LinuxQuestions.org

- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - pipes (https://www.linuxquestions.org/questions/programming-9/pipes-439432/)

hi all

I am trying to be much familier with pipes

I tried to write a c program that generates the following output

child will print the 0th character and send it to parent
a : from child .. waiting for parent to print same character
a : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
b : from child .. waiting for parent to print same character
b : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
c : from child .. waiting for parent to print same character
c : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
d : from child .. waiting for parent to print same character
d : from parent .. waiting for next character from child
child will print the 0th character and send it to parent
e : from child .. waiting for parent to print same character
e : from parent .. waiting for next character from child

but what I get was this :(
child will print the 0th character and send it to parent
a : from child .. waiting for parent to print same character
a : from parent .. waiting for next character from child

and here is my code

Code:

#include<stdio.h>

#include<string.h>

int main (void)

{

        int fd[2],nbytes,i;

        int childpid;

        char  *f1[5]={"a","b","c","d","e"};

        char f2[2];



        pipe(fd);

  

        childpid=fork();

        

        if(childpid==0)//start child process

        {



            for(i=0;i<5;i++)

            {

            

                printf("child will print the %dth  character and send it to parent\n",i);

                printf("%s : from child .. waiting for parent to print same character\n",f1[i]);



                  

                

                close(fd[0]);



                write(fd[1],f1[i],(strlen(f1[i])+1));

                

                exit(0);

              

            }

        }





        else

        {

                wait(NULL);



                close(fd[1]);



                nbytes=read(fd[0],f2,sizeof(f2));



                printf("%s : from parent .. waiting for next character from child\n",f2);

                

        }



        return 0;



}

I need your help to correct my code and thanx ;)

Likely this is because you put an exit(0) inside the for loop.

The biggest problem I see in the code is that you don't get an info when parent prints a character (the child gets no notification). Pipe is one-way. You can use another one for notifications. Without the second pipe (or any other mechanism with the same result) all you can get is something like:

Code:

child will print the 0th  character and send it to parent

a : from child .. waiting for parent to print same character

child will print the 1th  character and send it to parent

b : from child .. waiting for parent to print same character

child will print the 2th  character and send it to parent

c : from child .. waiting for parent to print same character

child will print the 3th  character and send it to parent

d : from child .. waiting for parent to print same character

child will print the 4th  character and send it to parent

e : from child .. waiting for parent to print same character

a : from parent .. waiting for next character from child

b : from parent .. waiting for next character from child

c : from parent .. waiting for next character from child

d : from parent .. waiting for next character from child

e : from parent .. waiting for next character from child

I have modified your code - removed exit(), added loop in child code.

Code:

#include<stdio.h>

#include<string.h>

int main (void)

{

  int fd[2],nbytes,i;

  int childpid;

  char  *f1[5]={"a","b","c","d","e"};

  char f2[2];



  pipe(fd);



  childpid=fork();



  if(childpid==0)//start child process

  {

    close(fd[0]);

    for(i=0;i<5;i++)

    {

      printf("child will print the %dth  character and send it to parent\n",i);

      printf("%s : from child .. waiting for parent to print same character\n",f1[i]);



      write(fd[1],f1[i],(strlen(f1[i])+1));

//    exit(0);

    }

  }  else  {

    wait(NULL);

    close(fd[1]);

    for(i=0;i<5;i++)

    {

                                                                                

      nbytes=read(fd[0],f2,sizeof(f2));

                                                                                

      printf("%s : from parent .. waiting for next character from child\n",f2);

    }                                                                            

  }                                                                          

  return 0;                                                                            

}

Quote:

Originally Posted by geeman2.0

Likely this is because you put an exit(0) inside the for loop.

even if I remove it, it does not give me what I want :confused:

Mara

thanx for explanning the mechanism

I have another question

if I have the following code

Code:

#include<stdio.h>

#include<string.h>



int main(void){

        int childpid,fd[2],nb,i,j;

        char line[BUFSIZ]="I want to print this line twice";

        char word[BUFSIZ] ;

        



        pipe(fd);



        childpid=fork();



        if(childpid==0)

                {

                        printf("from child:\n");

                        

                        close(fd[0]);



                        char *token=strtok(line," ");



                        while(token!=NULL)

                        {

                                printf(" %s\n",token);



                                write(fd[1],token,(strlen(token)+1));



                                token=strtok(NULL," ");



                        }

        

                }



        else

        {



                wait(NULL);

                printf("from parent:\n");

                

                



                close(fd[1]);

                

        for( i=0;i<7;i++){

                nb=read(fd[0],word,sizeof(word));

                printf("%s\n",word);

        

        }



        }



        return 0;



}

I want the output to be:

from child:
I
want
to
print
this
line
twice

from parent:
I
want
to
print
this
line
twice

what I get is
from child:
I
want
to
print
this
line
twice

from parent:
I
I
I
I
I
I
I

I think I am doing as you said, but instead of the first for loop I've used a while loop but I dont know why I did not get the correct results :confused: :confused:

thanx again for help

man strtok

strok splits a string up, and places a NULL \0 where the spaces are.
Hence when you print it you only see the I. The string STOPS PRINTING with the \0.

use strchr(line, ' ') instead.

if you are playing it may be easier to use printf() or puts() and gets() for the pipe.

I can't see why you are splitting the string.
strtok is a dangerous function if you don't know what you are doing ;)

It is ok for me to use strtok since I am dealing with rubbish data

I found a solution for that problem

thanx for your reply