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.
在Win32上，我相信它叫做_putenv.
如果你喜欢又长又难看的函数名，也可以看SetEnvironmentVariable.