forkpty/select timeout issue

12 Dec 2014

      I have some small code that calls forkpty, then in the child writes to stdout, and in the parent waits with select() for data to be available and then reads it all.

When testing on a 10.8 virtual machine and running the code repeatedly, I can get the select() to timeout about every other time I run the shell script. Also, sometimes the data is not read in the parent at all, read() just returns 0.

Here is the code:

===================

#include <stdio.h>
#include <util.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <assert.h>

int main()
{
    int fd;
    pid_t pid = forkpty(&fd, NULL, NULL, NULL);
    if (pid == -1) {
        printf("forkpty failed: %s\n", strerror(errno));
        return -1;
    } else if (pid == 0) {
        if (write(STDOUT_FILENO, "hello", 5) != 5) {
            _Exit(1);
        }
        _Exit(0);
    }

    // parent
    int flags = fcntl(fd, F_GETFL, 0);
    assert(flags != -1);
    assert(fcntl(fd, F_SETFL, flags | O_NONBLOCK) == 0);

    char output[1024];
    bzero(output, sizeof(output));

    for (;;) {
        // Wait for data to become available
        fd_set readfd;
        FD_ZERO(&readfd);
        FD_SET(fd, &readfd);
        struct timeval tv;
        tv.tv_sec = 1;
        tv.tv_usec = 0;
        printf("select...\n");
        int sel = select(fd+1, &readfd, NULL, NULL, &tv);
        if (sel == -1) {
            if (errno == EINTR) {
                printf("select EINTR\n");
                continue;
            }
            printf("select failed: %s\n", strerror(errno));
            return 1;
        } else if (sel == 0) {
            printf("select timed out\n");
            return 1;
        }
        // good
        assert(FD_ISSET(fd, &readfd));

        char foo[1024];
        bzero(foo, sizeof(foo));
        ssize_t num;
        for (;;) {
            num = read(fd, foo, sizeof(foo)-1);
            if (num == -1 && errno == EINTR) {
                printf("read EINTR\n");
                continue;
            }
            break;
        }
        printf("read: %d\n", (int)num);
        assert(num <= 5);
        if (num == -1) {
            printf("read failed: %s\n", strerror(errno));
            return 1;
        }
        if (num == 0) {
            // done
            break;
        }
        foo[num] = 0;
        strcat(output, foo);
    }

    printf("output: ->%s<-\n", output);
    assert(strcmp(output, "hello") == 0);

    int status;
    assert(waitpid(pid, &status, 0) > 0);
    assert(close(fd) == 0);
    assert(WIFEXITED(status) == 1);
    assert(WEXITSTATUS(status) == 0);

    return 0;
}

===================

Compile: cc test.c -o test

Here is the shell script to run repeatedly:

===================

set -e
for i in `seq 200`; do
  echo $i
  ./test
done

===================

This was stripped down from a larger code base to try and figure out what’s going on, but I don’t understand why it occasionally times out. I also reproduced on a 10.9 VM. I have reproduced the timeout on my native 10.10 system, but it is more rare (possibly due to being faster and more cores?).

Is there something I’m missing? I’ve been studying a lot of example code and cannot find anything that I should be doing extra.

Any help would be appreciated!

Kevin

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forkpty/select timeout issue

Kevin Wojniak