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SYSCONF(3)		   Linux Programmer's Manual		    SYSCONF(3)

       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  config-
       urable 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	 func-
       tion 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 con-
       stants.	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 sup-
       port 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  con-
       stant  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.

       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).

	      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	 vari-
	      able  is	obsolete. It was of course called CLK_TCK.  (Note: the
	      macro CLOCKS_PER_SEC does not give information:  it  must	 equal

	      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_SYM-
	      LOOP_MAX (8).

	      The maximum length of terminal device name, including the termi-
	      nating null byte.	 Must not  be  less  than  _POSIX_TTY_NAME_MAX

	      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

       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 trail-
	      ing newline.

	      The  maximum number of repeated occurrences of a regular expres-
	      sion 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 sup-

	      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.

	      The number of pages of physical memory.  Note that it is	possi-
	      ble for the product of this value and the value of _SC_PAGE_SIZE
	      to overflow.

	      The number of currently available pages of physical memory.

       If name is invalid, -1 is returned, and errno is set to EINVAL.	Other-
       wise,  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() is consumed by the user's environment

       Some returned values may be huge; they are not suitable for  allocating

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

GNU				  1993-04-18			    SYSCONF(3)
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