That would be at best a Linux extension, not some general C behavior you can assume when writing your program.

That said though, I can't even reproduce what you're saying on Linux:

  printf '%s\n' '#include <stdio.h>' 'int main() { setvbuf(stdout, NULL, _IONBF, 0); int r = fputs("Starting\n", stdout); fflush(stdout); fprintf(stderr, "%d\n", r); }' | cc -x c - && ./a.out >&-
  // Prints '0' instead of dying

That's a POSIX thing. It doesn't apply to all C implementations but it does apply to many more than just Linux-based ones. You've not got a closed pipe so you wouldn't see it, you've just got a closed file descriptor. Try running it as

  ./a.out | :

and you will probably see it. I say probably because there is a timing aspect as well, the write may happen before the pipe gets closed in which case it will not fail, but it is unlikely to.

Yeah I should've said POSIX, my bad. But yeah my point was it's not plain C behavior.

And yes on Linux I do see it with your no-op example now. Though for some reason not with 'head'... what's going on? Is it not closing the pipe when it exits?

  $ printf '%s\n' '#include <stdio.h>' '#include <unistd.h>' 'int main() { setvbuf(stdout, NULL, _IONBF, 0); int r = puts("Starting...\n"); r += fputs("First\n", stdout); fflush(stdout); usleep(1000000); fprintf(stderr, "%d\n", r); }' | cc -x c - && ./a.out | head -n 1
  Starting...
  19

Edit: D'oh, see below.

That would be the timing aspect of it. You have:

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.

Sorry, I think I edited my comment while you were replying. But I just noticed the problem in the most recent version was that I didn't write to stdout after the usleep(), so it never raised SIGPIPE. Thanks.

> it's not plain C behavior.

But pipes aren't a C thing in the first place. "unistd.h" is not a C thing, file descriptors aren't a C thing.

The C program would just be using stdio to interact with pipes.

And not every platform with pipes supports SIGPIPE with such behavior.