CODEWITHSUNDEEP
clinuxpipefork
user2041391
user2041391

Reputation: 15

Simple fork and piping

I am trying to understand forking and piping in my operating systems class. I am am not sure why the second child process will not print out the meminfo. It appears to write to the pipe if I print it out, but it will not read anything in the second child process. Am I missing something to allow the second process to read from the pipe?

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/wait.h>

#define R_FILE "/proc/meminfo"
#define BSIZE 256
#define READ_END 0
#define WRITE_END 1

int main()
{
  int status;
  pid_t pid_1, pid_2;

  int pipefd[2];
  pipe(pipefd);

  pid_1 = fork();
  if (pid_1 == 0) {
    /* process a */

    int rfd;
    size_t rsize;
    char buf[BSIZE];
    close(pipefd[READ_END]);
    if ((rfd = open(R_FILE, O_RDONLY)) < 0) {
      fprintf(stderr, "\nError opening file: %s. ERROR#%d\n", R_FILE, errno);
      return EXIT_FAILURE;
    }

    /* read contents of file and write it out to a pipe */
    while ((rsize = read(rfd, buf, BSIZE)) > 0) {
      /* This should write to a pipe */
      write(pipefd[1], buf, rsize);
    }

    close(pipefd[WRITE_END]);
    close(rfd);
    return 0;
  }

  pid_2 = fork();
  if (pid_2 == 0) {
    /* process b */
    size_t rsize;
    char buf[BSIZE];
    /* read from pipe and write out contents to the terminal */
    close(pipefd[WRITE_END]);
    /*  Thishis should read from a pipe */

      while ((rsize = read(pipefd[1], buf,BSIZE)) > 0) {
        write(STDOUT_FILENO, buf, rsize);

         }

    close(pipefd[READ_END]);

    return 0;
  }

  /* shell process */
  if ((waitpid(pid_1, &status, 0)) == -1) {
    fprintf(stderr, "Process 1 encountered an error. ERROR%d", errno);
    return EXIT_FAILURE;
  }

  if ((waitpid(pid_2, &status, 0)) == -1) {
    fprintf(stderr, "Process 2 encountered an error. ERROR%d", errno);
    return EXIT_FAILURE;
  }

  return 0;
}

Upvotes: 0

Views: 84

Answers (1)

myaut
myaut

Reputation: 11494

In process b you probably reading from wrong fd read(pipefd[1], buf,BSIZE). Replace it with read(pipefd[READ_END], buf,BSIZE).

Upvotes: 1

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