In Node.js how do I change (overwrite) a byte in a binary file (at a certain offset) without adding bytes in between and changing its length?
In C I would just do something like fopen() the file with "r+", fseek() to the offset where I want to change, then overwrite the bytes with fwrite(). How would the equivalent in Node.js look like?
Okay, I figured out it is actually pretty straight forward ^^
fs.open(filename, "r+", (err, fd) => {
if(!err) {
fs.write(
fd, new Uint8Array([byte]), 0, 1, offset,
(err, bw, buf) => {
if(!err) {
// succesfully wrote byte to offset
}
}
);
}
});
I had to do something like what you described recently. I had to update a URL in an executable without changing the size. The key is to use a Transform on a stream. The idea is that the Transform will read in and write out the exact data you want and only modify the bytes you specify.
Here's a Transform class that does a find and replace within a stream. The constructor takes arguments for what the start and end sequences of bytes are for the chunk that should be replaced. There is also a padValue argument that is used to maintain that same size.
import { Transform } from 'stream'
export default class FindAndReplaceTransform extends Transform {
constructor(startBuffer, endBuffer, replacementValueBuffer, padValue, options) {
super(options);
this.startBuffer = startBuffer;
this.endBuffer = endBuffer;
this.replacementValueBuffer = replacementValueBuffer;
this.padValue = padValue;
}
_findInBuffer(sourceBuffer, searchBuffer) {
let searchFound = -1;
let lengthOfPartialMatch = 0;
for (let i = 0; i < sourceBuffer.length; i++) {
for (let j = 0; j < searchBuffer.length; j++) {
if (i + j >= sourceBuffer.length) {
if (j > 0) {
lengthOfPartialMatch = j;
}
break;
}
if (sourceBuffer[i + j] !== searchBuffer[j]) {
break;
}
if (j === searchBuffer.length - 1) {
searchFound = i;
}
}
if (searchFound >= 0 || lengthOfPartialMatch > 0) {
break;
}
}
return { searchFound, lengthOfPartialMatch };
}
_doReplacement(length) {
let replacementValueBuffer = this.replacementValueBuffer;
if (this.padValue !== undefined) {
replacementValueBuffer = Buffer.concat([replacementValueBuffer, Buffer.alloc(length - replacementValueBuffer.length, this.padValue)], length);
}
this.push(replacementValueBuffer);
}
//override
_transform(data, encoding, done) {
if(this.lengthOfPartialStartMatch){
data = Buffer.concat([this.startBuffer.slice(0, this.lengthOfPartialStartMatch), data], this.lengthOfPartialStartMatch + data.length);
delete this.lengthOfPartialStartMatch;
}
if(this.lengthOfPartialEndMatch){
data = Buffer.concat([this.endBuffer.slice(0, this.lengthOfPartialEndMatch), data], this.lengthOfPartialEndMatch + data.length);
this.replacementBuffer = this.replacementBuffer.slice(0, this.replacementBuffer.length - this.lengthOfPartialEndMatch);
delete this.lengthOfPartialEndMatch;
}
let startAlreadyFound = !!this.replacementBuffer
let { searchFound: startIndex, lengthOfPartialMatch: lengthOfPartialStartMatch } = this._findInBuffer(data, this.startBuffer);
let tail = data.slice(startIndex >= 0 && !startAlreadyFound ? startIndex : 0);
let { searchFound: endIndex, lengthOfPartialMatch: lengthOfPartialEndMatch } = this._findInBuffer(tail, this.endBuffer);
if (!startAlreadyFound && startIndex >= 0) {
this.push(data.slice(0, startIndex))
this.replacementBuffer = Buffer.alloc(0);
startAlreadyFound = true;
}
if (startAlreadyFound) {
if (endIndex >= 0) {
let replacementLength = this.replacementBuffer.length + endIndex + this.endBuffer.length;
this._doReplacement(replacementLength);
delete this.replacementBuffer;
if (endIndex + this.endBuffer.length < tail.length) {
let remainder = tail.slice(endIndex + this.endBuffer.length)
this._transform(remainder, encoding, done);
return;
}
} else {
this.lengthOfPartialEndMatch = lengthOfPartialEndMatch;
this.replacementBuffer = Buffer.concat([this.replacementBuffer, tail], this.replacementBuffer.length + tail.length);
}
} else {
this.lengthOfPartialStartMatch = lengthOfPartialStartMatch;
this.push(data.slice(0, data.length - lengthOfPartialStartMatch))
}
done();
}
//override
_flush(done) {
if (this.replacementBuffer) {
this.push(this.replacementBuffer)
}
if(this.lengthOfPartialStartMatch){
this.push(this.startBuffer.slice(0, this.lengthOfPartialStartMatch));
}
delete this.replacementBuffer;
delete this.lengthOfPartialStartMatch;
delete this.lengthOfPartialEndMatch;
done()
}
}
To use the above transform, you could do something like this:
let stream = fs.createReadStream(inputFile);
let padding = 0x00;
let startSequence = Buffer.from('${', 'utf16le');
let endSequence = Buffer.from('}', 'utf16le');
let transform = new FindAndReplaceTransform(startSequence, endSequence, Buffer.from(replacementValue, 'utf16le'), paddingValue);
stream = stream.pipe(transform);
stream.pipe(fs.createWriteStream(outputFile));
Obviously, if all you want to do is change a byte at a certain offset, the Transform class will be significantly simpler. I provided that above code because I had it and if you want to do something a little more complex, you have it for reference.
The main method that you want to make sure to implement is the _transform method. You also may need to implement the _flush method depending on you implementation. The other class methods in the above code are for my implementation of the replacement code and are not needed for a Transform to work.
