Hands-On System Programming with Linux

The errno race

Consider this situation:

1. A process sets up a signal handler for several signals:
   • Let's say the signal handler for SIGUSR1 is called handle_sigusr.
2. Now the process is running a part of its code, a function foo:
   • foo issues a system call, say the open(2)
   • The system call fails returning -1
     • errno gets set to the positive integer 13 reflecting the error permission denied (errno macro EACCES).
   • The system call's error-handling code calls perror(3) to emit the English error message.

All this seems innocent enough, yes. However, now let's consider signals in the mix; check out the following scenario:

• <...>
  • foo issues a system call, say the open(2).
  • The system call fails returning -1.
    • errno gets set to the positive integer ...