Question

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

int main(int argc,char **argv)
{
    int fd[2];
    pid_t childpid;
    pipe(fd);
    childpid=fork();
    if (childpid == -1)
    {
        perror("Error forking...");
        exit(1);
    }
    if (childpid)   /*parent proces*/   //grep .c
    {
        wait(&childpid);        //waits till the child send output to pipe
        close(fd[1]);
        close(0);       //stdin closed
        dup2(fd[0],0);
        execlp(argv[2],argv[2],argv[3],NULL);

    }
    if (childpid==0)  //ls
    {
        close(fd[0]);   /*Closes read side of pipe*/
        close(1);       //STDOUT closed
        dup2(fd[1],1);
        execl(argv[1],NULL);
    }
    return 0;
}

If i give command line argument as “ls grep .c” i should get all the “.c” files displayed.

Pseudocode:-
My child process will run “ls” & parent process will run “grep .c”..
Parent process waits till the child process completes so that child writes to the pipe.

Test run:-

bash-3.1$ ls | grep .c
1.c
hello.c
bash-3.1$ ./a.out ls grep .c
bash-3.1$

Why is that happening?

Answer 1

A simple mistake: your execl call should actually be execlp. Besides, you can get rid of the wait and close statements. Then you should check the error code of execlp.