`memcpy`不复制所有字节

时间:2019-09-28 17:48:47

标签: c memory memcpy

我试图将结构的字节复制到充当缓冲区的char *上。为此,我将struct(vdnxfs_node)转换为char数组。该结构的长度为64个字节,而缓冲区为1024个字节。
但是,当执行memcpy时,仅将结构的前4个字节复制到缓冲区中。

文件:fs.c

int mkfs(uint8_t _dev, uint8_t _mode)
{
    // VDNXFS_BUFFER_SZ = 1024
    char* buf = (char*)malloc(VDNXFS_BUFFER_SZ);

    // Other code non relevant to the question...

    // vdnxfs_init_new_node(); returns an `vdnxfs_node*` initialized
    // as `calloc(1, sizeof(vdnxfs_node));`
    vdnxfs_node* node = vdnxfs_init_new_node();
    node->st_mode = VDNXFS_ST_IRWXU | VDNXFS_ST_IRGRP | VDNXFS_ST_IXGRP | VDNXFS_ST_IROTH | VDNXFS_ST_IXOTH;
    node->st_uid = 1;
    node->st_gid = 1;
    node->st_pid = 0;
    node->st_size = 0;
    node->st_slnk = 1;
    node->st_atime = 0xffff;
    node->st_mtime = 0xffff;
    node->st_ctime = 0xffff;
    strcpy(node->st_name, "ROOT_DIR\0");

    // With this I get the array of bytes(char) of the struct
    char* node_buf = encode_dnxfs_node(node);

    // Updating the buffer
    memcpy(buf, node_buf, sizeof(vdnxfs_node));

    // Here I write the content of `buf` to a file.
    if(!vdnxfs_write_disk(_dev, block, off, buf))
    {
        printf("Couldn't write superblock to disk `sda%d`.", _dev);
        return -1;
    }
    free(buf);
    free(node_buf);
}

文件:stat.h

typedef struct __vdnxfs_node
{
    uint16_t st_mode;
    uint16_t st_uid;
    uint16_t st_gid;
    uint32_t st_pid; // Parent ID
    uint32_t st_size;
    uint8_t st_slnk; // Number of symbolic links
    time_t st_atime;
    time_t st_mtime;
    time_t st_ctime;
    char st_name[16];
}vdnxfs_node;

文件:stat.c
简介:将节点的所有字节转换为字符数组

char* encode_dnxfs_node(vdnxfs_node* node)
{
    if(!node) // Node is null
    {
        return NULL;
    }
    char* buffer = (char*)malloc(sizeof(vdnxfs_node));

    if(!buffer)
    {
        printf("Failed to allocate buffer for encoding.");
        return NULL;
    }

    memcpy(buffer, node, sizeof(vdnxfs_node));

    return buffer;
}

文件:disk.c
简介:将_buf的内容写入文件。磁盘上写入的内容始终为1024 bytes中定义的VDNXFS_BUFFER_SZ_off是块内的偏移量。

char* vdnxfs_write_disk(uint8_t _dev, size_t _block, size_t _off, char* _buf)
{
    char* fl_name = (char*)calloc(1, 7);
    strcpy(fl_name, vdnxfs_disks[_dev]->fl_name);

    FILE* fp_buf = fopen(fl_name, "r+b");

    if(!fp_buf)
    {
        printf("DISK DRIVER: Couldn't open Disk %s\n", fl_name);
        return NULL;
    }

    // If succesful
    if(!fseek(fp_buf, (_block * VDNXFS_BUFFER_SZ) + _off, SEEK_SET))
    {
        fputs(_buf, fp_buf);
    }else { return NULL; }

    fclose(fp_buf);

    return _buf;
}

sizeof(vdnxfs_node)返回64,表示未填充。 我确实认为问题不在实际的memcpy中,而是在我的vdnxfs_write_disk(...)函数中,即使我已经通过编写空字符和随机字符进行了测试,并且可以按预期工作。在我的测试中,我在文件上写入了1024个a字符,并且可以正常工作。 因此,当我尝试将memcpy node_buf插入buf时出现我的问题,或者我丢失了一些东西。

注意:如果按字符打印node_buf char,我将得到以下结果:

φ                                           ROOT_DIR

但是,当使用十六进制编辑器分析文件时,该块的前4个字节为:

ED 01 01 00

其余全部为00

所有内容都以小端模式存储。 01ED的确是node->st_mode。我确定这是正确的,因为我使用的是Linux权限,并且7551ED的十六进制。

1 个答案:

答案 0 :(得分:0)

过了一会儿,我发现我传递给vdnxfs_write_disk的缓冲区大小是错误的(我传递的是VDXNFS_BUFFER_SZ而不是我想要写入的数据的大小)。我还将错误的偏移量和块传递给vdnxfs_write_disk。 我实际上是为块参数传递1,并且由于fseek的偏移量计算为(block * VDNXFS_BUFFER_SZ) + off,因此fseek的偏移量超过了1024 bytes,这是错误的

如果有人感兴趣,这是我对mkfs的最终(固定)结果:

int mkfs(uint8_t _dev, uint8_t _mode)
{
    char* buf = (char*)malloc(sizeof(size_t));
    char* signature_buf = vdnxfs_read_disk(_dev, 0, 4, 4);
    memcpy(buf, signature_buf, sizeof(size_t));
    free(signature_buf);

    // Signing the disk with the appopriate signature.
    size_t signature = _mode == vdnxfs_system ? VDNXFS_BDSK_SIGNATURE : VDNXFS_DDSK_SIGNATURE;
    // printf("Signature: %X\n", signature);

    for(int i = 0; i < 4; i++)
    {
        // Disk signature starts at 4th byte
        buf[i] = ((signature >> (8 * i)) & 0xff);
    }

    // Updating the content to disk
    if(!vdnxfs_write_disk(_dev, 0, 4, buf, 4))
    {
        printf("Couldn't sign disk `sda%d`.", _dev);
        return -1;
    }

    // Getting the Super Block based on the mode.
    // Boot/System FS stores the superblock from bytes 1024 to 2047
    // Data FS stores the superblock from bytes 512 to 1535
    size_t block = 0;
    size_t off = _mode == vdnxfs_system ? VDNXFS_BFS_SBLK_START : VDNXFS_DFS_SBLK_START;

    vdnxfs_node* node = (vdnxfs_node*)calloc(1, 64);
    node->st_mode = VDNXFS_ST_IRWXU | VDNXFS_ST_IRGRP | VDNXFS_ST_IXGRP | VDNXFS_ST_IROTH | VDNXFS_ST_IXOTH;
    node->st_uid = 1;
    node->st_gid = 1;
    node->st_pid = 0;
    node->st_size = 0;
    node->st_slnk = 1;
    node->st_atime = 0xffff;
    node->st_mtime = 0xffff;
    node->st_ctime = 0xffff;
    strcpy(node->st_name, "ROOT_DIR");
    // Updating the superblock to disk
    if(!vdnxfs_write_disk(_dev, block, off, encode_dnxfs_node(node), 64))
    {
        printf("Couldn't write superblock to disk `sda%d`.", _dev);
        free(node);
        free(buf);
        return -1;
    }

    free(node);
    free(buf);
}