sysconf manpage

Search topic Section

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
       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

       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 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	 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
	      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_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.

       For   an	  explanation	of   the  terms	 used  in  this	 section,  see

       |Interface | Attribute	  | Value	|
       |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

       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 4.04 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				  2015-08-08			    SYSCONF(3)