Cannot attach shared memory in shared library - c

Question

I would like to use shared memory in shared library to data exchange and operate socket.
In my library, when first process loads shared libary, shared memory is created in constructor.
Then, second process load shared library, I want to attach this shared memory.
But second process create other shared memory although key is same.
This is sample code.
char* sharedmemory;
__attribute__ ((constructor)) void attach_sharedmemory() {
key_t key = 10;
errno = 0
int seg_id = shmget(key, 1000, IPC_CREAT | IPC_EXCL | 0660);
if(seg_id == -1){
if(errno == EEXIST){
seg_id = shmget(key, 0, 0);
if(seg_id == -1){
exit(1);
}else{
shared_memory = (char*)(shmat(seg_id, 0, 0));
}
}else{
exit(1);
}
}else{
shared_memory = (char*)(shmat(seg_id, 0, 0));
}
}
void dllfunction1(){}
void dllfunction2(){}
ipcs command result is here.
Second process shmid is always first process's shmid + 1.
#result of ipcs
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000XXX 65536 user1 660 1000 1 dest <--- created first process
0x00000YYY 65537 user1 660 1000 1 dest <--- created second process
How can I fix this?
My enviroment is Ubuntu 20.04LTS.
Compiled GCC.

Answer 1

The second process calling shmget() fails due to the flag bit IPC_EXCL
as expected; although the next call to shmget() reused the same key and
flags changed to 0 (no IPC_EXCL), the size was 0 leading to a new segment.
By reusing the same key and size (again flags without IPC_EXCL), the second
process can get the same segment ID and attach. Sample code below is
based on the original with added shmem detach:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/shm.h>
static void *sharedmemory;
static int seg_id;
__attribute__ ((constructor))
void attach_shmem(void)
{
key_t key = 10;
size_t size = 1000;
seg_id = shmget(key, size, IPC_CREAT | IPC_EXCL | 0660);
if ((seg_id == -1) && (errno == EEXIST))
seg_id = shmget(key, size, 0);
if (seg_id == -1)
exit(1);
sharedmemory = shmat(seg_id, NULL, 0);
}
__attribute__ ((destructor))
void detach_shmem(void)
{
(void) shmdt(sharedmemory);
}