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

# putenv(3)

change or add an environment variable

## NAME

putenv - change or add an environment variable

## LIBRARY

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

## SYNOPSIS

```
#include <stdlib.h>
```

**int putenv(char \****string***);**

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

**putenv**():

```
    _XOPEN_SOURCE
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE
```

## DESCRIPTION

The **putenv**() function adds or changes the value of environment variables. The argument *string* is of the form *name*=*value*. If *name* does not already exist in the environment, then *string* is added to the environment. If *name* does exist, then the value of *name* in the environment is changed to *value*. The string pointed to by *string* becomes part of the environment, so altering the string changes the environment.

## RETURN VALUE

The **putenv**() function returns zero on success. On failure, it returns a nonzero value, and *errno* is set to indicate the error.

## ERRORS

- **ENOMEM**

  Insufficient space to allocate new environment.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **putenv**() | Thread safety | MT-Unsafe const:env |

## STANDARDS

POSIX.1-2008.

## HISTORY

POSIX.1-2001, SVr2, 4.3BSD-Reno.

The **putenv**() function is not required to be reentrant, and the one in glibc 2.0 is not, but the glibc 2.1 version is.

Since glibc 2.1.2, the glibc implementation conforms to SUSv2: the pointer *string* given to **putenv**() is used. In particular, this string becomes part of the environment; changing it later will change the environment. (Thus, it is an error to call **putenv**() with an automatic variable as the argument, then return from the calling function while *string* is still part of the environment.) However, from glibc 2.0 to glibc 2.1.1, it differs: a copy of the string is used. On the one hand this causes a memory leak, and on the other hand it violates SUSv2.

The 4.3BSD-Reno version, like glibc 2.0, uses a copy; this is fixed in all modern BSDs.

SUSv2 removes the *const* from the prototype, and so does glibc 2.1.3.

The GNU C library implementation provides a nonstandard extension. If *string* does not include an equal sign:

```
putenv("NAME");
```

then the named variable is removed from the caller's environment.

## SEE ALSO

[**clearenv**(3)](/clearenv/3.md), [**getenv**(3)](/getenv/3.md), [**setenv**(3)](/setenv/3.md), [**unsetenv**(3)](/unsetenv/3.md), [**environ**(7)](/environ/7.md)