0
我正試圖編寫一個程序，其中主進程分配共享內存，然後分叉4次。每個進程然後增加1個共享內存中的整數變量500次。所以整數變量最終保存2500.在訪問共享內存之前，進程鎖定一個信號量，並在寫入共享內存之後，該進程將其解鎖。但是程序給了我以下結果。我錯誤地使用了未命名的信號變量嗎？
[email protected]:~/thread$ ./a.out
shared memory value : 500 by 28488 process
shared memory value : 1000 by 28487 process
shared memory value : 2179 by 28490 process
shared memory value : 1500 by 28489 process
shared memory value : 2500 by 28491 process
[email protected]:~/thread$ ./a.out
shared memory value : 500 by 28493 process
shared memory value : 1000 by 28492 process
shared memory value : 2500 by 28495 process
shared memory value : 1500 by 28494 process
shared memory value : 2000 by 28496 process
[email protected]:~/thread$ ./a.out
shared memory value : 500 by 28498 process
shared memory value : 1000 by 28497 process
shared memory value : 1500 by 28501 process
shared memory value : 2000 by 28499 process
shared memory value : 2500 by 28500 process
第一個結果顯示奇怪的值2179.我也認爲中間結果應該是有序的，因爲一次只能有一個進程訪問共享內存。但第二個結果顯示500,1000,2500,1500,2000，這是亂序結果。
這裏是代碼片段。
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <unistd.h>
#include <semaphore.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>
#define KEY_NUM 9527
#define MEM_SIZE 4096
int main(int argc, char *argv[])
{
char buffer[1024];
char *c;
int i, n, status;
pid_t childpid = 0;
sem_t my_lock;
int counter = 0;
int shm_id;
void *shm_addr;
if((shm_id = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0666)) == -1)
{
printf("fail to allocate a shared memory.\n");
return -1;
}
shm_addr = shmat(shm_id, (void*)0, 0);
*(int*)shm_addr = 0; // initially 0.
if(sem_init(&my_lock, 1, 1) == -1)
{
perror("Could not initialize mylock semaphore");
exit(1);
}
for(i = 0; i < 4; i++)
if(0 == fork())
break; // child process exit for-loop.
// entry section
if(sem_wait(&my_lock) == -1)
{
perror("Semaphore invalid");
exit(1);
}
// start of critical section.
for(i = 0 ; i < 500; i++)
{
*(int*)shm_addr = *(int*)shm_addr + 1;
}
sprintf(buffer, "shared memory value : %d by %ld process\n", *(int*)shm_addr, (long)getpid());
write(STDOUT_FILENO, buffer, strlen(buffer));
// end of critical section.
// exit section
if (sem_post(&my_lock) == -1)
{
perror("Semaphore done");
exit(1);
}
// remainder section
if(childpid > 0)
{
for(i = 0 ; i < n ; i++) // wait for all children.
wait(&status);
shmctl(shm_id, IPC_RMID, 0); // deallocate the shared memory.
}
exit(0);
}
我沒有找到問題所在。我該怎麼辦？先謝謝你。