Is there a way to test whether pclose() will succeed?

Question

In my C++ application, I am seeing a pclose() that hangs because the pipe's process hung and never exited. Is there anyway I could do something like select() to test whether the pclose() will return because the child process has completed? I'd rather not do a fork() instead of popen() if possible. If fork() is the only solution, are there any examples of using fork() to replace a popen() / pclose() scenario?

Answer 1

Probably the easiest way, particularly if you only have one child process, is to catch SIGCHLD and set a flag that the process has terminated and pclose() can be called.

Here's a simple example:

sillyprog.c:

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

int main(void)
{
    printf("This is some data from the child.\n");
    fflush(stdout);
    sleep(5);
    return 0;
}

pc.c:

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

volatile sig_atomic_t child_done = 0;

void handler(int signum)
{
    if ( signum == SIGCHLD ) {
        child_done = 1;
    }
}

int main(void)
{
    /*  Set signal handler  */

    struct sigaction sa;
    sa.sa_handler = handler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    if ( sigaction(SIGCHLD, &sa, NULL) == -1 ) {
        perror("couldn't set signal handler");
        return EXIT_FAILURE;
    }

    /*  Open pipe  */

    FILE * fp = popen("./sillyprog", "r");
    if ( !fp ) {
        fprintf(stderr, "Couldn't open pipe\n");
        return EXIT_FAILURE;
    }

    /*  Get a line from pipe  */

    char buffer[100];
    if ( !fgets(buffer, 100, fp) ) {
        fprintf(stderr, "Error calling fgets()\n");
        return EXIT_FAILURE;
    }

    const size_t len = strlen(buffer);
    if ( len && buffer[len - 1] == '\n' ) {
        buffer[len - 1] = 0;
    }
    printf("Got '%s' from pipe.\n", buffer);

    /*  Wait for child to finish  */

    while ( !child_done ) {
        printf("Child not ready, waiting...\n");
        sleep(1);
    }

    /*  Close pipe  */

    if ( pclose(fp) == -1 ) {
        fprintf(stderr, "Error calling pclose()\n");
        return EXIT_FAILURE;
    }
    else {
        printf("pclose() successfully called.\n");
    }

    return 0;
}

which outputs:

paul@horus:~/src/sandbox$ ./pc
Got 'This is some data from the child.' from pipe.
Child not ready, waiting...
Child not ready, waiting...
Child not ready, waiting...
Child not ready, waiting...
Child not ready, waiting...
pclose() successfully called.
paul@horus:~/src/sandbox$