putenv(3) Library Functions Manual putenv(3)
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)):
putenv():
_XOPEN_SOURCE
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE
DESCRIPTION
The putenv() function adds or changes the value of environment vari-
ables. The argument string is of the form name=value. If name does not
already exist in the environment, then string is added to the environ-
ment. 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).
┌────────────────────────────────┬───────────────┬─────────────────────┐
│ 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 envi-
ronment. (Thus, it is an error to call putenv() with an automatic vari-
able 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), getenv(3), setenv(3), unsetenv(3), environ(7)
Linux man-pages 6.9.1 2024-05-02 putenv(3)
Generated by dwww version 1.16 on Tue Dec 16 04:02:52 CET 2025.