fs.read at position 0 followed by position null reads wrong data

[defunctzombie]

• Version: 6.4.0
• Platform: Ubuntu 14.04 x64
• Subsystem: fs

After reading from position 0 (using fs.read), subsequence read with position null reads the incorrect data.

I would expect a read from position 0 followed by a read from null position to read from the next set of data per the documentation:

position is an integer specifying where to begin reading from in the file. If position is null, data will be read from the current file position.

Save the following to a file and run it.

const fs = require('fs');
const assert = require('assert');

// change this to toggle failing vs passing behavior
const fail = true;

fs.open(__filename, 'r', (err, fd) => {
    assert(!err);

    const buff = new Buffer(4);

    // this is the line that causes incorrect (unexpected?) behavior
    // if 0 is passed to read, then the subsequent read with 'null'
    // results in the wrong data being read
    // when `null` is used I get `cons` followed by `t fs`
    // when 0 is used first, I get `cons` followed by `cons`
    fs.read(fd, buff, 0, 4, (fail) ? 0 : null, (err, bytes, buffer) => {
        assert(!err);
        assert.equal(buffer.toString(), 'cons');
        console.log(buffer.toString());
        fs.read(fd, buff, 0, 4, null, (err, bytes, buffer) => {
            assert(!err);
            assert.equal(buffer.toString(), 't fs');
            console.log(buffer.toString());
        });
    });
});

[mscdex]

What's happening here is that when you specify a position >= 0, pread() is used which does not modify the file pointer. When you pass a null position, read() is used which starts from the current file pointer.

[defunctzombie]

Should the docs be updated to make this clear? Or would it be useful to
make the core implementation track the position internally and behave
appropriately when using a null read after a positional read?

On Friday, September 2, 2016, Brian White [email protected] wrote:

What's happening here is that when you specify a position >= 0, pread()
is used which does not modify the file pointer. When you pass a null
position, then the read occurs from the current file pointer.

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[mscdex]

/cc @nodejs/collaborators

[bnoordhuis]

Should the docs be updated to make this clear?

The documentation for fs.read() seems clear enough to me:

`position` is an integer specifying where to begin reading from in the file.
If `position` is `null`, data will be read from the current file position.

Or would it be useful to make the core implementation track the position internally and behave appropriately when using a null read after a positional read?

Node.js has worked this way since forever so I don't think we would want to change that. It's covered by fs.createReadStream(path, { start: position }).

[mscdex]

I should also note that if you need/want seeking support (to set the file pointer), there are always addons like fs-ext which provide fs.seek()/fs.seekSync(). Functions like that might be nice to have in core finally and should work on both *nix and Windows.

[bnoordhuis]

fs.seek()/fs.seekSync()

Those are really bad anti-patterns when reading or writing asynchronously because there is no consistent ordering of events. Consider:

fs.read(...);
fs.seekSync(...);

Does the seek happen before or after the read? No one knows. No one can know.

[mscdex]

@bnoordhuis Sure but if you call fs.seekSync() within the fs.read() callback (and before the next fs.read() in the original example), then it's not a problem.

What happens when you do:

fs.read(fd, buf1, 0, 4, null, ...);
fs.read(fd, buf2, 0, 4, null, ...);

? Is there a defined order in that scenario?

[bnoordhuis]

There isn't, you should use positional reads in that case.

[defunctzombie]

The documentation for fs.read() seems clear enough to me:

Disagree. It is unclear for the original example I posted. If you do a read starting at position 0, you then think (rightfully so because nothing tells you otherwise) that the file position is now 0 + N bytes (4 in my example) and that a read using null will read from where the last read ended.

There isn't, you should use positional reads in that case.

I don't disagree that this is the better strategy (to be explicit in your read positions in these cases), which is why I think the documentation should make it clearer and guide the developer in understanding that position specified reading is disjoint from null position reading.

[Trott]

Documentation improvement PR would be welcome.

/cc @nodejs/documentation

If no one already reading this is inclined to submit a PR, then perhaps it makes sense to put a good first contribution label on the issue.

[dcharbonnier]

for me something has changed on 8.2 compared to 8.1, the documentation is confusing and #8397 (comment) is correct