confstr — get configuration dependent string variables


#include <unistd.h>
size_t confstr( int name,
  char *buf,
  size_t len);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


confstr() gets the value of configuration-dependent string variables.

The name argument is the system variable to be queried. The following variables are supported:

_CS_GNU_LIBC_VERSION (GNU C library only; since glibc 2.3.2)

A string which identifies the GNU C library version on this system (e.g., "glibc 2.3.4").

_CS_GNU_LIBPTHREAD_VERSION (GNU C library only; since glibc 2.3.2)

A string which identifies the POSIX implementation supplied by this C library (e.g., "NPTL 2.3.4" or "linuxthreads−0.10").


A value for the PATH variable which indicates where all the POSIX.2 standard utilities can be found.

If buf is not NULL and len is not zero, confstr() copies the value of the string to buf truncated to len − 1 bytes if necessary, with a null byte ('\0') as terminator. This can be detected by comparing the return value of confstr() against len.

If len is zero and buf is NULL, confstr() just returns the value as defined below.


If name is a valid configuration variable, confstr() returns the number of bytes (including the terminating null byte) that would be required to hold the entire value of that variable. This value may be greater than len, which means that the value in buf is truncated.

If name is a valid configuration variable, but that variable does not have a value, then confstr() returns 0. If name does not correspond to a valid configuration variable, confstr() returns 0, and errno is set to EINVAL.



The value of name is invalid.


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

Interface Attribute Value
confstr() Thread safety MT-Safe


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


The following code fragment determines the path where to find the POSIX.2 system utilities:

char *pathbuf;
size_t n;

n = confstr(_CS_PATH, NULL, (size_t) 0);
pathbuf = malloc(n);
if (pathbuf == NULL)
confstr(_CS_PATH, pathbuf, n);


getconf(1), sh(1), exec(3), fpathconf(3), pathconf(3), sysconf(3), system(3)


This page is part of release 5.11 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−pages/.

  Copyright (c) 1993 by Thomas Koenig (

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Modified Sat Jul 24 19:53:02 1993 by Rik Faith (

FIXME Many more values for 'name' are supported, some of which
are documented under 'info confstr'.
See <bits/confname.h> for the rest.
These should all be added to this page.
See also the POSIX.1-2001 specification of confstr()