setenv — change or add an environment variable


        #include <stdlib.h>
int setenv( const char *name,
  const char *value,
  int overwrite);
int unsetenv( const char *name);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
setenv(), unsetenv():
_POSIX_C_SOURCE >= 200112L || /* Glibc versions <= 2.19:


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)).

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.


The setenv() function returns zero on success, or −1 on error, with errno set to indicate the cause of the error.

The unsetenv() function returns zero on success, or −1 on error, with errno set to indicate the cause of the error.



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


Insufficient memory to add a new variable to the environment.


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

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


POSIX.1-2001, POSIX.1-2008, 4.3BSD.


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

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.


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


clearenv(3), getenv(3), putenv(3), environ(7)


This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright 1993 David Metcalfe (
and Copyright (C) 2004, 2007 Michael Kerrisk <>

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References consulted:
    Linux libc source code
    Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
    386BSD man pages
Modified Sat Jul 24 18:20:58 1993 by Rik Faith (
Modified Fri Feb 14 21:47:50 1997 by Andries Brouwer (
Modified 9 Jun 2004, Michael Kerrisk <>
    Changed unsetenv() prototype; added EINVAL error
    Noted nonstandard behavior of setenv() if name contains '='
2005-08-12, mtk, glibc 2.3.4 fixed the "name contains '='" bug