oracular (3) sysconf.3.gz

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NAME

       sysconf - get configuration information at run time

LIBRARY

       Standard C library (libc, -lc)

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 locales 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 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 │
       └──────────────────────────────────────────────────────────────────────────┴───────────────┴─────────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

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)