c - Print int from signal handler using write or async-safe functions

Question

I want to print a number into log or to a terminal using write (or any async-safe function) inside a signal handler. I would prefer not to use buffered I/O.

Is there an easy and recommended way to do that ?

For example in place of printf, below I would prefer write (or any asyn safe function).

void signal_handler(int sig)
{
  pid_t pid;
  int stat;
  int old_errno = errno;

  while((pid = waitpid(-1, &stat, WNOHANG)) > 0)
    printf("child %d terminated
", pid);

  errno = old_errno;
  return;
}

Printing strings is easy. In place of the printf above I can use (without printing pid):

write(STDOUT_FILENO, "child terminated", 16);

Answer 1

If you really insist on doing the printing from a signal handler, you basically have 2 options:

1. Block the signal except in a dedicated thread you create for handling the signal. This special thread can simply perform for (;;) pause(); and since pause is async-signal-safe, the signal handler is allowed to use any functions it wants; it's not restricted to only async-signal-safe functions. On the other hand, it does have to access shared resources in a thread-safe way, since you're now dealing with threads.

2. Write your own code for converting integers to decimal strings. It's just a simple loop of using %10 and /10 to peel off the last digit and storing them to a short array.

However, I would highly recommend getting this operation out of the signal handler, using the self-pipe trick or similar.