将posix信号量数组分配给共享内存

Question

I would like to write an array of unnamed semaphores to shared memory so I can access them in forked processes. Here's what I have so far:

int main(int argc, char *argv[]){

int num_process = atoi(argv[1]);
int i = 0;

int ppid = getpid();
void *addr;

int numsems = 512;
int object_size = numsems * sizeof(sem_t);

printf("declaring semaphores\n");
sem_t *sem[numsems];

//create shared memory and check that it worked
printf("opening shared memory\n");
int shmem_fd = shm_open("/my_shmem", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);

if(shmem_fd == -1){
    perror("Can't open shmem object");
    exit(-1);
}

//truncate memory object
if(ftruncate(shmem_fd, object_size) == -1){
    perror("failed to resize shmem object");
    exit(-1);
}

addr = mmap(NULL, object_size, PROT_READ | PROT_WRITE, MAP_SHARED, shmem_fd, 0);

if(MAP_FAILED == addr){
    perror("Map failed");
    exit(-1);
}

//store the semaphores at the start
printf("initializing semaphores\n");

for(i = 0; i<numsems; i++){
    sem[i] = addr + i*sizeof(sem_t);
    if(sem_init(sem[i], 1, 0) == -1){
            perror("sem_init failed");
            exit(-1);
    }
}

//create children
while((getpid() == ppid) && (i < num_process)){


    switch(fork()){
    case -1:
    printf("fork %d failed\n", i);
    break;

    case 0:
    //child process
    printf("child created\n");
    child_program( sem, numsems);
    printf("child done\n");
    break;

    default: //parent continues on to create next child
    break;

    }
i++;
}
}

Which compiles fine but when I try to run it I get a segmentation fault when it gets to the sem_t *sem[numsems] part. I've read elsewhere that you shouldn't create a pointer to semaphores but when I didn't and tried &sem = addr I got an error about needing a lvalue. Any help would be much appreciated.

Answer 1

First of all, why not create (and map) the shared memory segment in the parent process, and then open it in the child?

And also, instead of having an array of pointer, why not simply map the whole array, without using pointers:

Parent:

sem_t *sem;

shm_open("/my_shmem", O_CREAT | O_RDWR ...);

sem = mmap(NULL, object_size, ...);

Child:

semt_t *sem;

shm_open("/my_shmem", O_RDWR, 0);

sem = mmap(NULL, object_size, ...);