The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


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 putenv() function need not 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.

Although the space used by string is no longer used once a new string which defines name is passed to putenv(), if any thread in the application has used getenv() to retrieve a pointer to this variable, it should not be freed by calling free(). If the changed environment variable is one known by the system (such as the locale environment variables) the application should never free the buffer used by earlier calls to putenv() for the same variable.

The setenv() function is preferred over this function. One reason is that putenv() is optional and therefore less portable. Another is that using putenv() can slow down environment searches, as explained in the RATIONALE section for getenv.


Refer to the RATIONALE section in setenv.




exec, free, getenv, malloc, setenv

XBD <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.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0472 [167], XSH/TC1-2008/0473 [167], XSH/TC1-2008/0474 [273,438], and XSH/TC1-2008/0475 [273] are applied.

End of informative text.


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