How to drop root privileges on linux in C?

Question

I'd like to fork and exec and program as a non root user while the parent retains root permissions.

pseudo code:

  pid = fork();

  if (pid == 0) {
      if (RunAsUser(ConvertStringToUserId("John")) == false) {
          stop();
      }
      if (RunAsUser(ConvertStringToUserId("admin")) == true) {
          stop();
      }
      CreateProcess();
  }

Answer 1

If you want to drop privileges in C code, use the function setuid.