linux - Implementing pipe in C

Question

I am trying to implement pipe in C. eg - $ ls | wc | wc

I have written the following code -

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

void run_cmd(char *cmd, int* fd_in, int* fd_out)
{
    int c = fork();

    if (c==0)
    {
        if (fd_in != NULL)
        {
            close(fd_in[1]);
            dup2(fd_in[0], 0);
        }
        if (fd_out != NULL)
        {
            close(fd_out[0]);
            dup2(fd_out[1],1);
        }
        execlp(cmd, cmd, NULL);
    }
}

int main(int argc, char **argv)
{
    int fd_1[2], fd_2[2], i;
    pipe(fd_1);
    pipe(fd_2);

    run_cmd(argv[1], NULL, fd_1);

    for( i=2; i<argc-1; i++)
    {
        if (i%2 == 0)
            run_cmd(argv[i], fd_1, fd_2);
        else
            run_cmd(argv[i], fd_2, fd_1);
    }
    if (i%2 == 0)
        run_cmd(argv[i], fd_1, NULL);
    else
        run_cmd(argv[i], fd_2, NULL);
}

This works fine with two arguments, eg - $./a.out ls wc

But when I try with more than two arguments it does not work.

Would anyone please tell me what's wrong with my code, or any other way to do this?

Answer 1

This does virtually no error checking, but why so complicated?

int main (int argc, char ** argv) {
    int i;

    for( i=1; i<argc-1; i++)
    {
        int pd[2];
        pipe(pd);

        if (!fork()) {
            dup2(pd[1], 1); // remap output back to parent
            execlp(argv[i], argv[i], NULL);
            perror("exec");
            abort();
        }

        // remap output from previous child to input
        dup2(pd[0], 0);
        close(pd[1]);
    }

    execlp(argv[i], argv[i], NULL);
    perror("exec");
    abort();
}