---
name: "confstr"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/confstr"
---

# confstr(3)

get configuration dependent string variables

## NAME

confstr - get configuration dependent string variables

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <unistd.h>
```

**size\_t confstr(**size\_t size;                **int** *name***, char** *buf***\[***size***\], size\_t** *size***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**confstr**():

```
    _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE
```

## DESCRIPTION

**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").

- **\_CS\_PATH**

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

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

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

## RETURN VALUE

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 *size*, 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**.

## ERRORS

- **EINVAL**

  The value of *name* is invalid.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **confstr**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2008.

## HISTORY

POSIX.1-2001.

## EXAMPLES

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)
    abort();
confstr(_CS_PATH, pathbuf, n);
```

## SEE ALSO

[**getconf**(1)](/getconf/1.md), [**sh**(1)](/sh/1.md), [**exec**(3)](/exec/3.md), [**fpathconf**(3)](/fpathconf/3.md), [**pathconf**(3)](/pathconf/3.md), [**sysconf**(3)](/sysconf/3.md), [**system**(3)](/system/3.md)