is not

       *  Otherwise, the current value of the option or limit is returned.  This
          value will not be more restrictive than the corresponding value that
          was described to the application in <unistd.h> or <limits.h> when the
          application was compiled.

       EINVAL name is invalid.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                 Attribute     Value       │
       │sysconf()                                 │ Thread safety │ MT-Safe env │

       POSIX.1-2001, POSIX.1-2008.

       It is difficult to use ARG_MAX because it is not specified how much of
       the argument space for exec(3) is consumed by the user's environment

       Some returned values may be huge; they are not suitable for allocating

       bc(1), expr(1), getconf(1), locale(1), confstr(3), fpathconf(3),
       pathconf(3), posixoptions(7)

       This page is part of release 5.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                                2021-03-22                         SYSCONF(3)