Linux共享内存分段故障

Question

我正在学习有关的共享内存，并创造了这个示例程序来测试的事情了

//IPC - Shared Memory 

#include<stdio.h> 
#include<stdlib.h> 
#include<linux/ipc.h> 
#include<linux/msg.h> 
#include<linux/shm.h> 

int main(int argc, char* argv[]) 
{ 
    printf("setting up shared memory\n"); 

    key_t ipc_key; 
    int shmid; 
    int pid; 


    ipc_key = ftok(".",'b'); 

    if((shmid=shmget(ipc_key, 32, IPC_CREAT|0666))==-1) 
    { 
    printf("error creating shared memory\n"); 
    exit(1); 
    } 

    printf("shared memory created with id %d\n",shmid); 

    //fork a child process 
    pid = fork(); 
    printf("fork result %d\n",pid); 
    if(pid==0) 
    { 
    //child process 
    //attach the shared memory 
    int* shm_add_child = (int*)shmat(shmid, 0,0); 
    printf("child attached to shared mem at address %p\n",(void*)shm_add_child); 

    while(1) 
    { 
     printf("%d\n",*shm_add_child); 
     printf("a\n"); 
    } 

    //detach from shm 
    shmdt(shm_add_child); 
    } 
    else 
    { 
    //parent process 
    int* shm_add_parent; 

    shm_add_parent = (int*)shmat(shmid, 0,0); 
    printf("parent attached to shared mem at address %p\n",(void*)shm_add_parent); 
    *shm_add_parent = 10; 

    sleep(10); 

    //detach from shm 
    shmdt(shm_add_parent); 
    } 

    //remove shm 
    shmctl(shmid, IPC_RMID,0); 

    exit(0); 
} 

然而，当我运行它，我得到段错误。看来我对共享内存的指向是不对的。也没有什么是从子过程中的无限循环中打印出来的。

ankit@ankit-ThinkPad-W530:~/Desktop/week1_tasks$ ./ipc_sharedmem_a 
setting up shared memory 
shared memory created with id 5996570 
fork result 8703 
parent attached to shared mem at address 0xffffffff991aa000 
fork result 0 
child attached to shared mem at address 0xffffffff991aa000 
Segmentation fault (core dumped) 

这里怎么回事？

Answer 1

我不确定你为什么要包含最后三个头文件。它们不是正确的标题，会给你shm函数的错误定义。在我的系统的gcc甚至会产生警告它提供了一些线索，有一个问题：

test.c:38:26: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast] 
    int* shm_add_child = (int*)shmat(shmid, 0,0); 
         ^
test.c:55:22: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast] 
    shm_add_parent = (int*)shmat(shmid, 0,0); 

相反，你应该只包括在shmat man page指定的人。具体如下：

#include<stdio.h> 
#include<stdlib.h> 

// Remove these headers 
//#include<linux/ipc.h> 
//#include<linux/msg.h> 
//#include<linux/shm.h> 

// Include these instead 
#include <sys/types.h> 
#include <sys/shm.h>