Question

I’m trying to create a two-way communication between parent and child processes using 2 pipes using C on Linux. The parent is my program and the child is just a random program (say “cat”).

I try to uses read() in parent to read child output, but it gives me errno 9, which is Bad file descriptor.

The following is my code

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define Read            0
#define Write           1
#define ParentRead      read_pipe[1]
#define ParentWrite     write_pipe[0]
#define ChildRead       write_pipe[1]
#define ChildWrite      read_pipe[0]

int main()
{
    int data_processed;

    /** Pipe for reading for subprocess */
    int read_pipe[2];
    /** Pipe for writing to subprocess */
    int write_pipe[2];

    char buffer[100];
    memset(buffer, '\0', 100);

    if (pipe(read_pipe) == 0 && pipe(write_pipe) == 0)
    {
        pid_t pid = fork();
        if (pid == (pid_t)-1)
        {
            fprintf(stderr, "Fork failure");
            exit(EXIT_FAILURE);
        }
        else if (pid == (pid_t)0) //Child process
        {
            close(Read);
            close(Write);
            close(ParentRead);
            close(ParentWrite);
            dup(ChildRead);
            dup(ChildWrite);
            execlp("cat", (char*)NULL);
            exit(EXIT_FAILURE);
        }
        else { //Parent process
            close(ChildRead);
            close(ChildWrite);

            write(ParentWrite, "abc", 3);
            int r = read(ParentRead, buffer, 99);
            printf("%d %d", r, errno);
            puts(buffer);
        }
    }

    exit(EXIT_SUCCESS);
}

Answer 1

If you want to redirect stdin and stdout to pipes, you need to use dup2(2) system call.

dup2 (ChildRead, 0);
dup2 (ChildWrite, 1);

P.S.
Also I found wrong directions of reading/writing in pipes. Here is the correct way

#define ParentRead      read_pipe[0]
#define ParentWrite     write_pipe[1]
#define ChildRead       write_pipe[0]
#define ChildWrite      read_pipe[1]

Remember: pipe[0] is fd for reading, pipe[1] is fd for writing.

And one more error, in execlp. Do not forget to set the first argument you send to the executed programm as a name of the program

execlp("cat", "cat", (char*)NULL);