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 cer-
       tain options are supported, or what the value  is  of  certain  config-
       urable 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  func-
       tion  sysconf().   One  can ask for numerical values that may depend on
       the filesystem in which a file resides  using  fpathconf(3)  and  path-
       conf(3).  One can ask for string values using confstr(3).

       The  values obtained from these functions are system configuration con-
       stants.  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  con-
       stant  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  vari-
              able  is obsolete.  It was of course called CLK_TCK.  (Note: the
              macro CLOCKS_PER_SEC does not give information:  it  must  equal

              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.   (Some  sys-
              tems use PAGE_SIZE instead.)

              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_SYM-
              LOOP_MAX (8).

              The maximum length of terminal device name, including the termi-
              nating 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 revision.

   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  defini-
              tion file,

              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  stan-
              dard  input  or from a file.  This includes space for a trailing

              The maximum number of repeated occurrences of a regular  expres-
              sion 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  locates  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 possi-
              ble for the product of this value and the value of  _SC_PAGESIZE
              to overflow.

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

              The    number    of    processors    configured.     See    also

              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 cause of
          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 distin-
          guish 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

       |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), path-
       conf(3), posixoptions(7)

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