如何使用Mmap共享內存。 請更正我的代碼

Question

這里的問題是我希望我們的put為93。我希望所有線程共享該變量。 就像靜態變量是所有對象通用的一樣，我想要一個所有線程都通用的變量。

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

static int *glob_var; 

int main(void)
  {
  glob_var = (int*)mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE, 
                MAP_SHARED | MAP_ANONYMOUS, -1, 0);

  *glob_var = 1;
  int ppid =fork();

  if (ppid == 0) {
     *glob_var = 92;  printf("%d\n", *glob_var);

  } else if(ppid!=0){
     (*glob_var)++; /////I want a 93 over here???
      printf("%d\n", *glob_var); /////I want a 93 over here??? print
      munmap(glob_var, sizeof *glob_var);
  }
  return 0;
  }

Answer 1

這兩個過程都更新glob_var 。 您需要協調對該共享內存的訪問。 必須保證正確的數據修改順序，即應首先分配值92 。

在以下情況下， 信號燈通常用於同步操作：

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h> 
#include <sys/ipc.h>
#include <sys/sem.h>
#include <unistd.h>

// Binary semaphore implementation. Initial state 0
union semun 
{
  int val;
  struct semid_ds *buf;
  unsigned short int *array;
  struct seminfo *__buf;
};

int binary_semaphore_allocation (key_t key, int sem_flags)
{
  return semget (key, 1, sem_flags);
}

int binary_semaphore_deallocate (int semid)
{
  union semun ignored_argument;
  return semctl (semid, 1, IPC_RMID, ignored_argument);
}

int binary_semaphore_initialize (int semid)
{
  union semun argument;
  unsigned short values[1];
  values[0] = 0;
  argument.array = values;
  return semctl (semid, 0, SETALL, argument);
}

int binary_semaphore_wait (int semid)
{
  struct sembuf operations[1];
  operations[0].sem_num = 0;
  /* Decrement by 1. */
  operations[0].sem_op = -1;
  operations[0].sem_flg = SEM_UNDO;
  return semop (semid, operations, 1);
}

int binary_semaphore_post (int semid)
{
  struct sembuf operations[1];
  operations[0].sem_num = 0;
  /* Increment by 1. */
  operations[0].sem_op = 1;
  operations[0].sem_flg = SEM_UNDO;
  return semop (semid, operations, 1);
}

int main(void)
{
  key_t ipc_key;
  ipc_key = ftok(".", 'S');
  int sem_id;

  glob_var = (int*)mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE, 
                MAP_SHARED | MAP_ANONYMOUS, -1, 0);

  *glob_var = 1;

  if ((sem_id=binary_semaphore_allocation(ipc_key, IPC_CREAT|IPC_EXCL)) != -1) 
  {
    if (binary_semaphore_initialize(sem_id) == -1) 
    {
      printf("Semaphore initialization failed");
      return 2;
    }
  }

  int ppid = fork();
  if (ppid == 0)
  {
    if ((sem_id = binary_semaphore_allocation(ipc_key, 0)) == -1) 
    {
      printf("Child process failed to open semaphore");
      return 3;
    }
  }
  else
  {
    // Wait in parent process until child update glob_var
    binary_semaphore_wait(sem_id);
  }

  if (ppid == 0) 
  {
    *glob_var = 92;  
    printf("%d\n", *glob_var);
    binary_semaphore_post(sem_id);
  }
  else if(ppid!=0)
  {
    (*glob_var)++; 
    printf("%d\n", *glob_var); 
    munmap(glob_var, sizeof *glob_var);
    binary_semaphore_deallocate(sem_id);
  }

  return 0;
}

輸出：

92
93