Is it possible to write to an open pipe (from a C program) from the command line?

Question

Say I have a C program which creates a child throught fork . The parent processus ends before the child ends, and the child is blocked on a read system call because of an empty pipe.

If the processus ID of the child is 1500 , and by using the shell command ls -l /proc/1500/fd I see that a pipe is open, is it possible to write to this pipe from the terminal using a shell command so that the read system call unblocks and the child process finish its execution?

Answer 1

Been there done that.

Sh answer:

cat whatever >> /proc/pid/fd/0

C answer:

pid_t pid = whatever;
char buf[30];
sprintf(buf, "/proc/%d/fd/0", pid);
FILE *f = fopen(buf, "w");

But you appear to have the problem of have left the writing half of the pipe open in the child process. You should close it (see man 2 close ) it immediately after your fork() call that way the reader doesn't get stuck but can observe the end of pipe. The launcher should normally look something like this.

    int pipefd[2];
    pid_t pid;
    pipe(pipefd);

    if ((pid = fork()) == 0) {
        dup2(pipefd[0], 0);
        close(pipefd[0]);
        close(pipefd[1]);
        /* ... */
        /* usually this goes to exec but it doesn't have to */
        _exit(3);
    }
    close(pipefd[0]);
    if (pid < 0) {
        close(pipefd[1]);
        return ;
    }
    int pipefeed = pipefd[0];
    /* ... */
    /* I'm guessing in your case you don't wait() */