putenv(3) Library Functions Manual putenv(3)

putenv - change or add an environment variable

Standard C library (libc, -lc)

#include <stdlib.h>
int putenv(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


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

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.

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

Insufficient space to allocate new environment.

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

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


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:


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

clearenv(3), getenv(3), setenv(3), unsetenv(3), environ(7)

2024-05-02 Linux man-pages 6.9.1