posix-read

posix-read

Do POSIX read on files and sockets with Node.js.

Motivation

In Node.js, reading from sockets is performed using uv_read_start() from libuv. This consumes data from the file descriptor whenever it is available, but consumes it all. There is not built-in way to read a fixed amount, let's say n bytes.

This is a problem when you want to read only n bytes and leave the rest in the socket (so that, for instance, another process reads remaining data).

posix-read is a module to perform a POSIX read on a socket. That way, only n bytes are brought up to user-space and the rest remains in kernel-space, ready to be read(2) by any other process.

Usage

Warning

For posix-read to work, your code must prevent libuv to start reading from the socket. That means the socket must have the pauseOnCreate property.

In practice: if you get the socket from a net.Server, this server has to be created with the pauseOnConnect set to true.

Example

const net = require('net');
const posixRead = require('posix-read');

const server = net.createServer({ pauseOnConnect: true }, function (socket) {
    // Just got an incoming connection. Let's read 10 bytes from it (but DO NOT
    // consume more than 10 bytes from the socket).
    posixRead.read(socket, 10, function (err, buffer) {
        if (err && err.endOfFile)
            return process.stderr.write('peer sent less than 10 bytes\n');
        if (err)
            return process.stderr.write(`error: ${err}\n`);

        return process.stdout.write(`10 first bytes: ${buffer}\n`);
    });
}).listen(1234);

Error types

If a problem happens, the Error object passed to the callback has helpful properties:

• error.badStream === true if the socket is malformed or its file descriptor is not available
• error.endOfFile === true if the end-of-file was reached before having read all the bytes requested
• error.systemError === true in case of a system call error (in such a case, error.message should contain more useful information.

License

MIT license