Why does this C program generate SIGPIPE later than expected?

Question

This program generates SIGPIPE after piping it to "head -n 1", after a random time. I understand that because we're feeding more to "head -n 1" after the first line, we would expect it to generate SIGPIPE, but instead it will make it to a random number (usually > 20 and < 200) before exitting. Any idea why?

#include <stdio.h>
#include <stdlib.h>

main()
{
  int i;
  char *s = "ABCDEFGHIJKLMNOPQRSTUVWXYZ\n";

  i = 0;
  while (1) {
    fputs(s, stdout);
    fflush(stdout);
    fprintf(stderr, "Iteration %d done\n", i);
    i++;
  }
}

This is not homework, just something in my professor's notes that I do not understand.

Answer 1

It's the vagaries of scheduling.

Your producer — let's call it alphabeta — is able to run for some amount time before head is able to read and exit (thus breaking the pipe).

That "some amount of time", of course, is variable.

Sometimes alphabeta runs 20 times before head can read stdin and exit. Sometimes 200 times. On my system, sometimes 300 or 1000 or 2000 times. Indeed, it can theoretically loop up to the capacity of the pipe connecting producer and consumer.

For demonstration, let's introduce some delay so we can be reasonably sure that head is stuck in a read() before alphabeta produces a single line of output:

so$ { sleep 5; ./alphabeta; } | head -n 1
ABCDEFGHIJKLMNOPQRSTUVWXYZ
Iteration 0 done

(N.B. it's not guaranteed that alphabeta will only iterate once in the above. However, on an unloaded system, this is more-or-less always going to be the case: head will be ready, and its read/exit will happen more-or-less immediately.)

Watch instead what happens when we artificially delay head:

so$ ./alphabeta | { sleep 2; head -n 1; }
Iteration 0 done
...
Iteration 2415 done    # <--- My system *pauses* here as pipe capacity is reached ...
Iteration 2416 done    # <--- ... then it resumes as head completes its first read()
...
Iteration 2717 done    # <--- pipe capacity reached again; head didn't drain the pipe 
ABCDEFGHIJKLMNOPQRSTUVWXYZ

As an aside, @R.. is quite right in his remarks that SIGPIPE is synchronous. In your case, the first fflush-induced write to a broken pipe (after head has exited) will synchronously generate the signal. This is documented behavior.

Answer 2

I think it is simply because signals are asynchronous.

Update: As others pointed out they (more precisely many, including SIGPIPE) are not. This was an unthoughtful answer :)