NAME
putenv - change or add an environment variable
SYNOPSIS
#include <stdlib.h>
int putenv(char *string);
DESCRIPTION
The putenv() function adds or changes the value of environment
variables. The argument string is of the form name=value. If name does
not already exist in the environment, then string is added to the
environment. If name does exist, then the value of name in the
environment is changed to value. The string pointed to by string becomes
part of the environment, so altering the string changes the environment.
On Win32 it’s called _putenv I believe.
See SetEnvironmentVariable also if you’re a fan of long and ugly function names.