Linux终端命令与C代码中的管道

Question

I'm trying to execute the Linux command "ls -l | tail -n 2" with a simple pipe in ac code.

I added your tips and now this works but the output isn't exactly as it should be. It prints the output in a single line instead of two and waits for a user input to close. here is the new code:

#include "stdio.h"
#include "unistd.h"
#include "stdlib.h"
#include "sys/wait.h"
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

void main()
{
    char line[100];
    pid_t pid;
    int fd[2];
    int status;
    char* ls_arguments[] = {"ls", "-l", NULL};
    char* tail_arguments[] = {"tail", "-n", "2", NULL};
    pipe(fd);
    pid = fork();
    if(pid == 0)//ls client
    {
        close(1);
        dup(fd[1]);
        close(fd[0]);
        execvp("ls", ls_arguments);
    }
    pid = fork();
    if(pid == 0)//tail client
    {
        close(0);
    close(fd[1]);
        dup(fd[0]);
        execvp("tail", tail_arguments);
    }
    wait(pid, 0, WNOHANG);
    close(fd[0]);
    close(fd[1]);
}

this should run the "ls -l" command and output to the pipe and the next "tail" client would get it as input and run the "tail -n 2" command and print out the final output but the terminal prints nothing. Any help?

Answer 1

First of all, there is not such wait function, here is what the man says:

#include <sys/types.h>
#include <sys/wait.h>

pid_t wait(int *status);

pid_t waitpid(pid_t pid, int *status, int options);

I think you meant to use waitpid .

Then, you child process doesn't finish because the pipe is still opened somewhere: in the parent. Indeed you should first close the descriptors and then wait for your childs process. I would write:

  close(fd[0]);
  close(fd[1]);
  wait(NULL); // Wait for the first child to finish
  wait(NULL); // Wait fot the second one
  return 0;
}

Instead of:

  wait(pid, 0, WNOHANG);
  close(fd[0]);
  close(fd[1]);
}