Check if unix pipe closed without writing anything?

Question

Basically I have a parent process that forks a child and feeds it it's stdin through a pipe. The child process can terminate in one of two cases:

• the write end of the pipe is closed by the parent, meaning it reached the end of stdin thus receiving an EOF,
• or it receives a certain input through the pipe(-1 in this case) and exits

My parent code looks roughly like this:

close(pi[0]); // close input end
signal(SIGPIPE, SIG_IGN); // do not handle SIGPIPE
char buffer;
int ok = 1;
while(ok && read(STDIN_FILENO, &buffer, 1) > 0)  {
    int b_written = write(pi[1], &buffer, 1);
    if(b_written == -1) {
        if(errno == EPIPE) ok = 0;
        else perror("pipe write"); // some other error
    }
}

As you can see, I check whether the read end of a pipe is closed by checking for errno == EPIPE. However this means that the read loop does one extra iteration before closing. How could I possibly poll to see if the pipe is closed without necessarily writing something to it?

Answer 1

This snippet will check if the other end of a writable pipe is closed using poll(2). This works on Linux -- I'm not sure about other OSes or what POSIX says.

#include <poll.h>
#include <stdbool.h>
#include <stdio.h>
#include <unistd.h>

bool is_pipe_closed(int fd) {
    struct pollfd pfd = {
        .fd = fd,
        .events = POLLOUT,
    };

    if (poll(&pfd, 1, 1) < 0) {
        return false;
    }

    return pfd.revents & POLLERR;
}

Answer 2

The child could send a signal, such as SIGUSR1 when it detects it has finished. Parent could set a flag to when it receives SIGUSR1 signal, and check this flag before trying to read input. But I am not absolutely sure SIGUSR1 could not be received after checking the flag ans before reading input from stdin). So I prefer to use a control pipe, each time child know it will be able to read one more data it write a 1 in this control pipe. The result could be something like that:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>

#define STOP_VALUE 100
#define SIZE_STDIN_BUFFER 1024

static char can_read_more = 1;

static int handle_child(int *p_child_input_stream, int *p_control_stream)
{
    int pipefd[2][2];
    pid_t fk;

    if (pipe(pipefd[0]) < 0) // Pipe to read input from 
    {
        perror("pipe");
        return -1;
    }

    if (pipe(pipefd[1]) < 0) // Pipe to notifiate parent input can be processed
    {
        perror("pipe");
        close(pipefd[0][0]);
        close(pipefd[0][1]);
        return -1;
    }

    if ((fk = fork()) < 0)
    {
        perror("fork");
        close(pipefd[0][0]);
        close(pipefd[0][1]);
        close(pipefd[1][0]);
        close(pipefd[1][1]);
        return -1;
    }

    if (fk == 0)
    {
        close(pipefd[0][1]);
        close(pipefd[1][0]);
        write(pipefd[1][1], &can_read_more, sizeof(char)); // sizeof(char) == 1

        ssize_t nb_read = 0;
        char buffer;
        while (nb_read >= 0)
        {
            nb_read = read(pipefd[0][0], &buffer, sizeof(char));
            if (nb_read > 0)
            {
                printf("0x%02x\n", (unsigned int) buffer);
                if (buffer == STOP_VALUE)
                {
                    nb_read = -1;
                }
                else
                {
                    write(pipefd[1][1], &can_read_more, sizeof(char));
                }
            }
        }
        close(pipefd[0][0]);
        close(pipefd[1][1]);
        exit(0);
    }

    close(pipefd[0][0]);
    close(pipefd[1][1]);

    *p_child_input_stream = pipefd[0][1];
    *p_control_stream = pipefd[1][0];

    return 0;
}

int main()
{
    int child_input_stream;
    int control_stream;

    if (handle_child(&child_input_stream, &control_stream) < 0)
    {
        return 1;
    }

    char stdin_buffer[SIZE_STDIN_BUFFER];
    char buffer;
    int ok = 1;
    int child_available_input = 0;

    while(ok)
    {
        while (child_available_input <= 0 && ok)
        {
            ssize_t nb_control = read(control_stream, &buffer, sizeof(char));
            if (nb_control > 0)
            {
                child_available_input += buffer;
            }
            else
            {
                fprintf(stderr, "End of child reading its input detected.\n");
                ok = 0;
            }
        }

        if (ok)
        {
            if (fgets(stdin_buffer, SIZE_STDIN_BUFFER, stdin) == NULL)
            {
                ok = 0;
            }
            else
            {
                if (stdin_buffer[strlen(stdin_buffer) - 1] == '\n')
                {
                    stdin_buffer[strlen(stdin_buffer) - 1] = '\0';
                }

                char dummy;
                int input;
                if (sscanf(stdin_buffer, "%d%c", &input, &dummy) == 1)
                {
                    buffer = (char) input;
                    write(child_input_stream, &buffer, sizeof(char));
                    child_available_input--;
                }
            }
        }
    }

    return 0;
}