Re: select() and named pipes

[Enrico Forestieri]

On Sun, Feb 14, 2010 at 08:54:27PM -0500, Christopher Faylor wrote:

> I just checked in YA in my series of attempts to get this working right.
> It will behave marginally better now but there are still problems if you
> attempt to write to a fifo before anything is reading it and then try to
> do a non-blocking read.

Thank you for your efforts. It indeed works better with the 2010-02-15
snapshot.

In order to help you pinpointing another glitch, I attach here a slightly
slightly modified version of the original test case and a shell script.
Launching the program in a terminal and then the shell script in another
one, it seems that the program hangs on the write() call.
Apparently, the problem is the O_RDWR flag, as replacing it with O_WRONLY,
everything works. The test case works with O_RDWR both in cygwin 1.5 and
linux.

-- 
Enrico

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define FIFO_IN "/tmp/pipe.in"
#define FIFO_OUT "/tmp/pipe.out"
#define ACK "Message received\n"

int main(void)
{
    int infd;
    int outfd;
    int nsel;
    fd_set readfds;
    FD_ZERO(&readfds);

    if (mkfifo(FIFO_IN, 0600) < 0 || mkfifo(FIFO_OUT, 0600) < 0) {
        perror("mkfifo");
        remove(FIFO_IN);
        remove(FIFO_OUT);
        exit(1);
    }

    infd = open(FIFO_IN, O_RDONLY | O_NONBLOCK);
    outfd = open(FIFO_OUT, O_RDWR);

    if (infd < 0 || outfd < 0) {
        perror("open");
        if (infd >= 0)
            close(infd);
        if (outfd >= 0)
            close(outfd);
        remove(FIFO_IN);
        remove(FIFO_OUT);
        exit(2);
    }

    FD_SET(infd, &readfds);
    do {
        nsel = select(infd + 1, &readfds, 0, 0, 0);
    } while (nsel == -1 && (errno == EINTR || errno == EAGAIN));

    if (nsel == -1) {
        perror("select");
        exit(3);
    }

    if (FD_ISSET(infd, &readfds)) {
        char buf[100];
        int status;
        while ((status = read(infd, buf, sizeof(buf) - 1))) {
            if (status > 0) {
                buf[status] = '\0';
                printf("%s", buf);
            } else {
                printf("\n");
                if (errno != EAGAIN)
                    perror("read");
                break;
            }
        }
    }

    if (write(outfd, ACK, strlen(ACK)) < 0)
        perror("write");

    sleep(1);
    close(infd);
    close(outfd);
    remove(FIFO_IN);
    remove(FIFO_OUT);
    return 0;
}

fifotest.sh
Description: Bourne shell script

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