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我最近意识到我的 C 技能有点生疏了，所以决定玩一下。不过，我只是遇到了一个奇怪的行为——我请求的文件映射内存似乎没有刷新变量ifs.free_space。结果是如果你不注释掉这两//root行，程序总是从ifs.free_space等于零开始。但是，将其注释掉会导致计数器被保存。这里发生了什么事？

#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>

#define FILE_LENGTH 0x10000


void* offset_to_pointer(uint64_t offset, void* file_memory) {
    return (void *)((uint64_t)file_memory + offset);
}

uint64_t pointer_to_offset(void* pointer, void* file_memory) {
    return ((uint64_t)pointer - (uint64_t)file_memory);
}

void* allocate(uint64_t size, uint64_t* free_space, void* file_memory) {
    void* ret = offset_to_pointer(*free_space, file_memory);
    *free_space += size;
    return ret;
}

typedef struct dirent {
    uint64_t prev_offset;
    uint64_t next_offset;
    uint64_t size;
} dirent;

typedef struct idiotfs {
    void* file_memory;
    uint64_t* free_space;
    dirent* root;
} idiotfs;

int main (int argc, char* const argv[])
{
    int fd = open("file.bin", O_RDWR | O_DSYNC, S_IRUSR | S_IWUSR);
    if (fd == -1)
        return 2;
    idiotfs ifs;
    ifs.file_memory = mmap(0, FILE_LENGTH,
                           PROT_READ | PROT_WRITE,
                           MAP_SHARED, fd, 0);
    ifs.free_space = (uint64_t *)offset_to_pointer(0, ifs.file_memory);
    dirent* root = (dirent *)allocate(sizeof(struct dirent) + strlen("hi") + 1,
                                      ifs.free_space, ifs.file_memory);
    //root = (dirent *)allocate(sizeof(struct dirent) + strlen("hi") + 1,
    //                          ifs.free_space, ifs.file_memory);
    root->prev_offset = 0;
    root->next_offset = 0;
    root->size = 3;
    char* text = (char *)((uint64_t)root + sizeof(dirent));
    strcpy(text, "hi");
    close(fd);
}
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1 回答 1

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重叠结构

结构的位置ifs与第一个结构的位置重叠root。这是因为您从free_space0 开始，所以第一个root结构将在偏移 0 处分配，正好是ifs结构所在的位置。

在分配第一个root结构之前，您应该设置free_space为sizeof(ifs).

于 2015-05-07T00:55:39.417 回答