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       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 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

       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).

              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 (255).

              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 obsolete.  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   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_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

              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

       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  runtime utilities are

              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.

              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 limit.




       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),  fpathconf(3),  pathconf(3),


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