useratuniv useratuniv - 7 days ago 6
C Question

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

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\n", 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);

R.. R..
Answer

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.

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