c - 连接共享内存段时出错

Question

我有以下代码，我想通过共享内存段在两个进程之间进行通信。我的问题是我在附加内存段时出错，但我不知道为什么。

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
#include "process.h"

int main(int argc, char** argv)
{
    /*- Check the command-line arguments -*/
    if(argc != 2)
    {
        printf("\n--- Wrong input at command-line arguments ---\n\n");
        printf("--- The program terminate --- \n");
        return 1;
    }

    int N = atoi(argv[1]);      // The amount of total threads to be created
    int status;         // for the wait() function
    char* data;         // a string to use for shared memory segment

    /*- Variables for the shared memory segment -*/
    key_t key = 1003;       // for the shmget() key argument
    int shmid;          // to store the returned value from shmget()
    int size = 1024;        // for the shmget() size argument


/* ************************************************************************************/

    pid_t pid = fork(); // create second process

    if(pid < 0)     // if something going wrong
    {
        printf("\n\n---- Error in function fork!----\n");
        exit(1);
    }
    else if(pid == 0)   // the child process (P) 
    {
        // create the shared memory segment for the P process
        shmid = CreateShmSegment(key, size, IPC_CREAT);

        if(shmid == -1)     // check if creating shared memory return an error
        {
            printf("\n---Error at creating the memory segment! ---\n");
            exit(1);
        }

        // attach the shared memory segment
        data = AttachShmSegment(shmid);

        if(data == (char*)-1)   // check if attached the shared memory return an error
        {
            printf("\n---Error at attaching the memory segment! ---\n");
            exit(1);
        }   

        char* str = data;
        sprintf(str, "testing");

//      ChildProcess();
        printf("\n** Child process! ** \n");
        printf("N = %d\n", N);
        printf("write: %s\n",str);

        // detach the shared memory segment
        if(shmdt(data) == -1)       //check for error
        {
            printf("\n---Error at detaching memory segment! ---\n");
            exit(1);
        }
    }
    else            // the parent process (C)
    {   
        // create the shared memory segment for the C process
        shmid = CreateShmSegment(key, size, IPC_CREAT);

        if(shmid == -1)     // check if creating shared memory return an error
        {
            printf("\n---Error at creating the memory segment! ---\n");
            exit(1);
        }

        // attach the shared memory segment
        data = AttachShmSegment(shmid);
        if(data == (char*)-1)   // check if attached the shared memory return an error
        {
            printf("\n---Error at attaching the memory segment! ---\n");
            exit(1);
        }   

//      ParentProcess();
        wait(&status);
        printf("\n** Parent process! **\n");
        printf("N = %d\n",N);   
        printf("read from segment: %s\n", data);

        // detach the shared memory segment
        if(shmdt(data) == -1)       //check for error
        {
            printf("\n---Error at detaching memory segment! ---\n");
            exit(1);
        }

        // deallocate the shared memory segment
        if(DeallocateShmSegment(shmid) == -1)   //check for error
        {
            printf("\n---Error at destroy memory segment! ---\n");
            exit(1);
        }
    }

    return 0;
}

我引用了另外两个文件进行编译。

进程.h:

#ifndef __Processes_H_
#define __Processes_H_
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>

void    ChildProcess();
void    ParentProcess();
int CreateShmSegment(key_t, size_t, int);
void*   AttachShmSegment(int);
int DetachShmSegment(const void*);
int     DeallocateShmSegment(int);

#endif

和process.c:

#include "process.h"

void ChildProcess()
{

}


/***************************************************************/

void ParentProcess()
{

}

/****************************************************************/

int CreateShmSegment(key_t key, size_t size, int flag)
{
    int id;
    id = shmget(key, size, flag);
    return id;
}

/*****************************************************************/

void* AttachShmSegment(int id)
{
    char* data = NULL;
    data = (char*) shmat(id, NULL, 0);
    return data;
}

/*****************************************************************/

int DetachShmSegment(const void* addr)
{
    return shmdt(addr);
}

/*****************************************************************/

int DeallocateShmSegment(int id)
{
    return shmctl(id, IPC_RMID, NULL);
}

我不知道连接内存出了什么问题。我在网上搜索并对 shmat() 的参数进行了一些更改，但我无法解决它。

Answer 1

创建段时没有指定任何模式标志。因此，我希望它的创建对任何人(包括其创建者)都没有读或写访问权限。这本身并没有错误，但非特权进程在具有该模式时附加该段的所有尝试都应该失败，并显示 EACCES。

大概，您希望在创建段时在标志中至少包含 S_IRWXU，或者在创建段后使用 shmctl() 来修改段的模式允许所有者进行读写访问。 (请注意，执行权限对于此目的没有意义，因此它相当于使用 S_IRUSR | S_IWUSR。)例如，

    shmid = CreateShmSegment(key, size, IPC_CREAT | S_IRWXU);

您可能想授予更广泛的访问权限。

还要注意的是，先 fork 然后让父进程和子进程创建并附加共享内存段，这很奇怪。附加的共享内存段是通过 fork() 继承的，因此在 fork 之前创建和附加段会更干净。