Ndtyjky

#include <stdio.h>

#include <stdlib.h>

#include <sys/wait.h>

#include <string.h>

#include <fcntl.h>

#include <sys/shm.h>

#include <sys/stat.h>

#include <sys/time.h>

#include <unistd.h>

#include <sys/mman.h>



int main()

{

    const int SIZE = 500;

    const char *name = "name";

    int fd;

    char *ptr = NULL;

    pid_t pid;

    pid = fork();



    if (pid < 0) {

        fprintf(stderr, "Fork Failed");

        return 1;

    }

    else if (pid == 0) {

        fd = shm_open(name,O_CREAT | O_RDWR,0666);

        ftruncate(fd, SIZE);

        ptr = (char *)mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

        sprintf(ptr, "%s", "Hello, World!n");

        return 0;

    }

    else {

        wait(NULL);

        fd = shm_open(name, O_RDONLY, 0666);

        ptr = (char *)mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

        printf("%sn", (char *)ptr);

    }

    return 0;

}

I am basically looking to create some shared memory in the child process and access it from the parent.

In the child process, the mmap works fine. When I print using the pointer returned by mmap it does in fact print Hello, World!, but the same print gives a seg fault from the parent.

In the parent (pid != 0) you opened the object O_RDONLY, but mmapped it with PROT_WRITE, MAP_SHARED. Remove the | PROT_WRITE and you are fine.
You might want to check the return values for errors the odd time.

The crash is due to this excerpt from man:

O_RDONLY   Open the object for read access.  A shared memory object

           opened in this way can be mmap(2)ed only for read

           (PROT_READ) access.

You've attempted to:

fd = shm_open(name, O_RDONLY, 0666);

//                  ^^^^^^^^

ptr = (char *)mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

//                                    ^^^^^^^^^^^^ incorrect!

Another remark: your name should follow the man recommendation for portability:

For portable use, a shared memory object should be identified by a name

of the form /somename; that is, a null-terminated string of up to

NAME_MAX (i.e., 255) characters consisting of an initial slash,

followed by one or more characters, none of which are slashes.

Lastly, you have some unnecessary (char *) casts and always error check your return values.