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

       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 cer-
       tain 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().	  One  can ask for numerical values that may depend on
       the filesystem a	file is	in using  the  calls  fpathconf(3)  and	 path-
       conf(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  de-
       fined  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 de-
       fined  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

       For variables or	limits,	typically, there is a constant _FOO, maybe de-
       fined  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 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).

       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  sys-
	      tems 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 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  defini-
	      tion 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, either from	 stan-
	      dard  input  or from a file.  This includes space	for a trailing

	      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  run-time	utilities  are

	      indicates	 whether  the  POSIX.2	creation  of  locates  via lo-
	      caledef(1) is supported.

       POSIX2_SW_DEV - _SC_2_SW_DEV
	      indicates	whether	the POSIX.2 software development utilities op-
	      tion 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_PAGESIZE
	      to overflow.

	      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.	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(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), confstr(3), fpathconf(3),	 path-
       conf(3),	posixoptions(7)

       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at

GNU				  2014-03-20			    SYSCONF(3)


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