Missing data when reading from C++ named pipe in node.js

Question

I cobbled together a simple C++ app that dumps HID keycodes from /dev/input/event[x] into a named pipe on Linux. It logs to the console fine but when I read the named pipe from my node.js app, it randomly misses data events.

Relevant C++ code:

int fd;
char * myfifo = "/tmp/testfifo";
mkfifo(myfifo, 0660);
fd = open(myfifo, O_WRONLY);

while (1){

    value = ev[0].value;

    if (value != ' ' && ev[1].value == 1 && ev[1].type == 1) {

        string s = to_string(ev[1].code);
        char const *sop = (s + "\n").c_str();

        cout << sop;
        write(fd, sop, sizeof(sop));
    }
}

Relevant node.js code:

var fifo = '/tmp/testfifo';    
var fd = fs.openSync(fifo, 'r+');

fs.createReadStream(null, {fd:fd}).on('data', function (d) {
    console.log(d);
});

I'm guessing my method for reading the named pipe is flawed since the C++ output looks good but I know almost nothing about C++ so am not sure if I'm flushing the pipe properly on the C++ side or there is some sort of read throttle I need to tweak on the node.js side. Any ideas?

Value looks good. I think it's definitely on the node.js side because when I use spawn to tail the same pipe it works great. Very interesting. — mr.freeze
– mr.freeze, Commented Sep 26, 2017 at 0:08
This javascript works for me, even with simpler fs.createReadStream('/tmp/testfifo').on(...). — Maxim Egorushkin
– Maxim Egorushkin, Commented Sep 26, 2017 at 17:24

Maxim Egorushkin · Accepted Answer · 2017-09-25 15:53:06Z

1

A couple of errors:

Statement char const *sop = (s + "\n").c_str(); produces a dangling reference because the temporary string produced by (s + "\n") gets destroyed after the statement has been evaluated.
write(fd, sop, sizeof(sop)); writes sizeof(char const*) bytes, whereas it should write strlen(sop) bytes.

A fix:

std::string sop = s + "\n";
write(fd, sop.data(), sop.size());

edited Sep 25, 2017 at 15:53

answered Sep 25, 2017 at 15:47

Maxim Egorushkin

138k19 gold badges201 silver badges293 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

mr.freeze Over a year ago

Thank you for the reply. Unfortunately that change didn't have any effect.

Collectives™ on Stack Overflow

Missing data when reading from C++ named pipe in node.js

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related