---
name: "setenv"
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/setenv/3"
---

# setenv(3)

change or add an environment variable

## NAME

setenv - change or add an environment variable

## LIBRARY

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

## SYNOPSIS

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

**int setenv(const char \****name***, const char \****value***, int** *overwrite***);** **int unsetenv(const char \****name***);**

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

**setenv**(), **unsetenv**():

```
    _POSIX_C_SOURCE >= 200112L
        || /* glibc <= 2.19: */ _BSD_SOURCE
```

## DESCRIPTION

The **setenv**() function adds the variable *name* to the environment with the value *value*, if *name* does not already exist. If *name* does exist in the environment, then its value is changed to *value* if *overwrite* is nonzero; if *overwrite* is zero, then the value of *name* is not changed (and **setenv**() returns a success status). This function makes copies of the strings pointed to by *name* and *value* (by contrast with [**putenv**(3)](/putenv/3.md)).

The **unsetenv**() function deletes the variable *name* from the environment. If *name* does not exist in the environment, then the function succeeds, and the environment is unchanged.

## RETURN VALUE

**setenv**() and **unsetenv**() functions return zero on success, or -1 on error, with *errno* set to indicate the error.

## ERRORS

- **EINVAL**

  *name* is NULL, points to a string of length 0, or contains an '=' character.

- **ENOMEM**

  Insufficient memory to add a new variable to the environment.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **setenv**(), **unsetenv**() | Thread safety | MT-Unsafe const:env |

## STANDARDS

POSIX.1-2008.

## HISTORY

POSIX.1-2001, 4.3BSD.

Prior to glibc 2.2.2, **unsetenv**() was prototyped as returning *void*; more recent glibc versions follow the POSIX.1-compliant prototype shown in the SYNOPSIS.

## CAVEATS

POSIX.1 does not require **setenv**() or **unsetenv**() to be reentrant.

## BUGS

POSIX.1 specifies that if *name* contains an '=' character, then **setenv**() should fail with the error **EINVAL**; however, versions of glibc before glibc 2.3.4 allowed an '=' sign in *name*.

## SEE ALSO

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