Different shared memory values between processes - c

Question

I create a buffer via this function.
int create_buffer(const char *shmem_name, void **shmem_obj)
{
int returnval = 0;
int fd;
fd = shm_open(shmem_name, O_CREAT | O_RDWR, 0666);
ftruncate(fd, sizeof(ShareStruct));
*shmem_obj = (ShareStruct *)mmap(NULL, sizeof(ShareStruct), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (shmem_obj < 0)
{
returnval = -1;
}
return returnval;
}
Then when I use the pointer in process 1 and its functions, everything works.
In process 2 I fetch the buffer using this function:
int fetch_buffer(const char *shmem_name, void **shmem_obj)
{
int returnval = 0;
int fd;
fd = shm_open(shmem_name, O_RDWR, 0666);
ftruncate(fd, sizeof(ShareStruct));
*shmem_obj = (ShareStruct *)mmap(NULL, sizeof(ShareStruct), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (shmem_obj < 0)
{
returnval = -1;
}
printf("buffer fetched \n");
return returnval;
}
But the contents of the buffer are different (weird values).
What am I doing wrong?
EDIT:
process 1 code:
#define SHMEM_NAME "OS"
int main(void) {
int exitcode = 0;
void *shmem_obj;
/* Initialize the buffer and create the shared memory */
if( create_buffer(SHMEM_NAME, &shmem_obj) != 0) {
fprintf(stderr, "Something went wrong while initializing the buffer.\n");
exitcode = 1;
} else {
printf("Shared buffer created.\n");
initParameters(&shmem_obj);
subscribe(0, &shmem_obj);
printf("\n[PRESS ENTER TO CLOSE THE SHARED BUFFER]");
getchar();
/* unlink the shared memory */
if(destroy_buffer(SHMEM_NAME) != 0) {
fprintf(stderr, "Something went wrong while destroying the buffer.\n");
exitcode = 2;
} else {
printf("Shared buffer closed.\n");
}
}
return exitcode;
}
Process 2 code:
#define SHMEM_NAME "OS"
int main(void) {
void *shmem_obj;
/* fetch the buffer from shared memory */
fetch_buffer(SHMEM_NAME, &shmem_obj);
subscribe(1, &shmem_obj);
printf("Subscriber 0 subscribed\n");
}
Other code:
typedef struct ShareStruct
{
Subscriber subscriberList[20];
int subscriberActiveList[20];
unsigned int writepointer_tail;
unsigned int readpointer_head;
Task taskList[21];
sem_t freePostions;
pthread_mutex_t mutex;
unsigned int subscriberCount;
} ShareStruct;
int initParameters(void *shmem_obj)
{
ShareStruct *shmem = shmem_obj;
int returnval = 0;
shmem->writepointer_tail = 0;
shmem->readpointer_head = 0;
shmem->subscriberCount = 0;
pthread_mutex_init(&shmem->mutex, NULL);
sem_init(&shmem->freePostions, 1, 20);
return returnval;
}
int destroy_buffer(const char *shmem_name)
{
int returnval = 0;
returnval = shm_unlink(shmem_name);
return returnval;
}
int subscribe(subscriber_id subscriber, void *shmem_obj)
{
int returnval = 0;
ShareStruct *shmem = shmem_obj;
int value;
sem_getvalue(&shmem->freePostions, &value);
printf("sem value : %d\n", value);
pthread_mutex_lock(&shmem->mutex);
int freeposition = returnNextFreePosition(shmem_obj);
if (freeposition != -1)
{
printf("subscribe logic");
}
else
{
printf("Subscriber list is full \n");
}
pthread_mutex_unlock(&shmem->mutex);
return returnval;
}

Answer 1

The problem is that you're being inconsistent in the number of layers of pointer indirection there are, and your use of void * rather than more specific pointer types is preventing the compiler from realizing and telling you that you're doing that. Replace all of your void *s with ShareStruct *s, and then fix the resulting errors and warnings your compiler gives you, and your code will start to work.