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       sysconf - get configuration information at run time


       #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().  One
       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 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).

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

              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).

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

              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.)

              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

              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 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

              The maximum length of a utility's input line, either from standard input or from  a
              file.  This includes space 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 supported.

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

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

              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.

              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.


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

       │InterfaceAttributeValue       │
       │sysconf() │ Thread safety │ MT-Safe env │


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


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


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