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NAME

       sysconf - Get configuration information at runtime

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().  On can ask for numerical values that may depend on
       the  filesystem  a  file  is  in  using  the  calls  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 runtime 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 more 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()  parameter
       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() family of
              functions.  Must not be less than _POSIX_ARG_MAX (4096).

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

       HOST_NAME_MAX - _SC_HOST_NAME_MAX
              Max  length  of  a  hostname,  not  including  the final NUL, 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 final NUL.  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  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.  (Some systems
              use PAGE_SIZE instead.)

       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
              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 final NUL.
              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 length, either from
              standard  input  or  from  a  file.  This  includes length 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  runtime  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_PAGE_SIZE to overflow.

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

RETURN VALUE

       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 limit.

CONFORMING TO

       POSIX.1.

BUGS

       It  is difficult to use ARG_MAX because it is not specified how much of
       the argument space for exec() 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), locale(1), fpathconf(3), pathconf(3), posixoptions(7)