How to tell if a downstream process in a Unix pipe has crashed

Question

I have a Linux process (let's call it the main process) whose standard output is piped to another process (called the downstream process) by means of the shell's pipe operator (|). The main process is set up to receive SIGPIPE signals if the downstream process crashes. Unfortunately, SIGPIPE is not raised until the main process writes to stdout. Is there a way to tell sooner that the downstream process has terminated?

One approach is to write continuously to the downstream process, but that seems wasteful. Another approach is to have a separate watchdog process that monitors all relevant processes, but that is complex. Or perhaps there is some way to use select() to trigger the signal. I am hoping that the main process can do all this itself.

Is it necessary to know immediately when the downstream process crashes, or would it be enough to know that it has crashed the next time you have something to write to it? — zwol, Nov 10 '11 at 03:24
As I mention in the question, I'd like to know that the downstream process crashes sooner, rather than on the next write (which might come at an indeterminate time in my context). — Randall Cook, Nov 10 '11 at 04:30

score 3 · Accepted Answer · answered Nov 10 '11 at 03:12

It appears the stdout file descriptor becomes "ready for reading" when the receiver crashes:

$ gcc -Wall select-downstream-crash.c -o select-downstream-crash
$ gcc -Wall crash-in-five-seconds.c -o crash-in-five-seconds
$ ./select-downstream-crash | ./crash-in-five-seconds
    ... five seconds pass ...
stdout is ready for reading
Segmentation fault

select-downstream-crash.c

#include <err.h>
#include <stdio.h>
#include <sys/select.h>
#include <unistd.h>

int main(void)
{
    fd_set readfds;
    int rc;

    FD_ZERO(&readfds);
    FD_SET(STDOUT_FILENO, &readfds);

    rc = select(STDOUT_FILENO + 1, &readfds, NULL, NULL, NULL);
    if (rc < 0)
        err(1, "select");

    if (FD_ISSET(STDOUT_FILENO, &readfds))
        fprintf(stderr, "stdout is ready for reading\n");

    return 0;
}

crash-in-five-seconds.c

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

int main(void)
{
    sleep(5);
    putchar(*(char*)NULL);
    return 0;
}

I tried this on Linux, but don't know if it'll work elsewhere. It would be nice to find some documentation explaining this observation.

score 0 · Answer 2 · answered Nov 10 '11 at 00:10

0

If the main process forks the other processes, then it will get SIGCHLD notifications when they exit.

answered Nov 10 '11 at 00:10

zwol

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Right. Unfortunately, the downstream process is created by the shell and connected by the pipe operator: `main | downstream`. – Randall Cook Nov 10 '11 at 01:30

How to tell if a downstream process in a Unix pipe has crashed

2 Answers2