Thread: Shared memory, cannot read last element in a struct

Hi,

I have trouble reading the last element in a struct.
I do get the correct value for the first two elements. In my example that is:
a = 11 and c = H
however I get:
b = 0
but I am expecting b=2

I got two files to handle this.
The first one is writing data to memory -

Code:

#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    int  a;
    char c;
    int  b;
} MyObject;


int main() {

    int      shmid;
    key_t    key;
    char     *shm;
    MyObject obj = {11,'H',2};

    key = ftok("/tmp",'k');
    printf("key in server: %d\n",key);

    // create the segment.
    shmid = shmget(key, sizeof(obj), IPC_CREAT | SHM_R | SHM_W);
    printf("shmid:         %d\n",shmid);

    // attach the segment to our data space.
    shm = (char*)shmat(shmid, NULL, 0);
    printf("shm:           %d\n",shm);

    // put some things into the memory for the other process to read.
    memcpy(shm,&obj,sizeof(obj));
    printf("Data written to memory\n");

    return 0;
}

And the second one is reading from memory -

Code:

#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    int  a;
    char c;
    int  b;
} MyObject;

int main() {

    int      shmid;
    key_t    key;
    char     *shm;
    MyObject *obj2;

    key = ftok("/tmp",'k');
    printf("key in client: %d\n",key);

    // locate the segment.
    shmid = shmget(key, sizeof(obj2), SHM_R | SHM_W);
    printf("shmid:         %d\n",shmid);

    // attach the segment to our data space.
    shm = (char*)shmat(shmid, NULL, 0);
    printf("shm:           %d\n",shm);

    obj2 = (MyObject*)malloc(sizeof(obj2));

    // read what the server put in the memory.
    memcpy(obj2,shm,sizeof(obj2));

    printf("a: %d\n",obj2->a);
    printf("c: %c\n",obj2->c);
    printf("b: %d\n",obj2->b);

    return 0;

}

So my question is why I cannot get correct value for the third element in my struct?
In the second file where I'm reading from memory I allocate some space. Is this incorrect in some way?
I'm running this on a Linux machine.

Aha! That whas the problem.
Yes, sizeof(obj2) gave me 8 and sizeof(*obj2) gave me 12.

Thank you for your help.

Now it's working.