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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.
            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.
            indicates  whether  the  POSIX.2  software development utilities
            option is supported.
     These values also exist, but may not be standard.

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.

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)


     This  page  is  part of release 4.16 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 2017-11-26 SYSCONF(3)

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