A stdin-stdout passthrough which ignores SIGPIPE

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Blog 2025/5/8

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A stdin-stdout passthrough which ignores SIGPIPE

One of the problems with using bash's set -o pipefail is that a command may fail with SIGPIPE if the next command in the pipepine exits before receiving all of its input.

For example:

command_which_creates_lots_of_output | grep -q foo

Here, grep will exit as soon as it encounters the first instance of "foo" on stdin.

If command_which_creates_lots_of_output still has more output to write, it will receive SIGPIPE, causing the pipeline to fail.

We can work around this by writing a C program which copies stdin to stdout but ignores SIGPIPE.

Compiling:

gcc -std=c99 -Wall -Werror -o nosigpipe nosigpipe.c

Revisiting our above example:

command_which_creates_lots_of_output | ./nosigpipe | grep -q foo

nosigpipe.c

/* a stdin-stdout passthrough which ignores SIGPIPE. */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <stdbool.h>

bool did_epipe = false;

#define BUFF_SIZE 1024
char* buff[BUFF_SIZE];

void sigpipe_handler(int signum) {
    ; /* do nothing. */
}

int main(void) {
    /* register to receive SIGPIPE. */
    int ret = 0;
    struct sigaction sa;
    ret = sigemptyset(&sa.sa_mask);
    if (ret == -1) {
        perror("sigemptyset");
        exit(1);
    }
    sa.sa_handler = sigpipe_handler;
    ret = sigaction(SIGPIPE, &sa, NULL);
    if (ret == -1) {
        perror("sigaction");
        exit(1);
    }

    /* copy stdin to stdout. */
    ssize_t nr;
    ssize_t nw;
    while (true) {
        nr = read(STDIN_FILENO, buff, BUFF_SIZE);
        if (nr == -1) {
            perror("read");
            exit(1);
        }
        if (nr == 0) {
            exit(0);
        }
        if (did_epipe) {
            /* if we've hit SIGPIPE, read and discard any remaining input. */
            continue;
        }

        nw = 0;
        while (nr > 0) {
            nw = write(STDOUT_FILENO, buff + nw, nr);
            if (nw == -1) {
                if (errno == EINTR) {
                    errno = 0;
                    continue;
                }
                if (errno == EPIPE) {
                    did_epipe = true;
                    errno = 0;
                    break;
                }
                perror("write");
                exit(1);
            }
            nr -= nw;
            nw = 0;
            continue;
        }
    }

    return 0;
}