gex/tests/integration/programs/background.c

/* A child that stays alive after fork (like `cmd &`), works in the parent
 * while it does, exchanges messages over pipes, and reaps it at the end.
 *
 * Every line is ordered by an event -- a pipe read, a write, an exit status --
 * never by which process happens to be scheduled.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>

static void put(const char *s) {
    /* Unbuffered, because the interleaving between two processes is the point
     * and stdio would reorder it into whichever flushed last. */
    write(1, s, strlen(s));
}

int main(void) {
    int down[2], up[2];
    if (pipe(down) != 0 || pipe(up) != 0) {
        put("pipe failed\n");
        return 1;
    }

    pid_t kid = fork();
    if (kid < 0) {
        put("fork failed\n");
        return 1;
    }
    if (kid == 0) {
        close(down[1]);
        close(up[0]);
        /* Nothing is printed before this read: the parent must run to send
         * anything, so reaching the end of it is the whole test. A line before
         * it would be ordered by scheduling alone. */
        char buf[32];
        ssize_t n = read(down[0], buf, sizeof buf - 1);
        if (n <= 0) {
            put("child: the parent never wrote\n");
            _exit(2);
        }
        buf[n] = 0;
        put("child: got ");
        put(buf);
        put("\n");
        write(up[1], "ok", 2);
        _exit(7);
    }

    close(down[0]);
    close(up[1]);
    /* The parent keeps working while the child is alive. */
    put("parent: still running\n");
    long acc = 0;
    for (long i = 0; i < 200000; i++) {
        acc += i % 7;
    }
    if (acc == 0) {
        put("parent: impossible\n");
    }
    put("parent: sending work\n");
    write(down[1], "hello", 5);

    char reply[8];
    ssize_t n = read(up[0], reply, sizeof reply - 1);
    reply[n > 0 ? n : 0] = 0;
    put("parent: child replied ");
    put(reply);
    put("\n");

    int status = 0;
    if (waitpid(kid, &status, 0) != kid) {
        put("parent: waitpid did not find the child\n");
        return 1;
    }
    printf("parent: child exited %d\n", WIFEXITED(status) ? WEXITSTATUS(status) : -1);
    return 0;
}