spawn_process_and_grab_output.c

#include <unistd.h>
#include <stdbool.h>
#include <assert.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/wait.h>
#include <sys/poll.h>

#ifdef __APPLE__
static inline int pipe2(int *fds, int flags)
{
    if(pipe(fds) == -1)
        return -1;
    fcntl(fds[0], F_SETFL, fcntl(fds[0], F_GETFL) | flags);
    fcntl(fds[1], F_SETFL, fcntl(fds[1], F_GETFL) | flags);
    return 0;
}
#endif

static inline bool wrapClose(int* pfd)
{
    assert(pfd);

    if (*pfd < 0)
        return false;
    close(*pfd);

    return true;
}
#define FF_AUTO_CLOSE_FD __attribute__((__cleanup__(wrapClose)))

int main(void)
{
    int pipes[2];
    const int timeout = 5000;

    if(pipe2(pipes, O_CLOEXEC) == -1)
        return 1; // "pipe() failed";

    pid_t childPid = fork();
    if(childPid == -1)
    {
        close(pipes[0]);
        close(pipes[1]);
        return 2; // "fork() failed";
    }

    //Child
    if(childPid == 0)
    {
        int nullFile = open("/dev/null", O_WRONLY | O_CLOEXEC);
        dup2(pipes[1], STDOUT_FILENO);
        dup2(nullFile, STDERR_FILENO);
        setenv("LANG", "C", 1);
        char* exe = "/usr/bin/ssh-agent";
        execvp(exe, (char* []) { exe, NULL });
        _exit(127);
    }

    //Parent
    close(pipes[1]);

    int FF_AUTO_CLOSE_FD childPipeFd = pipes[0];
    char str[8192];

    while(1)
    {
        if (timeout >= 0)
        {
            struct pollfd pollfd = { childPipeFd, POLLIN, 0 };
            if (poll(&pollfd, 1, timeout) == 0)
            {
                kill(childPid, SIGTERM);
                waitpid(childPid, NULL, 0);
                return 3; // "poll(&pollfd, 1, timeout) timeout";
            }
            else if (pollfd.revents & POLLERR)
            {
                kill(childPid, SIGTERM);
                waitpid(childPid, NULL, 0);
                return 4; //"poll(&pollfd, 1, timeout) error";
            }
        }

        ssize_t nRead = read(childPipeFd, str, sizeof(str));
        if (nRead > 0)
            fwrite(str, (size_t) nRead, 1, stdout);
        else if (nRead == 0)
        {
            int stat_loc = 0;
            if (waitpid(childPid, &stat_loc, 0) == childPid)
            {
                if (!WIFEXITED(stat_loc))
                    return 5; //"child process exited abnormally";
                if (WEXITSTATUS(stat_loc) == 127)
                    return 6; //"command was not found";
                return 0;
            }
            return 7; // "waitpid() failed";
        }
        else if (nRead < 0)
            break;
    };

    return 8; // "read(childPipeFd, str, FF_PIPE_BUFSIZ) failed";
}