Reading from pipes

(regarding the attached program segment, below)

> Is it ok to read an empty pipe?  It seems that when I try to read from
> an empty pipe, instead of getting -1 or 0 returned, the program halts.
> Can you tell me why?

Your program is halting because read() is, by default, a blocking system
call.  Since you are reading from a pipe before anything has been written
to it, the read() call will wait indefinitely.

Try reversing the order of the read() and write() to see what happens.

- Jeremy

/* program.c */
#define MSG "hello, world"
#define MSG_LEN 13  /* number of characters plus 1 */

int main () {
        int pfd[2];     /* pipe file descriptors */
        char inbuf[MSG_LEN], outbuf[MSG_LEN];
        int nbytes;

        if (-1 == pipe(pfd)) {
                perror ("pipe");
                exit(1);
        }

        nbytes = read (pfd[0], inbuf, MSG_LEN);
        if (MSG_LEN != nbytes) {
                perror ("read did not receive message properly");
                exit(1);
        }

        printf ("sent message to myself: %s\n", inbuf);

        strcpy (outbuf, MSG, MSG_LEN);
        nbytes = write (pfd[1], outbuf, MSG_LEN);
        if (MSG_LEN != nbytes) {
                perror ("write did not send message properly");
                exit(1);
        }

        close (pfd[0]);
        close (pfd[1]);
        return 0;
}