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


setenv - add or change environment variable


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

int setenv(const char *
envname, const char *envval, int overwrite); [Option End]


The setenv() function shall update or add a variable in the environment of the calling process. The envname argument points to a string containing the name of an environment variable to be added or altered. The environment variable shall be set to the value to which envval points. The function shall fail if envname points to a string which contains an '=' character. If the environment variable named by envname already exists and the value of overwrite is non-zero, the function shall return success and the environment shall be updated. If the environment variable named by envname already exists and the value of overwrite is zero, the function shall return success and the environment shall remain unchanged.

The setenv() function shall update the list of pointers to which environ points.

The strings described by envname and envval are copied by this function.

The setenv() function need not be thread-safe.


Upon successful completion, zero shall be returned. Otherwise, -1 shall be returned, errno set to indicate the error, and the environment shall be unchanged.


The setenv() function shall fail if:

The envname argument points to an empty string or points to a string containing an '=' character.
Insufficient memory was available to add a variable or its value to the environment.

The following sections are informative.




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


Unanticipated results may occur if setenv() changes the external variable environ. In particular, if the optional envp argument to main() is present, it is not changed, and thus may point to an obsolete copy of the environment (as may any other copy of environ). However, other than the aforementioned restriction, the standard developers intended that the traditional method of walking through the environment by way of the environ pointer must be supported.

It was decided that setenv() should be required by this version because it addresses a piece of missing functionality, and does not impose a significant burden on the implementor.

There was considerable debate as to whether the System V putenv() function or the BSD setenv() function should be required as a mandatory function. The setenv() function was chosen because it permitted the implementation of the unsetenv() function to delete environmental variables, without specifying an additional interface. The putenv() function is available as part of the XSI option.

The standard developers considered requiring that setenv() indicate an error when a call to it would result in exceeding {ARG_MAX}. The requirement was rejected since the condition might be temporary, with the application eventually reducing the environment size. The ultimate success or failure depends on the size at the time of a call to exec, which returns an indication of this error condition.

See also the RATIONALE section in getenv.




exec, getenv, putenv, unsetenv

XBD <stdlib.h>, <sys/types.h>, <unistd.h>


First released in Issue 6. Derived from the IEEE P1003.1a draft standard.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/55 is applied, adding references to exec in the APPLICATION USAGE and SEE ALSO sections.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0549 [167], XSH/TC1-2008/0550 [185], XSH/TC1-2008/0551 [167], and XSH/TC1-2008/0552 [38] are applied.

End of informative text.


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