The Posix solution is to use pipes. Basically, you'll want the parent process to set up a pipe for stderr, fork, then the child process uses the write end of the stderr while the parent reads from the other end. Not sure what the Windoze solution is. Alternatively, the cheap and easy way is to use redirects:
system("dmd bla.d 2>error.log");
If an error is thrown, read from error.log.
