Sstrhsrtj

I want to create N processes from one parent and this child processes have to read that parent write, but what i have only the first process reads correctly:

#include <stdio.h>

#include <unistd.h>

#include <sys/types.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>



int main(int argc, char *argv){





pid_t pid;



int fd[2];

char buff[50];



char str = "Hello";



if(pipe(fd) == -1){

    fprintf(stderr, "pipe Failed");

    return 1;

}



for(int num_process = 0; num_process < 3; num_process++){

    pid = fork();

    if(pid < 0){

        perror("Error");

        exit(1);

    }



    if(pid == 0){ //child code

        printf("Child %i (pid= %i)n", num_process, getpid());





    close(fd[1]);

    read(fd[0], buff, sizeof(buff)); //read from pipe

    printf("Read child = %sn", buff);

    close(fd[0]);

    exit(0);

    }



    else{//parent

        printf("Im parent %in",getpid());

        close(fd[0]);

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

        printf("Parent send %sn", str);

        close(fd[1]);

        for(int num_process = 0; num_process < 3; num_process++){

        wait(NULL);

        }

    }

}



return 0;

}

output:

Im parent 5863

Parent send Hello

Child 0 (pid= 5864)

Read child = Hello

Im parent 5863

Parent send Hello

Child 1 (pid= 5865)

Read child = 

Im parent 5863

Parent send Hello

Child 2 (pid= 5866)

Read child = 

read(fd[0], buff, sizeof(buff)); //read from pipe

printf("Read child = %sn", buff);

If you did check the return value of read (as you always should) you would've seen that the 2nd and 3rd time it returns -1 and sets errno to EBADF. That's because you've already closed the read end of the pipe in the first iteration of the loop, in the "parent" part, here:

   printf("Im parent %in",getpid());

   close(fd[0]);

The same thing with the write following it:

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

   printf("Parent send %sn", str);

   close(fd[1]);

fd[1] will be closed in the 1st iteration of the loop, and the write will fail with EBADF the 2nd and 3rd time.

the following proposed code:

1. handles each child separately


2. performs the desired functionality


3. incorporates the comments to the OPs question


4. caveat: the closing of the pipe ends is somewhat careless in the proposed code

and now the proposed code:

#include <stdio.h>

#include <unistd.h>

#include <sys/types.h>

#include <stdlib.h>

#include <string.h>

#include <sys/wait.h>



#define MAX_CHILDREN 3



int main( void )

{

    pid_t pid;



    int fd[2];





    char str = "Hello";



    for(int num_process = 0; num_process < MAX_CHILDREN; num_process++)

    {

        if(pipe(fd) == -1)

        {

            perror( "pipe Failed" );

            continue;

        }



        pid = fork();



        if(pid < 0)

        {

            perror("fork failed");

            exit(1);

        }



        if(pid == 0)

        { //child code

            char buff[50];

            printf("Child %i (pid= %i)n", num_process, getpid());

            close(fd[1]);



            if ( read( fd[0], buff, sizeof(buff)) <= 0) //read from pipe

            {

                perror( "read failed" );

                exit( EXIT_FAILURE );

            }



            printf("Read child = %sn", buff);

            exit(0);

        }



        else{//parent

            printf("Im parent %in",getpid());

            close(fd[0]);

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

            printf("Parent send %sn", str);

            wait(NULL);

        }

    }



    return 0;

}

A typical run of the code results in:

Im parent 26451

Parent send Hello

Child 0 (pid= 26452)

Read child = Hello

Im parent 26451

Parent send Hello

Child 1 (pid= 26453)

Read child = Hello

Im parent 26451

Parent send Hello

Child 2 (pid= 26454)

Read child = Hello