因此，下面的程序访问位于“shmid”位置的一块共享内存，并将其附加到指针“total”。然后创建一个子进程来访问和修改这块内存，然后一旦完成，另一个子进程被创建并执行相同的操作，然后是第三个子进程。一旦这 3 个子进程修改完值并执行完毕，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”块会产生预期的效果，但似乎没有。
if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1)
{
perror ("shmctl");
exit (-1);
}
我不相信它，因为在这个块之后我有以下在执行期间打印出与以前相同的值。这是否表明指针仍在访问同一位内存，还是我弄错了？
printf("value after memory release:%d\n", total->value);
谢谢您的意见！
#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*****/