kernel-lite-mini-extend-file-lit-guide.md

# 开发指导<a name="ZH-CN_TOPIC_0000001152860497"></a>

移植LittleFS到新硬件设备上，需要申明lfs\_config：

```
const struct lfs_config cfg = {
    // block device operations
    .read  = user_provided_block_device_read,
    .prog  = user_provided_block_device_prog,
    .erase = user_provided_block_device_erase,
    .sync  = user_provided_block_device_sync,

    // block device configuration
    .read_size = 16,
    .prog_size = 16,
    .block_size = 4096,
    .block_count = 128,
    .cache_size = 16,
    .lookahead_size = 16,
    .block_cycles = 500,
};
```

其中.read，.prog，.erase，.sync分别对应该硬件平台上的底层的读写\\擦除\\同步等接口。

read\_size 每次读取的字节数，可以比物理读单元大以改善性能，这个数值决定了读缓存的大小，但值太大会带来更多的内存消耗。

prog\_size 每次写入的字节数，可以比物理写单元大以改善性能，这个数值决定了写缓存的大小，必须是read\_size的整数倍，但值太大会带来更多的内存消耗。

block\_size 每个擦除块的字节数，可以比物理擦除单元大，但此数值应尽可能小因为每个文件至少会占用一个块。必须是prog\_size的整数倍。

block\_count 可以被擦除的块数量，这取决于块设备的容量及擦除块的大小。

### 示例代码<a name="section4670115211265"></a>

代码实现如下：

```
int main(void) {
    // mount the filesystem
    int err = lfs_mount(&lfs, &cfg);

    // reformat if we can't mount the filesystem
    // this should only happen on the first boot
    if (err) {
        lfs_format(&lfs, &cfg);
        lfs_mount(&lfs, &cfg);
    }

    // read current count
    uint32_t boot_count = 0;
    lfs_file_open(&lfs, &file, "boot_count", LFS_O_RDWR | LFS_O_CREAT);
    lfs_file_read(&lfs, &file, &boot_count, sizeof(boot_count));

    // update boot count
    boot_count += 1;
    lfs_file_rewind(&lfs, &file);
    lfs_file_write(&lfs, &file, &boot_count, sizeof(boot_count));

    // remember the storage is not updated until the file is closed successfully
    lfs_file_close(&lfs, &file);

    // release any resources we were using
    lfs_unmount(&lfs);

    // print the boot count
    printf("boot_count: %d\n", boot_count);
}
```