How to pass a string between parent and child?

Question

#include<stdio.h>
#include<stdlib.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<sys/types.h>
#include<string.h>
#include<sys/stat.h>
#define SIZE 100

void main()
{
    int shmid,status;
    pid_t pid;
    int i;
    char *a,*b,d[100];
    shmid=shmget(IPC_PRIVATE,SIZE,S_IRUSR | S_IWUSR);
    pid=fork();


    if(pid==0)
    {
        b=(char *) shmat(shmid,NULL,0);
        printf("enter");
        printf("%c",*b);
        shmdt(b);
    }
    else
    {
        a=(char *) shmat(shmid,NULL,0);
        printf("enter a string");
        scanf("%s",&d);
        strcpy(a,d);
        shmdt(a);
    }
}

I was trying to pass a string from the parent process to the child process. But before scanning the value into "d" the program switches to the child process. How should I correct this logical error? And how should I pass this string "d" to the child process?

Why are you using shared memory? A pipe is a much better choice. — Filipe Gonçalves, Sep 26 '15 at 18:41
@FilipeGonçalves: As an exercise in learning about shared memory, perhaps? This is obviously a toy program. — Nate Eldredge, Sep 26 '15 at 21:07

score 1 · Answer 1 · edited May 23 '17 at 12:06

1

After call to fork was made you never know which process will execute first. You must simply assert your code handles properly interprocess communication, whatever happens now.

You can use pipe(2) or shared memory to pass data between different processes on same host.

#include <unistd.h>

int pipe(int pipefd[2]);

But you can also read data into global variable before call to fork. Fork will create a copy of global data in new process.

Shared memory using shmget example.

edited May 23 '17 at 12:06

Community

1
1

answered Sep 26 '15 at 18:43

4pie0

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i do not want to use pipes. i want to use shared memory .pass a string from user and child prints the string .the string should be stored in shared memory. – rishabh agarwal Sep 26 '15 at 18:51
@rishabhagarwal attached a link to shmget example – 4pie0 Sep 26 '15 at 20:28

score -1 · Answer 2 · answered Sep 26 '15 at 18:49

Fork is a system call and it creates two processes one is called the parent and the other is called the child! To enable them to communicate you need to apply interprocess communication techniques You could use

     1.Pipes
     2.FIFO-also known as Named pipes 
     3.Shared Memory
     4.Message Queue
     5.Semaphore

Everything you need to know to use them is mentioned here!The sample codes are written after the description

How to pass a string between parent and child?

2 Answers2