Wsrtjtyk

I have been trying to write some code that redirects writes to STDOUT_FILENO, via write(1, line, strlen(line)) or printf(), to the STDIN_FILENO of another process. The other process will be /usr/bin/less. Nothing seems to work, despite trying quite a few attempts on this site, a lot of man page reading and trying every combination of close() and dup2().

Appreciate your help, thanks.

#include  <fcntl.h>

#include  <stdio.h>

#include  <stdlib.h>

#include  <string.h>

#include  <sys/types.h>

#include  <sys/wait.h>

#include  <sys/stat.h>

#include  <termios.h>

#include  <unistd.h>

#define INPUT_END 1

#define OUTPUT_END 0

int main(int argc, char* argv){

   pid_t pid1;

   pid_t pid2;

   int fd1[2], fd2[2];

   int x;

   pipe(fd1);

   pipe(fd2);

   pid1 = fork();



   if (pid1 == -1) {

      fprintf(stderr, "pid errorn");

      exit(1);

   }



   if(pid1 == 0) {

      close(fd1[INPUT_END]);

      dup2(fd1[OUTPUT_END], STDIN_FILENO);

      close(fd1[OUTPUT_END]);

      close(fd2[OUTPUT_END]);

      dup2(fd2[INPUT_END], STDOUT_FILENO);

      close(fd2[INPUT_END]);

      execlp("less", "-r",(char*) NULL);



   }else {

      close(fd1[OUTPUT_END]);

      dup2(fd1[INPUT_END], STDOUT_FILENO);

      close(fd1[INPUT_END]);

      close(fd2[INPUT_END]);

      dup2(fd2[OUTPUT_END], STDIN_FILENO);

      close(fd2[OUTPUT_END]);



      for(x=0;x<100;x++) {

         write(STDOUT_FILENO, "AAAn", 4);

         printf("AAAn");

         fflush(stdout);

      }



      close(fd1[OUTPUT_END]);

      close(fd1[INPUT_END]);

      close(fd2[OUTPUT_END]);

      close(fd2[INPUT_END]);

      waitpid(-1, NULL, 0);

   }

}

Your program does exactly what you programmed it to do: It creates two pipes and a child running less, using the pipes to redirect stdout of the program to stdin of less AND stdout of less to stdin of the program. It then writes a bunch of data (which less will copy to its stdout -- stdin of the program) and the waits for less to exit. But that will never happen, as less is waiting for more input.

If you want things to stop, you'll need to close(STDOUT_FILENO) before the call to waitpid -- if you add this, less will exit, and the waitpid will return and your program will then proceed (and exit). You still won't see anything on your terminal, as nothing is ever being written to the terminal -- less is writing to stdin of your program (which you are never reading).

If you increase the amount of data being written, you will see a deadlock -- as you are never reading stdin, once the fd2 pipe fills up, less will block writing, and then once fd1 also fills up, your program will block. The default pipe sizes on Linux are quite large, however, so this will take a lot of data.

Note that you could just as well use cat as less -- less essentially devolves to cat when stdout is not a terminal.

If what you want is to output data to your terminal, using less to mediate that (stopping after a page, etc), then you need to leave stdout of less pointing at your terminal. Simply delete fd2 and every line referring to fd2 from your above code, and it should work -- though it will still hang at the end if you don't close(STDOUT_FILENO)