setenv man page

Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

setenv — add or change environment variable

Synopsis

#include <stdlib.h>

int setenv(const char *envname, const char *envval, int overwrite);

Description

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.

Return Value

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

Errors

The setenv() function shall fail if:

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

The following sections are informative.

Application Usage

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

Rationale

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().

See Also

exec, getenv(), putenv(), unsetenv()

The Base Definitions volume of POSIX.1‐2008, <stdlib.h>, <sys_types.h>, <unistd.h>

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2013 IEEE/The Open Group POSIX Programmer's Manual