CODEWITHSUNDEEP
c++linuxbashpipe
yolob 21
yolob 21

Reputation: 396

C++ program to simulate ls|wc|wc using pipes does not display output after running successfully

The program executes, but the final output is not being displayed. I seem to be messing up the pipe lining of outputs.

Removed the checks for failure of execlp(),pipe(),fork() since they aren't contributing to the said problem.

#include "iostream"
#include "stdlib.h"
#include "unistd.h"
#include "wait.h"
#include "stdio.h"
#include "fcntl.h"
using namespace std;
int main(int argc, char **argv)
{
    int fd[2],status,status2,fd1[2];
    pipe(fd);
    pipe(fd1);
    char buf[12];
    switch(fork())
    {
        case 0:
                close(fd[0]);
                dup2(fd[1],1);
                close(fd[1]);
                execlp("ls","ls",NULL);
                exit(0);
                break;
        default:
                waitpid(-1,&status,0);
                close(fd[1]);
                close(fd1[0]);
                dup2(fd1[1],1);
                close(fd1[1]);
                dup2(fd[0],0);
                close(fd[0]);
                execlp("wc","wc",NULL);
    }

    switch(fork())
    {
        case 0:
        close(fd1[1]);
        dup2(fd1[0],0);
        close(fd1[0]);
        execlp("wc","wc",NULL); //this is not being redirected to STDOUT
        exit(0);
        break;
     default:
      waitforpid(-1,&status2,0);
    }
    return 0;
}

Upvotes: 1

Views: 856

Answers (2)

Anatoly Trosinenko
Anatoly Trosinenko

Reputation: 754

The exec family of functions does not spawn separate processes, they replaces the current process instead. As stated in the man page for execlp, execlp returns control only in case of an error.

So, in case all fork and execlp calls succeed, your program cannot reach the third invocation of execlp, since one of cases in the first switch is always executed, and either of them replaces your process.

Meanwhile, I suggest you not to parse the ls output in real life problems, since file names in UNIX can contain whitespace characters, newlines, etc., that you probably cannot parse, there are other options such as the find command that can separate file names with '\0'.

Upvotes: 1

nicoeft
nicoeft

Reputation: 58

Seems like the second fork() won't be executed since the two process will mutate in the execlp instruction to the code of the programs 'ls' and 'wc' . Check if it's that, maybe just changing the execlp instruccion in the default of the first switch to the second switch should be enough.

Upvotes: 1

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