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

       sysconf - Get configuration information at runtime

       #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().
       On can ask for numerical values that may depend on the file system  a  file  is  in
       using  the calls fpathconf(3) and pathconf(3).  One can ask for string values using

       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 func-
       tions and headers exist, but one has to ask at runtime what degree  of  support  is
       available.  If it is defined to a value other than -1 or 0, then the option is sup-
       ported.  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 posixoptions(7).

       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 guar-
       anteed 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 max number of simultaneous processes per user ID.  Must not be less than
              _POSIX_CHILD_MAX (25).

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

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

       clock ticks - _SC_CLK_TCK
              The number of clock ticks per second.  The corresponding variable  is  obso-
              lete.   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.  (Some systems use
              PAGE_SIZE instead.)

              The number of repeated occurrences of a BRE permitted by regexec(3) and reg-
              comp(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 (9).

              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 bc(1).

              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 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 length, either from standard
              input or from a file.  This includes length 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 YYYYMML.

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

              indicates whether the POSIX.2 FORTRAN development utilities are supported.

              indicates whether the POSIX.2 FORTRAN runtime utilities are supported.

              indicates whether the POSIX.2 creation of locates via localedef(1)  is  sup-

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

       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_PAGE_SIZE to overflow.

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

              The number of processors configured.

              The number of processors currently online (available).

       If  name  is  invalid,  -1 is returned, and errno is set to EINVAL.  Otherwise, the
       value returned is the value of the system resource and errno is  not  changed.   In
       the case of options, a positive value is returned if a queried option is available,
       and -1 if it is not.  In the case of limits, -1 means that  there  is  no  definite


       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 variables.

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

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

GNU                               2007-12-12                        SYSCONF(3)

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