a) a.out writes line 1

b) a.out writes line 2

c) head reads line 1

d) head closes the pipe

We know that b and c both happen after a, and that d happens after c. However, we do not know whether b happens before c, between c and d, or after d. Your a.out process will only get killed by SIGPIPE if it happens after d.

On my system, running a.out under strace is enough to slow it down enough to affect the timing and see the SIGPIPE you were expecting. You may alternatively insert artificial delays in your test program such as by calling the sleep() function between the two lines of output to see the same result.