具有2个信号量组和共享内存的进程

Question

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <errno.h>
#include <sys/wait.h>

int sum =0;

int compute(int data){
    sum += data;
    return sum;
}

int main(){
    int i;
    int shm_id;
    int data;
    pid_t pid;
    key_t shm_key;
    sem_t *sem;
    // unsigned int sem_value =2;
    shm_key = ftok("/dev/null", 65);
    shm_id = shmget(shm_id, sizeof(int), 0644 | IPC_CREAT);
    if (shm_id < 0){
        perror("shmgget");
        exit(EXIT_FAILURE);
    }
    // data = shmat(shm_id, NULL, 0);
    sem = sem_open("semaphore", O_CREAT | O_EXCL, 0644, 2);

    for (i = 0; i < 2; i++){
        pid = fork();
        if (pid < 0)
        {
            perror("fork");
            sem_unlink("semaphore");
            sem_close(sem);
            exit(EXIT_FAILURE);
        }
        else if (pid == 0)
        {
            break;
        }
    }
    if (pid == 0)
    {
        puts("Enter the data:");
        scanf("%d", &data);
        //child process
        sem_wait(sem);
        printf("Child - %d is in critical section\n", i);
        sleep(1);

        puts("Enter the data:");
        scanf("%d", &data);
        // *shrd_value += data;
        printf("Child - %d: new value of data = %d\n", i, data);
        printf("Child - %d: sum of whole data by far = %d\n", i, compute(data));
        sem_post(sem);
        exit(EXIT_SUCCESS);
    }
    else if (pid > 0)
    {
        //parent process
        while (pid = waitpid(-1, NULL, 0))
        {
            if (errno == ECHILD)
            {
                break;
            }
        }
        puts("All children exited");
        shmdt(&data);
        shmctl(shm_id, IPC_RMID, 0);
        sem_unlink("semaphore");
        sem_close(sem);         
        exit(0);
    }
}

Enter the data:
Enter the data:
2
Child - 0 is in critical section
1Enter the data:

Child - 1 is in critical section
Enter the data:
3
Child - 0: new value of data = 3
Child - 0: sum of whole data by far = 3
2
Child - 1: new value of data = 2
Child - 1: sum of whole data by far = 2
All children exited

Answer 1

我还修改了它们写入共享内存的方式：它们直接在代码中缺少的shmat调用给出的地址处写入。我修复了一些bug，简化了代码（删除了添加数组的详细日志记录，特别是在进入critial部分之前和之后）：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <errno.h>
#include <sys/wait.h>

int main(){
    int i;
    int shm_id;
    pid_t pid;
    int *addr; 
    int data;
    pid_t current_pid;
    key_t shm_key;
    sem_t *sem;

    shm_key = ftok("/dev/null", 65);
    shm_id = shmget(shm_key, sizeof(int), 0644 | IPC_CREAT);
    if (shm_id < 0){
        perror("shmget");
        exit(EXIT_FAILURE);
    }

    sem_unlink("semaphore");
    sem = sem_open("semaphore", O_CREAT, 0644, 1);
    if (sem == SEM_FAILED) {
        perror("sem_open");
        exit(EXIT_FAILURE);
    } 

    addr = (int *) shmat(shm_id, (void *) 0, 0);   
    if (addr == (void *) -1) {
        perror("shmat");
        exit(EXIT_FAILURE);
    }
    *addr = 0;

    for (i = 0; i < 2; i++){
        pid = fork();
        if (pid < 0)
        {
            perror("fork");
            sem_close(sem);
            sem_unlink("semaphore");
            exit(EXIT_FAILURE);
        }
    }


    if (pid == 0)
    {
    current_pid = getpid();
        printf("Child %d: waiting for critical section \n", current_pid);
        sem_wait(sem);
        printf("Child %d: enters in critical section \n", current_pid);
        printf("child %d: Enter the data:\n", current_pid);
        scanf("%d", &data);
        printf("Child %d: new value of data = %d\n", current_pid, data);
        printf("Child %d: sum of whole data so far = %d\n", current_pid, *addr += data);
        sem_post(sem);
    printf("Child %d exits from critical section\n", current_pid);
        exit(EXIT_SUCCESS);
    }
    else if (pid > 0)
    {
        //parent process
        while (pid = waitpid(-1, NULL, 0))
        {
            if (errno == ECHILD)
            {
                break;
            }
        }
        puts("All children exited");
        shmdt(addr);
        shmctl(shm_id, IPC_RMID, 0);
        sem_close(sem);         
        sem_unlink("semaphore");
        exit(0);
    }

    exit(0);
}

请注意，信号量初始值必须为1，才能有2个进程的真正临界区段。

我还删除了睡眠调用，我们可以看到其中一个过程正在等待：

Child 22514: waiting for critical section 
Child 22514: enters in critical section 
child 22514: Enter the data:
Child 22515: waiting for critical section 
333
Child 22514: new value of data = 333
Child 22514: sum of whole data so far = 333
Child 22514 exits from critical section
Child 22515: enters in critical section 
child 22515: Enter the data:
666
Child 22515: new value of data = 666
Child 22515: sum of whole data so far = 999
Child 22515 exits from critical section
All children exited
All children exited