How to leave (interrupt) the waitpid() function?

Question

Currently I am programming a shell and I use the waitpid() function for my children processes.

I also installed a signal handler so I can trap the SIGINT (CTRL+C) signal.

So what I want now is when someone presses the SIGINT (CTRL+C) signal it should leave the waitpid() functions and keep going as usually.

I am looking for a function who can help me about that.

Don’t set SA_RESTART in the flags when you call `sigaction()`. — Jonathan Leffler, May 19 '18 at 13:35
... `sigaction()` is preferred over using `signal()` to install a signal handler. — alk, May 19 '18 at 13:37
Consider also handling the `SIGCHLD` signal, see [signal(7)](http://man7.org/linux/man-pages/man7/signal.7.html) & [signal-safety(7)](http://man7.org/linux/man-pages/man7/signal-safety.7.html) — Basile Starynkevitch, May 19 '18 at 14:28

score 0 · Answer 1 · answered May 19 '18 at 13:38

0

Install your signal handler without the SA_RESTART flag:

void handler( int sig, siginfo_t *si, void *arg )
{
    ...
}

...

struct sigaction newact;
memset( &newact, 0, sizeof( newact ) );
sigemptyset( &newact.sa_mask );
newact.sa_flags = SA_SIGINFO;  // note that **lack** of SA_RESTART
newact.sa_siginfo = handler;

sigaction( SIGINT, &newact, NULL );

You'll have to add error checking to that, and note that you will now have to handle interrupted calls in your code.

answered May 19 '18 at 13:38

Andrew Henle

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OT: I'd use `struct sigaction newact = {0};` instead of calling `memset()`. – alk May 19 '18 at 13:40
@alk I've been around too long... :-) – Andrew Henle May 19 '18 at 13:41
Also I doubt that explicitly emptying `sa_mask` is necessary. Also^2 why do you set `SA_SIGINFO` for a handler which actually does nothing. So assuming `void handler(int sig);` then the short version could be `{ sigaction(SIGINT, (struct sigaction){handler}, NULL); }` :-) – alk May 19 '18 at 13:44
Thanks for your help, but I dont understand what you calling, do you have a reference or something like that? – Nado Ba May 19 '18 at 13:45
@alk I tend to be paranoid about making sure all my variables are properly initialized - a defensive coding measure against the bugs I'm going to put in (two kinds of programmers: those who create bugs, and liars). As far as `SA_SIGINFO`, we don't know what he OP's signal handler does, so I provided an example of the more complex usage. – Andrew Henle May 19 '18 at 13:49

score 0 · Answer 2 · answered May 19 '18 at 13:51

A process blocking on waitpid() would return on reception of a signal and set errno to EINTR.

For this to work the signal handler my not have been installed with the option SA_RESTART.

To install a signal handle without setting the option SA_RESTART the prefered function is sigaction().

void handle(int sig)
{
  /* Do nothing. */
}

int main(void)
{
  sigaction(SIGINT, (struct sigaction){handler}, NULL);

  /* fork and bla ... */

  if (-1 == waitpid(..., /*no WNOHANG here */))
  {
    if (EINTR == errno)
    {
      /* process received signal */
      ...

How to leave (interrupt) the waitpid() function?

2 Answers2