Question

I have the following code. When I compile it with the gnu extensions (-std=gnu99), the program will catch 5 SIGINT before ending (which I would expect). When compiled without it (-std=c99) ends after the second (and only outputs one line).

What am I missing?

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

int int_stage = 0;
int got_signal = 0;

void sigint(int parameter)
{
  (void)parameter;
  got_signal = 1;
  int_stage++;
}

int main()
{
  signal(SIGINT,sigint);

  while(1)
  {
    if (got_signal)
    {
      got_signal = 0;
      puts("still alive");
      if (int_stage >= 5) exit(1);
    }
  }
  return 0;
}

Answer 1

Use sigaction(2) rather than signal(2).

The Linux man page has this, in particular, in the Portability section:

In the original UNIX systems, when a handler that was established using signal() was invoked by the
delivery of a signal, the disposition of the signal would be reset to SIG_DFL, and the system did
not block delivery of further instances of the signal. System V also provides these semantics for
signal(). This was bad because the signal might be delivered again before the handler had a chance
to reestablish itself. Furthermore, rapid deliveries of the same signal could result in recursive
invocations of the handler.

BSD improved on this situation by changing the semantics of signal handling (but, unfortunately,
silently changed the semantics when establishing a handler with signal()). On BSD, when a signal
handler is invoked, the signal disposition is not reset, and further instances of the signal are
blocked from being delivered while the handler is executing.

The situation on Linux is as follows:

• The kernel’s signal() system call provides System V semantics.

• By default, in glibc 2 and later, the signal() wrapper function does not invoke the kernel system
  call. Instead, it calls sigaction(2) using flags that supply BSD semantics. This default behav‐
  ior is provided as long as the _BSD_SOURCE feature test macro is defined. By default, _BSD_SOURCE
  is defined; it is also implicitly defined if one defines _GNU_SOURCE, and can of course be explic‐
  itly defined.
  On glibc 2 and later, if the _BSD_SOURCE feature test macro is not defined, then signal() provides
  System V semantics. (The default implicit definition of _BSD_SOURCE is not provided if one
  invokes gcc(1) in one of its standard modes (-std=xxx or -ansi) or defines various other feature
  test macros such as _POSIX_SOURCE, _XOPEN_SOURCE, or _SVID_SOURCE; see feature_test_macros(7).)

Using std=gnu99, you’re getting BSD semantics. Using -std=c99, you’re getting System V semantics. So the signal handler is “reinstalled” in one case (BSD), and the signal disposition is reset back to SIG_DFL in the other (System V).