Question

Two way piped process

Asked by: k7t266

Hi,

I have some problems with my program. I need to run a program from another(i'm using fork and execlp). I need to redireting standart input and output of the child cause i can't modify it. I can modify the input or the output with a dup2 on a file descriptor created by pipe(). But i cant modify the input and the output in the same time: the program is blocked. I'm using two file descriptor: one for parent->child and the other for child->parent. First i write a number to the child who "scanf" it and after it returns me the double by a printf that i read in the parent.

Here the code(its a trying program):

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

int
main (void)
{int fd[2],fd2[2];
 int pid;    
 char buff[5];
 pipe(fd);
 pipe(fd2);
 pid=fork();
 strcpy(buff,"     ");
 if(pid==0)
      {dup2(fd2[1],1);
      dup2(fd[0],0);
      close(fd[1]);
      close(fd2[0]);
      execlp("enfant","enfant",0);}
 else
      {close(fd2[1]);
      close(fd[0]);
      write(fd[1],"12",2);
      read(fd2[0],buff,5);
      printf("s:%s\n",buff);
         }      
}

enfant:

#include<stdio.h>

int main(void){long i;
     char buff[9];
     scanf("%s",buff);
     buff[8]='\0';
     i=atol(buff)*2;
     printf("c:%d\n ",i);
     }
Please gimme an answer i need this to succeed in my year im a student!

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Asked On
2002-11-25 at 02:55:18ID20407047
Tags

dup2

,

execlp

,

pipe

Topics

Programming Languages

,

Shell Scripting

Participating Experts
3
Points
50
Comments
6

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Answers

 

by: zebadaPosted on 2002-11-25 at 10:31:24ID: 7495142

Try this...

int pfd1[2],pfd2[2];
...
pipe(pfd1);
pipe(pfd2);
...
if ( ( pid=fork())==0 ) {
  /* Duplicate STDIN to pipe */
  close(0);
  dup2(pfd1[0],0);
  close(pfd1[0]);
  close(pfd1[1]);
  /* Duplicate STDOUT to pipe */
  close(1);
  dup2(pfd2[1],1);
  close(pfd2[0]);
  close(pfd2[1]);
}

 

by: zebadaPosted on 2002-11-25 at 10:41:26ID: 7495191

Missed a few lines of code...

int pfd1[2],pfd2[2];
char buf[5];
...
pipe(pfd1);
pipe(pfd2);
...
if ( ( pid=fork())==0 ) {
  // Child code
  // Duplicate STDIN to pipe
  close(0);
  dup2(pfd1[0],0);
  close(pfd1[0]);
  close(pfd1[1]);
  // Duplicate STDOUT to pipe
  close(1);
  dup2(pfd2[1],1);
  close(pfd2[0]);
  close(pfd2[1]);
  // Execute process
  execlp("enfant","enfant",0);}
  _exit(0);
}

// Parent code...
// Close the "other" ends of the pipes - they are not needed.
close(pfd1[0]);
close(pfd2[1]);

// Write to child
write(pfd1[1],"12",2);
// Read from child
read(pfd2[0],buf,5);
...
// When child is finished
// Close the pipes.
close(pfd1[1]);
close(pfd2[0]);


 

by: ozoPosted on 2002-11-26 at 12:23:04ID: 7500168

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

int
main (void)
{int fd[2],fd2[2];
int pid;
char buff[5];
pipe(fd);
pipe(fd2);
pid=fork();
strcpy(buff,"     ");
if(pid==0)
     {dup2(fd2[1],1);
     dup2(fd[0],0);
     close(fd[1]);
     close(fd2[0]);
     execlp("./enfant","enfant",0);}
else
     {close(fd2[1]);
     close(fd[0]);
     write(fd[1],"12",2);
     close(fd[1]);
     read(fd2[0],buff,5);
     printf("s:%s\n",buff);
     }
}

 

by: CleanupPingPosted on 2003-10-02 at 03:31:09ID: 9474958

k7t266:
This old question needs to be finalized -- accept an answer, split points, or get a refund.  For information on your options, please click here-> http:/help/closing.jsp#1
EXPERTS:
Post your closing recommendations!  No comment means you don't care.

20120131-EE-VQP-002

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