The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


putenv - change or add a value to an environment


[XSI] [Option Start] #include <stdlib.h>

int putenv(char *
string); [Option End]


The putenv() function shall use the string argument to set environment variable values. The string argument should point to a string of the form " name= value ". The putenv() function shall make the value of the environment variable name equal to value by altering an existing variable or creating a new one. In either case, the string pointed to by string shall become part of the environment, so altering the string shall change the environment. The space used by string is no longer used once a new string which defines name is passed to putenv().

The putenv() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.


Upon successful completion, putenv() shall return 0; otherwise, it shall return a non-zero value and set errno to indicate the error.


The putenv() function may fail if:

Insufficient memory was available.

The following sections are informative.


Changing the Value of an Environment Variable

The following example changes the value of the HOME environment variable to the value /usr/home.

#include <stdlib.h>
static char *var = "HOME=/usr/home";
int ret;

ret = putenv(var);


The putenv() function manipulates the environment pointed to by environ, and can be used in conjunction with getenv().

See exec() , for restrictions on changing the environment in multi-threaded applications.

This routine may use malloc() to enlarge the environment.

A potential error is to call putenv() with an automatic variable as the argument, then return from the calling function while string is still part of the environment.

The setenv() function is preferred over this function.


The standard developers noted that putenv() is the only function available to add to the environment without permitting memory leaks.




exec(), getenv(), malloc(), setenv(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The type of the argument to this function is changed from const char * to char *. This was indicated as a FUTURE DIRECTION in previous issues.

A note indicating that this function need not be reentrant is added to the DESCRIPTION.

Issue 6

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/48 is applied, clarifying wording in the DESCRIPTION and adding a new paragraph into APPLICATION USAGE referring readers to exec.

End of informative text.

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