Shared memory in the task of the producer and the consumer

Good day, I caught the eye of the code that implements this task, but I can not understand how it is performed and why. In particular, how producer () and consumer () are called and executed. As I see, the memory itself is allocated using System V functions, when the semaphores are used by POSIX. That's actually the main ():

#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/wait.h> #include <semaphore.h> #include <sys/shm.h> #include <time.h> #define N 10 int L = 0; struct sharedmem { sem_t mutex; sem_t buffer_full; sem_t buffer_empty; FIFO buff; // тут наша очередь, куда наш производитель кладёт буквы //для очереди реализованны банальные функции типо // Pop(FIFO & val) - достать элемент из очереди val // Push(FIFO & val, char c) - положить элемент в очередь val }; struct sharedmem *shared; int main() { srand((unsigned int)time(NULL)); int semid; semid = semget(1024, 3, IPC_CREAT | 0600); int shmid; shmid = shmget(1024, sizeof(shared), IPC_CREAT | 0600); shared = (struct sharedmem*) shmat(shmid, NULL, 0); shared->count = 0; sem_init(&shared->mutex, 1, 1); sem_init(&shared->buffer_full, 1, 0); sem_init(&shared->buffer_empty, 1, N); pid_t children[2]; children[0] = fork(); if(children[0] == 0) consumer(); children[1] = fork(); if(children[1] == 0) producer(); waitpid(children[0], NULL, 0); waitpid(children[1], NULL, 0); return 0; } 

This is a manufacturer function.

 void producer() { char let; while(1) { let = random_letter(); sem_wait(&shared->buffer_empty); sem_wait(&shared->mutex); Push(&shared->buff, let); sem_post(&shared->mutex); sem_post(&shared->buffer_full); printf("Producer added: %s\n", &let); Sleep();// засыпает на рандомное время } } 

This is a consumer

 void consumer() { char let; while(1) { sem_wait(&shared->buffer_full); sem_wait(&shared->mutex); let = Pop(&shared->buff); sem_post(&shared->mutex); sem_post(&shared->buffer_empty); printf("Consumer read and deleted: %s\n", &let); Sleep();// засыпает на рандомное время } } 

approximate result of the program: