如何使用node.js从（大）文件中进行随机访问读取？

Question

Am I missing something or does node.js's standard file I/O module lack analogs of the usual file random access methods?

• seek() / fseek()
• tell() / ftell()

How does one read random fixed-size records from large files in node without these?

Answer 1

tell is not, but it is pretty rare to not already know the position you are at in a file, or to not have a way to keep track yourself.

seek is exposed indirectly via the position argument of fs.read and fs.write . When given, the argument will seek to that location before performing its operation, and if null , it will use whatever previous position it had.

Answer 2

node doesn't have these built in, the closest you can get is to use fs.createReadStream with a start parameter to start reading from an offset, (pass in an existing fd to avoid re-opening the file).

http://nodejs.org/api/fs.html#fs_fs_createreadstream_path_options

Answer 3

I suppose that createReadStream creates new file descriptor over and over. I prefer sync version:

function FileBuffer(path) {
const fd = fs.openSync(path, 'r');

function slice(start, end) {
    const chunkSize = end - start;
    const buffer = new Buffer(chunkSize);

    fs.readSync(fd, buffer, 0, chunkSize, start);

    return buffer;
}

function close() {
    fs.close(fd);
}

return {
    slice,
    close
}

}

Answer 4

Use this:

fs.open(path, flags[, mode], callback)

Then this:

fs.read(fd, buffer, offset, length, position, callback)

Read this for details:

https://nodejs.org/api/fs.html#fs_fs_read_fd_buffer_offset_length_position_callback