win_env.h

/* Replacements for setenv, unsetenv on windows */
 
/* Add #include <stdlib.h> to your function to then include this in your 
   module implementation file (*.c) not in a header */
 
int setenv(const char *name, const char *value, int overwrite)
{
   int errcode = 0;
   if(!overwrite) {
      size_t envsize = 0;
      errcode = getenv_s(&envsize, NULL, 0, name);
      if(errcode || envsize) return errcode;
   }
   return _putenv_s(name, value);
}
 
int unsetenv(const char *name)
{
   const char* value = "";
   return _putenv_s(name, value);  // empty string removes the value
}