共享内存和 memcpy 问题

Question

我正在学习使用 C (linux) 中的共享内存进行编程。我需要在将使用 fork() 创建的几个进程之间共享一些结构。不幸的是，当我尝试初始化新共享的地址空间时，我在 memcpy 调用中收到一个提示错误（控制台中没有输出）。

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>

#define ROWS 10000
#define COLS 15000

struct mm_shared {
    int test;
    unsigned char matrix[ROWS][COLS];
};

int main(void) {

    int fd;
    if ((fd = shm_open("/mm", O_CREAT | O_RDWR, 0777)) == -1) {
        printf(stderr, "shm_open failed! %d - %s\n", errno, strerror(errno));
    }

    if (ftruncate(fd, sizeof(struct mm_shared) == -1)) {
        printf(stderr, "ftruncate failed! %d - %s\n", errno, strerror(errno));
    }

    struct mm_shared * shared;
    if ((shared = mmap(NULL, sizeof(struct mm_shared), PROT_READ
            | PROT_WRITE, MAP_SHARED, fd, 0)) == -1) {
        printf("mmap failed! %d, %s\n", errno, strerror(errno));
    }
    struct mm_shared * init = (struct mm_shared *) malloc(sizeof(struct mm_shared));

    memcpy(shared, init, sizeof(struct mm_shared)); <-- here lies the problem!


    shm_unlink("/mm");
    return EXIT_SUCCESS;
}

调试共享指针时，调试信息（eclipse调试器）显示：

Failed to execute MI command:
-data-evaluate-expression (shared)->test
Error message from debugger back end:
Cannot access memory at address 0x7ffff7ffc000

我不知道这是否有帮助。另外，我想问一下我在结构中存储大矩阵的方法是否正确（它应该在堆中分配对吗？因为即使矩阵本身不是指针，我也会得到一个指向结构的指针）。

对此的任何帮助将不胜感激！

Answer 1

经过一个小时的调试和审查周期终于发现了错误，而且是这个

second argument for ftruncate seems to be evaluating to wrong value

if (ftruncate(fd, sizeof(struct mm_shared) == -1)) {

改为

if (ftruncate(fd, sizeof(struct mm_shared)) == -1) {

这里是有改动的代码

int main(void) {

    int fd;
    if ((fd = shm_open("/mm", O_CREAT | O_RDWR, 0777)) == -1) {
        fprintf(stderr, "shm_open failed! %d - %s\n", errno, strerror(errno)); // should be fprintf
    exit(1);// possibly exit here
    }

    //if (ftruncate(fd, sizeof(struct mm_shared) == -1)) {
    if (ftruncate(fd, sizeof(struct mm_shared)) == -1) {
        fprintf(stderr, "ftruncate failed! %d - %s\n", errno, strerror(errno)); // should be fprintf
    goto out;   // remove all before leaving    
    }

    struct mm_shared * shared;
    if ((shared = mmap(NULL, sizeof(struct mm_shared) , PROT_READ
            | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED ) {  // Change -1 to MAP_FALED
        fprintf(stderr, "mmap failed! %d, %s\n", errno, strerror(errno)); // should be fprintf and stderr 
    goto out;   // remove all before leaving
    }
    struct mm_shared * init = (struct mm_shared *) malloc(sizeof(struct mm_shared));

    //memcpy(shared, init, sizeof(struct mm_shared)); //<-- here lies the problem!
    memcpy(init, shared, sizeof(struct mm_shared)); //<-- here lies the problem!

out:
    shm_unlink("/mm");
    return EXIT_SUCCESS;
}