NAME
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.
If the application modifies environ or the pointers to which it points,
the behavior of setenv() is undefined. 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 reentrant. A function that is not
required to be reentrant is not required to 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 name argument is a null pointer, 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.
EXAMPLES
None.
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 developers of
IEEE Std 1003.1-2001 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 revision
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 an XSI extension.
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.
FUTURE DIRECTIONS
None.
SEE ALSO
exec() , getenv() , unsetenv() , the Base Definitions volume of
IEEE Std 1003.1-2001, <stdlib.h>, <sys/types.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .