Question

I have a problem with a simple program im making with fork and pipes for learning purpose.
I want a child that send some data to the parent and this (the parent) send it again to the child.

The result is that the parent acts like the pipe is non-blocking and the child acts if the pipe is blocking. But i didnt use any code to tell that the pipes are non-blocking at all.

Let me show it:

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <conio.h>

void WriteStr(char* inStr)
{
    write(1,inStr,strlen(inStr));
}

int main(void){

    char *cad1 = "Introdueix un valor:  ";
int bytesr = 0;
char valor[256];

//Un pipe serveix solament per una comunicació unidereccional, 
//si es necessari que pare i fill es comuniquin en amdos sentits, 
//s'hauran d'utilitzar dos pipes.

int pip_fill[2];    //pip_fp[0]: file descriptor de lectura
                //pip_fp[1]: file descriptor d'escriptura

int pip_pare[2];    //pip_pf[0]: file descriptor de lectura
                //pip_pf[1]: file descriptor d'escriptura
int pid, nbytes = 0;
char cad[256];
int num = 0;

if (pipe (pip_fill)<0){
    write (1,"Pipe error!!!!\n",15);
    exit (1);
}

if (pipe (pip_pare)<0){
    write (1,"Pipe error!!!!\n",15);
    exit (1);
}

fflushnou();

pid = fork();
switch (pid){
    case -1:    
            //Error
            write (2,"Tractar_Valor: Error!!!!\n",25);
            exit (-1);
    case 0:
            //Fill
            //printf("Fill: fp[0]:%d fp[1]:%d pf[0]:%d pf[1]:%d \n",pip_fp[0],pip_fp[1],pip_pf[0],pip_pf[1]);
            close(pip_fill[0]);
                close(pip_pare[1]);

            while(1){
                bytesr = read(0, valor, 256);

                valor[bytesr] = '\0';

                write(pip_fill[1],valor,strlen(valor)); //el fill escriu a la pipe la var valor

                WriteStr("Fill avans\n");
                nbytes = read(pip_pare[0],cad,strlen(cad)); //el fill llegeix de la pipe la var un cop ha estat tractada per el pare
                WriteStr("Fill despres\n");

                if (nbytes != 0){   //vol dir que hem llegit algo per la pipe pip_pf
                    write(1,cad,strlen(cad));   //pintem cad per pantalla
                }

                sleep(1);
            }
    default:
            //Pare
            close(pip_fill[1]);
            close(pip_pare[0]);
            close(0);

            while(1){   

                nbytes = read(pip_fill[0],valor,strlen(valor));//el pare llegeix de la pipe la var valor 
                //WriteStr("Pare despres\n");
                if (nbytes != 0){   //vol dir que hem llegit algo per la pipe pip_fp
                    //tractem la variable valor
                    num = atoi(valor);
                    num = num*2;

                    sprintf(cad,"Valor actual de la variable: %d \n\n",num);

                    write(1,cad,strlen(cad));

                    write(pip_pare[1],cad,strlen(cad)); //el pare escriu a la pipe la var tractada
                }

                sleep(1);
            }
}
return 0;
}

The actual behaviour is that child accepts input, and then stucks reading ‘pip_pare[0]’. At the same time, parent process is looping and reading all the time from ‘pip_fill[0]’ a value of 0.

So, i am little confused about that, why parent is reading and loopinf without bloking in the ‘read’ function ??

Any suggestion to fix it ?

Thanks for the help 🙂

LLORENS

Answer 1

nbytes = read(pip_pare[0],cad,strlen(cad));

I think you probably meant sizeof(cad) here.

write(1,cad,strlen(cad));

And nbytes here.

nbytes = read(pip_fill[0],valor,strlen(valor));

And this along similar lines, but there’s a hidden trap in this version which I shall leave as an exercise!