---
name: "clearenv"
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/clearenv"
---

# clearenv(3)

clear the environment

## NAME

clearenv - clear the environment

## LIBRARY

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

## SYNOPSIS

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

**int clearenv(void);**

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

**clearenv**():

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

## DESCRIPTION

The **clearenv**() function clears the environment of all name-value pairs and sets the value of the external variable *environ* to NULL. After this call, new variables can be added to the environment using [**putenv**(3)](/putenv/3.md) and [**setenv**(3)](/setenv/3.md).

## RETURN VALUE

The **clearenv**() function returns zero on success, and a nonzero value on failure.

## ATTRIBUTES

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

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

## STANDARDS

- **putenv**()

  POSIX.1-2008.

- **clearenv**()

  None.

## HISTORY

- **putenv**()

  glibc 2.0. POSIX.1-2001.

- **clearenv**()

  glibc 2.0.

Various UNIX variants (DG/UX, HP-UX, QNX, ...). POSIX.9 (bindings for FORTRAN77). POSIX.1-1996 did not accept **clearenv**() and [**putenv**(3)](/putenv/3.md), but changed its mind and scheduled these functions for some later issue of this standard (see §B.4.6.1). However, POSIX.1-2001 adds only [**putenv**(3)](/putenv/3.md), and rejected **clearenv**().

## NOTES

On systems where **clearenv**() is unavailable, the assignment

```
environ = NULL;
```

will probably do.

The **clearenv**() function may be useful in security-conscious applications that want to precisely control the environment that is passed to programs executed using [**exec**(3)](/exec/3.md). The application would do this by first clearing the environment and then adding select environment variables.

Note that the main effect of **clearenv**() is to adjust the value of the pointer [**environ**(7)](/environ/7.md); this function does not erase the contents of the buffers containing the environment definitions.

The DG/UX and Tru64 man pages write: If *environ* has been modified by anything other than the [**putenv**(3)](/putenv/3.md), [**getenv**(3)](/getenv/3.md), or **clearenv**() functions, then **clearenv**() will return an error and the process environment will remain unchanged.

## SEE ALSO

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