Child process still does not run after receiving signal

Question

The code is one of the answers to this question .

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


void
sigusr1( int pidno )
{
  fprintf(stderr, "Caught\n");
}

int
main()
{
  pid_t pid;

  signal( SIGINT, sigusr1 );
  if( (pid = fork()) == 0 ){
    pause();
    fprintf(stderr, "Child\n");
  }
  else
  {
    fprintf(stderr, "Parent\n");
    kill( pid , SIGINT ); //parent sends signal to child
  }
  pause();
  return 0;
}

The child waits for a signal, and after receiving it continues execution.

Running it I get

Parent
Caught

It seems that the child does not runs after receiving the signal. Pressing Ctrl+c:

Parent
Caught
^CCaught
Caught
Child

Can somebody explain me what is happening here?

Answer 1

pause() gets invoked after the signal has been received and handled by the child. Unfortunately, pause() easily introduces this kind of race condition :

time you parent                   child
---- --- ------                   -----
  0      signal(SIGINT,handler)
  1      fork()                   (hello)
  2      print("Parent")          (waking up)
  3      kill(..., SIGINT)        <<SIGINT>>               # RACE kill v pause
  4                               handler: print("Caught")
  5      pause()                  pause()
  6   ^C <<SIGINT>>               <<SIGINT>>
  7      handler: print("Caught") handler: print("Caught")
  8      exit                     pause()
  9                               (still running)

To check what exactly goes on, try adding %i, getpid() to printfs, and maybe also one more printf() before the pause() call in the child branch.

sigsuspend() and explicit signal masking might be a better choice here.