int unsetenv(const char *name);
The unsetenv() function shall remove an environment variable from the environment of the calling process. The name argument points to a string, which is the name of the variable to be removed. The named argument shall not contain an '=' character. If the named variable does not exist in the current environment, the environment shall be unchanged and the function is considered to have completed successfully.
If the application modifies environ or the pointers to which it points, the behavior of unsetenv() is undefined. The unsetenv() function shall update the list of pointers to which environ points.
The unsetenv() function need not be reentrant. A function that is not required to be reentrant is not required to 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 unsetenv() function shall fail if:
The name argument is a null pointer, points to an empty string,
or points to a string containing an '='
The following sections are informative.
Refer to the RATIONALE section in setenv() .
COPYRIGHTPortions 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 .