unsetenv (string);
__unsetenv (string);
putenv (string)     const char *string;{  const char *const name_end = strchr (string, '=');  if (name_end != NULL)    {#ifdef _LIBC      char *name = strndupa (string, name_end - string);#else      char *name = alloca (name_end - string + 1);      memcpy (name, string, name_end - string);      name[name_end - string] = '\0';#endif      return setenv (name, name_end + 1, 1);    }  unsetenv (string);  return 0;}