"Turn" initialized memory into shared memory with the SYSV API? - c

Question

To create a new and empty shared memory segment I can do something like:
int shmid = shmget(IPC_PRIVATE, nbytes, IPC_CREAT | 0777);
int* new_array = (int*)shmat(shmid, NULL, 0);
Now suppose int* array owns a chunk of pre-existing data. Something like
int* array = malloc(2 * sizeof(int));
array[0] = 42;
array[1] = 24;
I'd like to turn this memory segment into a shared memory segment with the System V shared memory C API, on Linux.
How can I do this?
I've tried stuff like
int shmid = shmget(IPC_PRIVATE, nbytes, IPC_CREAT | 0777);
void* attach = shmat(shmid, array, SHM_RND);
and like
int shmid = shmget(IPC_PRIVATE, nbytes, IPC_CREAT | 0777);
array = (int*)shmat(shmid, 0, 0);
but either shmat returns (void*)-1 or int* array gets all its values set to zero.
Here's a full program that shows the failures:
/*
gcc shm0.c -o shm0 && ./shm0
*/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define NELEMENTS 3
void array_show(int* array, size_t nelements){
puts("");
for(int i=0; i < nelements; ++i)
printf("%d\n", *(array + i));
}
int main(){
const size_t nbytes = NELEMENTS * sizeof(int);
int* array;
posix_memalign((void**)&array, 16*16*16, nbytes);
int shmid = shmget(IPC_PRIVATE, nbytes, IPC_CREAT | 0777);
// 1st try...
void* attach = shmat(shmid, array, SHM_RND);
if(attach == (void*)-1)
puts("shmat() failed!");
// 2nd try...
memset(array, 1, nbytes); // Fill array with dummy data!
array_show(array, NELEMENTS);
array = (int*)shmat(shmid, 0, 0);
array_show(array, NELEMENTS);
if(array[1] == 0)
puts("array was reset!");
// 3rd try
array = (int*)shmat(shmid, array, 0); // Now accessing, say, array[0] yields a segfault!
shmdt(array);
}
Why SYSV shared memory and not mmap()? Because I'm using the X11 MIT shared memory extension, and, according to the docs:
... to be able to use this extension, your system must provide the SYSV shared memory primitives. There is not an mmap-based version of this extension. To use shared memory on Sun systems, you must have built your kernel with SYSV shared memory enabled ...
(If someone knows a way around this, or how to do this using shm_open(), I'd be delighted to hear about it.)