Question

因此，下面的程序访问“shmid”位置的一块共享内存，并将其附加到指针“total”。然后创建一个访问和修改这段内存的子进程，然后一旦完成，创建另一个子进程并执行相同的操作，然后创建第三个子进程。一旦这3个孩子完成修改值并完成执行，父进程就会释放共享内存，然后程序退出。我的印象是这个“shmctl”块会产生预期效果，但似乎没有。

if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1)
{
  perror ("shmctl");
 exit (-1);
}

我不相信它，因为在这个块后面我有以下内容，在执行期间打印出与以前相同的值。这不是表明指针仍在访问同一位内存，还是我错了？

printf("value after memory release:%d\n", total->value);

感谢您的投入！

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdlib.h>
#include <unistd.h>

/* change the key number */
#define SHMKEY ((key_t) 5600) //7890

typedef struct
{
  int value;
} shared_mem;

shared_mem *total;

/*----------------------------------------------------------------------*
 * This function increases the value of shared variable "total"
 *  by one all the way to 100000
 *----------------------------------------------------------------------*/

void process1 ()
{
  int k = 0;

  while (k < 100000)
  {
      k++;
      total->value = total->value + 1;
  }
  printf ("From process1 total = %d\n", total->value); //process 1 prints out total and  returns to main() function
}

/*----------------------------------------------------------------------*
 * This function increases the vlaue of shared memory variable "total"
 *  by one all the way to 170000
 *----------------------------------------------------------------------*/

void process2 ()
{
  int k = 0;

  while (k < 170000)
  {
  k++;
  total->value = total->value + 1;
  }
  printf ("From process2 total = %d\n", total->value); //process 2 prints out total and returns to main() function
}
/*----------------------------------------------------------------------*
 * This function increases the vlaue of shared memory variable "total"
 *  by one all the way to 200000
 *----------------------------------------------------------------------*/

void process3 ()
{
  int k = 0;

  while (k < 200000)
  {
    k++;
    total->value = total->value + 1;
  }
  printf ("From process3 total = %d\n", total->value); //process 3 prints out total and returns to main() function
}

/*----------------------------------------------------------------------*
 * MAIN()
 *----------------------------------------------------------------------*/

int main()
{
  int shmid;
  int pid1;
  int pid2;
  int pid3;
  int ID;
  int status;

  char *shmadd;
  shmadd = (char *) 0;

/* Create and connect to a shared memory segmentt*/

if ((shmid = shmget (SHMKEY, sizeof(int), IPC_CREAT | 0666)) < 0)
{
  perror ("shmget");
  exit (1);
}


if ((total = (shared_mem *) shmat (shmid, shmadd, 0)) == (shared_mem *) -1)
{
  perror ("shmat");
  exit (0);
}

total->value = 0;

if ((pid1 = fork()) == 0) //first child created
{
  process1(); //first child process begins
}

while((ID = wait(&status)) != -1) //parent loops until 1st child is finished
printf("child %d is finished\n", ID); //parent prints out returned value after child is finished


if ((pid1 != 0) && ((pid2 = fork()) == 0)) //second child created
{
  process2(); //second child process begins
}   

while((ID = wait(&status)) != -1) //parent loops until 2nd child is finished
printf("child %d is finished\n", ID); //parent prints out returned value after child is finished

if ((pid1 != 0) && (pid2 != 0) && ((pid3 = fork()) == 0)) //third child created
{
  process3(); //third child process begins
}

while((ID = wait(&status)) != -1) //parent loops until 3rd child is finished
printf("child %d is finished\n", ID); //parent prints out returned value after child is finished



if ((pid1 != 0) && (pid2 != 0) && (pid3 != 0)) 
{
  if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1)
  {
 perror ("shmctl");
 exit (-1);
  }

  printf ("\t\t  End of Program.\n"); //prints after all children have finished

  printf("value after memory release:%d\n", total->value);
}
return 0;
} 

/***** Note:  loop for parent to wait for child processes to finish and print ID of each child*****/

Answer 1

从shmctl手册页：

IPC_RMID

标记要销毁的细分。该细分市场实际上只是   在最后一个过程分离后（即，当   相关结构shmid_ds的shm_nattch成员为零）。该   呼叫者必须是所有者或创建者，或具有特权。如果是一个细分   已被标记为销毁，然后是（非标准）SHM_DEST标志   检索到的关联数据结构中的shm_perm.mode字段   将设置IPC_STAT。

调用者必须确保段最终被销毁;   否则其出现故障的页面将保留在内存或   交换。

即。您的代码标记了要销毁的段，但在程序退出之前不会将其分离。

如果您想提前分离，可以致电shmdt(total);。如果在最后一次printf之前执行此操作，则printf可能会导致seg错误。

释放共享内存

1 个答案: