输入重定向和管道

Question

I understand piping where you want to run a command like ls -l | wc -l:

int pipes[2];
pipe(pipes); 

if (fork() == 0){ //first fork
  dup2(pipes[1],1); 
  close(pipes[0]);
  close(pipes[1]);

  execvp(arr1[0], arr1); //arr1[0] = "ls" and  arr1[1] = "-l" and arr1[2] = 0
  perror("Ex failed");
  exit(1);
}

if (fork() == 0){ //2nd fork
  close(pipes[1]);
  dup2(pipes[0],0); 
  close(pipes[0]);

  execvp(arr2[0], arr2); //arr2[0] = "wc" and  arr2[1] = "-l" and arr2[2] = 0
  perror("Ex failed");
  exit(1);
}

But, how would you include input and output redirection? lets say I want to cat < foo.txt | wc -l

I understand the first fork needs to be modified, but I don't understand what is needed (another dup2()?). I would greatly appreciate some help.

Thanks.

Answer 1

But, how would you include input and output redirection? lets say I want to cat < foo.txt | wc -l

In the case of input redirection, you open the file for reading, and then use dup2(2) to duplicate the file descriptor into standard input. The file descriptor of stdin is STDIN_FILENO , defined in unistd.h . So, something like this:

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

// ...

filename = "foo.txt";
int fd;
if ((fd = open(filename, O_RDONLY)) == -1) {
    perror("open %s: %s\n", filename, strerror(errno));
    exit(EXIT_FAILURE);
}
if (dup2(fd, STDIN_FILENO) == -1) {
    perror("dup2: %s -> stdin: %s\n", filename, strerror(errno));
    exit(EXIT_FAILURE);
}
if (close(fd) == -1) {
    perror("close %s: %s\n", filename, strerror(errno));
    exit(EXIT_FAILURE);
}

// Now, reading from stdin will read from the file.
// Do the normal pipe operations here.

int pipes[2];
// ...

Note that your code doesn't have error handling - not one - this is very bad because when something goes wrong, you will ignore it and the code will crash in mysterious ways. Pretty much every function you called can return with an error; considering handling the error to clearly show to the user what went wrong where.