unsetenv

unsetenv is a function provided by the C standard library on POSIX-compliant systems that removes an environment variable from the calling process’s environment. By modifying the process-wide environment, it affects the set of variables visible to code running in the same process and to subsequently executed programs.

The typical prototype is int unsetenv(const char *name); and it is usually declared in a standard header such as stdlib.h. The function removes the environment variable named by name from the process’s environment, which is managed as a global environment array (often accessible as environ).

Return value and error handling: On success, unsetenv returns 0. If an error occurs, it returns -1 and sets errno. A common error condition is an invalid argument, such as a NULL pointer, an empty name, or a name containing an '=' character, in which case errno is set to EINVAL. If the named variable does not exist, many implementations treat the call as a no-op and still return 0.

Relationship to setenv: unsetenv is the counterpart to setenv. While setenv creates or updates an environment variable, unsetenv removes it from the environment. After calling unsetenv, subsequent getenv or access to that variable will reflect its absence, and it will not be inherited by child processes created via fork/exec.

Shell usage: In many Unix shells, unsetenv is not a built-in; shells such as bash and sh use a different command (unset) to remove shell variables or exported environment variables. Some shells in the C shell family provide a built-in named unsetenv that operates within the shell environment.

See also: getenv, setenv, environ, execve.