I've encountered an issue where sending data from a typed array to a subprocess's stdin gets truncated if the data in the typed array is changed. Below is a small example demonstrating this problem.
The subprocess in question is a simple C++ app that reads all data from stdin and outputs the number of read bytes to stdout. The code for the C++ subprocess is as follows:
#include <iostream>
#include <limits>
int main(int argc, char **args) {
std::cin.ignore(std::numeric_limits<std::streamsize>::max());
std::cout << std::cin.gcount() << std::flush;
return 0;
}
The Node.js app repeatedly calls this executable, passes 128 bytes of data to stdin, and checks if the response from the child process's stdout is "128". The code for the Node.js app is as follows:
const execFile = require('child_process').execFile;
var data = new Float64Array(16);
var generateError = true;
generateError && setInterval(_ => {
for (var i = 0; i < 16; i++) {
data[i] = Math.random();
}
}, 1);
setInterval(_ => {
var subproc = execFile('a.exe', [ '-c' ], (error, stdout, stderr) => {
if (stdout.trim() !== '128' || error) {
console.log('ERR: ' + stdout);
} else {
console.log('OK');
}
});
subproc.stdin.write(Buffer.from(data.buffer)) || console.log('Use drain'); // Write Binary Data (128 Byte)
subproc.stdin.end();
}, 100);
The output I receive is as follows:
OK
OK
OK
OK
OK
OK
ERR: 69
ERR: 114
OK
OK
OK
ERR: 41
What puzzles me are the following points:
- When I set
generateErrorto false, everything works as expected. - Using "
wc.exe -c" (Word/Bytecount Tool from Cygwin) also yields the expected results, even whengenerateErroris set to true.
My question is: What's causing this issue? Is it the C++ code or the Node.js code? Are we not allowed to change the buffer after passing it to subproc.stdin.write? If so, why does the variant with wc.exe work seamlessly?