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NAME

       sysconf - get configuration information at run time

SYNOPSIS

       #include <unistd.h>

       long sysconf(int name);

DESCRIPTION

       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 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 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).

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

       HOST_NAME_MAX - _SC_HOST_NAME_MAX
              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).

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

       NGROUPS_MAX - _SC_NGROUPS_MAX
              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.)

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

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

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

       RE_DUP_MAX - _SC_RE_DUP_MAX
              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).

       STREAM_MAX - _SC_STREAM_MAX
              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).

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

       TTY_NAME_MAX - _SC_TTY_NAME_MAX
              The  maximum  length  of terminal device name, including the terminating null byte.
              Must not be less than _POSIX_TTY_NAME_MAX (9).

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

       _POSIX_VERSION - _SC_VERSION
              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.

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

       BC_DIM_MAX - _SC_BC_DIM_MAX
              indicates the maximum value of elements permitted in an array by bc(1).

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

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

       COLL_WEIGHTS_MAX - _SC_COLL_WEIGHTS_MAX
              indicates the maximum numbers of weights that can be assigned to an  entry  of  the
              LC_COLLATE order keyword in the locale definition file,

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

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

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

       POSIX2_VERSION - _SC_2_VERSION
              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 supported.

       POSIX2_FORT_DEV - _SC_2_FORT_DEV
              indicates whether the POSIX.2 FORTRAN development utilities are supported.

       POSIX2_FORT_RUN - _SC_2_FORT_RUN
              indicates whether the POSIX.2 FORTRAN run-time utilities are supported.

       _POSIX2_LOCALEDEF - _SC_2_LOCALEDEF
              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 possible 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.

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

        - _SC_NPROCESSORS_ONLN
              The    number    of    processors   currently   online   (available).    See   also
              get_nprocs_conf(3).

RETURN VALUE

       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 distinguish an indeterminate limit  from  an
          error,  set errno to zero before the call, and then check whether errno is nonzero when
          -1 is returned.)

       *  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 supported.

       *  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.

ERRORS

       EINVAL name is invalid.

ATTRIBUTES

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

       ┌──────────┬───────────────┬─────────────┐
       │InterfaceAttributeValue       │
       ├──────────┼───────────────┼─────────────┤
       │sysconf() │ Thread safety │ MT-Safe env │
       └──────────┴───────────────┴─────────────┘

CONFORMING TO

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

BUGS

       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.

SEE ALSO

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

COLOPHON

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