C: Stuck with dup2 () :-(

Question

I prepared a program that emulates the shell (cmd) interface using pipes. There are two versions of the program: 1. Using one pipe (using a channel from parent to child message) 2. Using a double pipe (using two channels from parent to child and from child to parent for communication).

So, the first program provides the desired interface and works the way I want, but I cannot achieve the same result (interface) in the second program (using dup2 () and similar).

So, I give you the help and put both codes below.

BS: You can compile and try both programs in the same way using the following commands:

$ gcc prog1.c -o prog1

Next, run:

$. / prog1

Then start a new terminal and try writing some data to the input.txt file:

$ echo pwd> input.txt

And then look at the result in the first terminal.

(This works fine for the first program, but I need this working interface to be the same interface in the second program)

FIRST PROGRAM CODE (WORK LESSON):

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>

void do_child(int data_pipe[]) {
    int c;
    int rc;
    close(data_pipe[1]);

    dup2(data_pipe[0], 0); /* This string provides the desired interface of the program */

    char* cmd[] = { "bash", (char *)0 };
    execvp("bash", cmd);

    while ((rc = read(data_pipe[0], &c, 1)) > 0) 
    {
        putchar(c);
    }
    exit(0);
}

void do_parent(int data_pipe[])
{
    int c;
    int rc;
    FILE *in;

    close(data_pipe[0]);

    while (1)
    {
        in = fopen("input.txt", "r");
        while ((c = fgetc(in)) > 0) 
        {
            rc = write(data_pipe[1], &c, 1);
            if (rc == -1) 
            {
                perror("Parent: write");
                close(data_pipe[1]);
                exit(1);
            }
        }
        fclose(in);
    }
    close(data_pipe[1]);
    exit(0);
}

int main(int argc, char* argv[])
{
    int data_pipe[2];
    int pid;
    int rc;

    umask(0);
    mknod("input.txt", S_IFIFO|0666, 0);

    rc = pipe(data_pipe);
    if (rc == -1) 
    {
        perror("pipe");
        exit(1);
    }
    pid = fork();
    switch (pid) 
    {
    case -1:
        perror("fork");
        exit(1);
    case 0:
        do_child(data_pipe);
    default:
        do_parent(data_pipe);
    }
    return 0;
}

SECOND PROGRAM CODE (NEEDED TO CORRECT LITTLE BIT):

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>

/* Original version got from http://www.iakovlev.org */

int parent_to_child[2];
int child_to_parent[2];

void do_parent()
{
    int c;
    char ch;
    int rc;
    FILE *in;

    close(child_to_parent[1]); /* we don't need to write to this pipe.  */
    close(parent_to_child[0]); /* we don't need to read from this pipe. */

    while (1)
    {
        in = fopen("input.txt", "r");
        while ((c = fgetc(in)) > 0) {
            ch = (char)c;
            /* write to child */
            rc = write(parent_to_child[1], &ch, 1);
            if (rc == -1) {
                perror("child: write");
                close(child_to_parent[0]);
                close(parent_to_child[1]);
                exit(1);
            }
            /* read back from child */
            rc = read(child_to_parent[0], &ch, 1);
            c = (int)ch;
            if (rc <= 0) {
                perror("parent: read");
                close(child_to_parent[0]);
                close(parent_to_child[1]);
                exit(1);
            }
            putchar(c);
        }
        fclose(in);
    }
    close(child_to_parent[0]);
    close(parent_to_child[1]);
    exit(0);
}

void do_child()
{
    int c;
    char ch;
    int rc;

    close(parent_to_child[1]); /* we don't need to write to this pipe.  */
    close(child_to_parent[0]); /* we don't need to read from this pipe. */

    //dup2(parent_to_child[0], STDIN_FILENO);
    //dup2(child_to_parent[1], STDOUT_FILENO);

    /* Some dup2() routines must be added here 
    to get this working as the first program above */

    char* cmd[] = { "bash", (char *)0 };
    execvp("bash", cmd);

    while (read(parent_to_child[0], &ch, 1) > 0) {
        c = (int)ch;
        ch = (char)c;
        putchar(ch);
        rc = write(child_to_parent[1], &ch, 1);
        if (rc == -1) {
            perror("child: write");
            close(parent_to_child[0]);
            close(child_to_parent[1]);
            exit(1);
        }
    }
    close(parent_to_child[0]);
    close(child_to_parent[1]);
    exit(0);
}

int main(int argc, char* argv[])
{
    int pid;
    int rc;

    umask(0);
    mknod("input.txt", S_IFIFO|0666, 0);    

    rc = pipe(parent_to_child);
    if (rc == -1) {
        perror("main: pipe parent_to_child");
        exit(1);
    }

    rc = pipe(child_to_parent);
    if (rc == -1) {
        perror("main: pipe child_to_parent");
        exit(1);
    }

    pid = fork();
    switch (pid) {
    case -1:
        perror("main: fork");
        exit(1);
    case 0:
        do_child();
    default:
        do_parent();
    }
    return 0;
}

Answer 1

The main difference is here:

    while ((c = fgetc(in)) > 0) {
        ch = (char)c;
        /* write to child */
        rc = write(parent_to_child[1], &ch, 1);
        /* .... */
        /* read back from child */
        rc = read(child_to_parent[0], &ch, 1);
        /* .... */
        putchar(c);
    }

/ , , read(). (bash ) . , .

, (), , - . O_NONBLOCK child_to_parent [0] fcntl (F_SETFL), errno == EAGAIN, read(). , .

Edit1.. BTW. : do_parent() poll() child_to_parent[0] in, - (read() ), () .

Answer 2

, .

, do_parent:

void do_parent()
{
    int c;
    char ch;
    int rc;
    FILE *in;

    struct pollfd fds[2];
    int pol_ret;

    fds[0].fd = child_to_parent[0];

    close(child_to_parent[1]); /* we don't need to write to this pipe.  */
    close(parent_to_child[0]); /* we don't need to read from this pipe. */

    while (1)
    {   
        in = fopen("input.txt", "r");
        fds[1].fd = fileno(in);
        pol_ret = poll(fds, 2, 500);

        while ((c = fgetc(in)) > 0) {
            ch = (char)c;
            /* write to child */
            rc = write(parent_to_child[1], &ch, 1);
            if (rc == -1) {
                perror("child: write");
                close(child_to_parent[0]);
                close(parent_to_child[1]);
                exit(1);
            }
            /* read back from child */
            if (fds[0].revents & POLLIN)
            {
                rc = read(child_to_parent[0], &ch, 1);
                c = (int)ch;
                if (rc <= 0) {
                    perror("parent: read");
                    close(child_to_parent[0]);
                    close(parent_to_child[1]);
                    exit(1);
                }
                putchar(c);
            }
        }
        fclose(in);
    }
    close(child_to_parent[0]);
    close(parent_to_child[1]);
    exit(0);
}

do_child():

dup2(parent_to_child[0], STDIN_FILENO);