不能减少到（1）

Question

I'm trying to write a program that should works as " printenv | sort | less ". However the last pipe to less does not work. I can pipe to cat with the expected result (environment variables are displayed) but when i pipe to less nothing is displayed. The program runs less but nothing happens. If I go with more instead the program cats the output until the bottom of the window and after that nothing happens. I'm not in a pager.

Any ideas?

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

#define WRITE 1
#define READ 0

#define DEBUG 1

int p1[2], p2[2], p3[2];

void err(char* msg){
  perror(msg);
  exit(EXIT_FAILURE);
}

void closePipe(int pipeEnd[2]){

  if (close(pipeEnd[READ]) == -1)
    err("error when closing pipe, read");

  if (close(pipeEnd[WRITE]) == -1)
    err("error when closing pipe, write");
}

int main(int argc, char** argv, char** envp){

  int pid1, pid2, pid3;


  /*Creating pipes*/
  if (pipe(p1) == -1){
    err("pipe1");
  }
  if (pipe(p2) == -1){
    err("pipe2");
  }

  /*
  if (pipe(p3) == -1){
    err("pipe3");
  }
  */

  /*First fork*/
  pid1 = fork();
  if (pid1 == -1){
    err("fork1");
  }

  /**FIRST CHILD*************PRINTENV****************/
  else if (pid1 == 0){ 

    if (dup2(p1[WRITE],STDOUT_FILENO)== -1){
      err("dup2 miss");
    }

    closePipe(p2);
    closePipe(p1);

    if (execlp("printenv", "printenv", NULL) == -1){
      err("execlp printenv does not like you!");
    }
  }

  /**PARENT*******************************************/
  else {

    pid2 = fork();
    if (pid2 == -1){
      err("fork2");
    }

    /**SECOND*CHILD***********SORT*******************/
    else if (pid2 == 0){

      if (dup2(p1[READ],STDIN_FILENO) == -1){
    err( "dup2 p1 read" );
      }
      if (dup2(p2[WRITE], STDOUT_FILENO) == -1){
    err("dup2 p2 write");
      }

      closePipe(p2);
      closePipe(p1);

      if (execlp("sort", "sort", NULL) == -1){
    err("execlp sort does not like you!");
      }
     }

    /**PARENT*****************************************/
    else {

      pid3=fork();
      if (pid3 == -1){
    err("fork3");
      }
      /**THIRD CILD***************LESS****************/
      else if (pid3 == 0){

    if (dup2(p2[READ], STDIN_FILENO) == -1){
      err("err dup2 read");
    }

    closePipe(p2);
    closePipe(p1);

    char *args4exec[] = {"less", NULL};

    if (execvp(args4exec[0], args4exec) == -1){
      err("err in exelp pager");
    }
      }
    }
  }  
  return 0;
}

Answer 1

Two things. First off, in the parent you also need to close the pipes. Second, your parent process should wait for the less process to finish before exiting itself. Try adding this to your code:

/* ... */

  closePipe(p1);
  closePipe(p2);
  waitpid(pid3, NULL, 0);

  return 0;
}

The first issue causes the child never to be able to properly receive an EOF, and the 2nd will cause less to exit right away because the parent goes away.