CODEWITHSUNDEEP
cunix
Plexind
Plexind

Reputation: 5

Bad file descriptor. WHY?

I have searched everywhere and the only reason I know of that I would be getting a bad file descriptor for fdUp[READ] is if I had previously closed it. I don't believe I have done this and I cannot figure out why.

bool start_process(void) {
    int fdDown[2];
    int fdUp[2];
    FILE* in;
    FILE* out;

    if(pipe(fdDown) && pipe(fdUp)) {
        return false; //pipe failed
    }

    switch(fork()) {
        case -1:
            return false; //fork failed
        case 0:
            // child
            close(fdDown[WRITE]);
            dup2(fdDown[READ], STDIN_FILENO);
            close(fdDown[READ]);

            close(fdUp[READ]);
            dup2(fdUp[WRITE], STDOUT_FILENO);
            close(fdUp[WRITE]);

            execlp("./process", "process1", (char *) 0);
            return false; //exec failed
        default:
            //parent
            close(fdDown[READ]);
            out = fdopen(fdDown[WRITE], "w");

            close(fdUp[WRITE]);
            in = fdopen(fdUp[READ], "r");
            if(in==NULL) printf("%s\n", strerror(errno)); //BAD FILE DESCRIPTOR

            fprintf(out, "msg from parent\n");
            fflush(out);

            char buf[100];
            fgets(buf, 100, in);
            printf("%s", buf);

            wait(NULL);
    }
    return true;
}

Upvotes: 0

Views: 234

Answers (1)

P.P
P.P

Reputation: 121427

Your descriptors in fdUp are indeed invalid because of the way you initialize them.

   if(pipe(fdDown) && pipe(fdUp)) {
        return false; //pipe failed
    }

If pipe(fdDown) succeeds (i.e., returns 0) then pipe(fdUp) won't be executed at all (due to short-circuit evaluation). You really want:

   if(pipe(fdDown) || pipe(fdUp)) {
        return false; //pipe failed
    }

Another way with better diagnostics could be:

   if(pipe(fdDown)) {
    perror("pipe: fdDown");
        return false;
    }


   if(pipe(fdUp)) {
    perror("pipe: fdUp");
        return false;
    }

Upvotes: 1

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