exec() and pipe() between child process in C

Question

I'm trying to implement this using pipe() and fork() :

ls | wc

First I checked if pipe works fine, and here is the code.

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

int main(void){

    char *param1[]={"ls",NULL};
    char *param2[]={"wc",NULL};

    int p[2];
    pipe(p);

    pid_t process1, process2;

    if((process1=fork())==0){ // child 1
        dup2(p[1],1); // redirect stdout to pipe
        close(p[0]);
        execvp("ls", param1);
        perror("execvp ls failed");
    }
    else if(process1>0){ // parent
        wait(NULL);
    }

    if((process2=fork())==0){ // child 2
        dup2(p[0],0); // get stdin from pipe
        close(p[1]);

        char buff[1000]={0};
        read(STDIN_FILENO,buff,250);
        printf("---- in process2 -----\n%s\n",buff);
    }
    else if(process2>0){ // parent
        wait(NULL);
    }
    return 0;
}

I checked it works fine. I replaced read() and printf() to exec(), like :

if((process2=fork())==0){ // child 2
    dup2(p[0],0); // get stdin from pipe
    close(p[1]);

    execvp("wc",param2);
    perror("execvp wc failed");
}

My terminal just hang. I think wc process doesn't get any input. So my question is, why read() and printf() works, and execvp() doesn't work?

You have no error handling if the first process failed to start! Additionally, have you tried waiting for the processes after *both* of them executed (prevents the pipe running full)? — Aconcagua, Dec 12 '17 at 08:31
Just a recommendation: if you use `STDIN_FILENO` for `read`, just use these macros, too, for `dup2`... And why do you provide a buffer of size 1000 if you only use 250 bytes anyway? Would rather select buffer sizes in powers of two (1024, 256, ...). — Aconcagua, Dec 12 '17 at 08:35
@Aconcagua No I haven't. thanks. And I just wanted to check my simple pipe working, regardless of size. — wanttobepro, Dec 12 '17 at 08:39
here is a similar question possibly a duplicate https://stackoverflow.com/questions/13801175/classic-c-using-pipes-in-execvp-function-stdin-and-stdout-redirection — asio_guy, Dec 12 '17 at 08:41
Possible duplicate of [Classic C. Using pipes in execvp function, stdin and stdout redirection](https://stackoverflow.com/questions/13801175/classic-c-using-pipes-in-execvp-function-stdin-and-stdout-redirection) — asio_guy, Dec 12 '17 at 08:42
By the way, you should check the return value of [`pipe`](http://man7.org/linux/man-pages/man2/pipe.2.html), too... — Aconcagua, Dec 12 '17 at 08:52

score 1 · Accepted Answer · edited Dec 12 '17 at 08:37

1

You need not to wait every time you create a new process and also close the descriptor in the parent, something like:

if((process1=fork())==0){ // child 1
    dup2(p[1],1); // redirect stdout to pipe
    close(p[0]);
    execvp("ls", param1);
    perror("execvp ls failed");
}
else if(process1==-1){ // fork failed
    exit(1);
}
close(p[1]); // no need for writing in the parent

if((process2=fork())==0){ // child 2
    dup2(p[0],0); // get stdin from pipe

    char buff[1000]={0};
    read(STDIN_FILENO,buff,250);
    printf("---- in process2 -----\n%s\n",buff);
}
else if(process2==-1){ // second fork failed
    close(p[0]); // ensure there is no reader to the pipe
    wait(NULL); // wait for first chidren
    exit(1);
}
close(p[0]); // no need for reading in the parent

wait(NULL);
wait(NULL); // wait for the two children

edited Dec 12 '17 at 08:37

Aconcagua

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answered Dec 12 '17 at 08:34

Jean-Baptiste Yunès

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Thanks, It works. I have a question : is `close()` necessary? I can check if I didn't use `close()`, it doesn't work – wanttobepro Dec 12 '17 at 08:58
1

@wanttobepro `close()` is necessary on the input to `wc` because that is how it knows to stop counting words. – JeremyP Dec 12 '17 at 09:33
@wanttobepro As Jeremy says closing in the parent is a necessity, pipe semantic is to provide synchronization between ends, so if you don't close it may happen that a children think that there is somebody else on the other end of the pipe. – Jean-Baptiste Yunès Dec 12 '17 at 09:39

exec() and pipe() between child process in C

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