waitpid和信号处理程序，C

Question

I tried to find an answer to my question at this post: Signal handler and waitpid coexisting but for me isn't very clear at the moment.

I try to explain my problems:

I'm trying to write a C program that concerns IPC between a parent process and its children. The parent process creates N child processes, then it waits for the termination in a loop like this:

while((pid_term = waitpid(-1, &status, 0)) != -1)

After X seconds, parent receives SIGALRM, then with the sigaction system call, it catches the alarm:

struct sigaction act;
act.sa_handler = alarmHandler;
sigemptyset(&act.sa_mask);

act.sa_flags = 0;
sigaction(SIGALRM, &act, NULL);  

But, when the handler function returns, the waitpid also returns -1, and the parent process exits from the while loop above. At the moment, the handler function has an empty body.

I ask myself what happened — why did waitpid() return -1 after the handler invocation even though most of the children are still alive? Why doesn't this happen with signal() function?

Answer 1

The default behavior of signal handlers established by sigaction is to interrupt blocking system calls; if you check errno after the alarm fires you should observe it to be set to EINTR . This behavior is almost never what you want; it's only the default for backward compatibility's sake. You can make it not do this by setting the SA_RESTART bit in sa_flags :

struct sigaction act;
act.sa_flags = SA_RESTART;
act.sa_handler = alarmHandler;
sigemptyset(&act.sa_mask);
sigaction(SIGALRM, &act, 0);

One of the most important reasons to use sigaction instead of signal , is that when you use signal it is unpredictable whether or not the signal handler will interrupt blocking system calls. (The System V lineage picked one semantic and the BSD lineage picked the other.)