A simple Native Message Host for Chromium that echoes received messages.

NativeHostEcho.cc

/*
* This is a simple Native Message Host application. It echoes any messages
* it receives. After receiving and echoing a `{"quit":1}` message, the app
* will exit.
*/
#include <fcntl.h>
#include <io.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef union {
    unsigned long as_ulong;
    unsigned char as_bytes[4];
} MessageLength;

int main(int argc, char argv[])
{
    // Set stdio streams to unbuffered binary mode to avoid
    // corrupting messages.
    _setmode(_fileno(stdin), _O_BINARY);
    setvbuf(stdin, NULL, _IONBF, 0);
    _setmode(_fileno(stdout), _O_BINARY);
    setvbuf(stdout, NULL, _IONBF, 0);

    size_t bytes_read = 0;
    MessageLength message_length;
    message_length.as_ulong = 0;
    char* message_body = nullptr;

    // Read the size from stdin.
    bytes_read = fread(message_length.as_bytes, 1, 4, stdin);

    // Read messages in a loop.
    while (bytes_read == 4)
    {
        bool got_exit_signal = false;
        int iLen = (int)message_length.as_ulong;
        if (iLen > 0) {
            // Read the message body.
            message_body = (char*)malloc(iLen);
            bytes_read = fread(message_body, 1, iLen, stdin);

            // Check for the exit signal.
            got_exit_signal = !_strnicmp(message_body, "{\"quit\":1}", iLen);

            // Reply by echo'ing the message.
            fwrite(message_length.as_bytes, 1, 4, stdout);
            fwrite(message_body, 1, iLen, stdout);
            fflush(stdout);
        }
        else {
            break;
        }

        if (message_body != nullptr) free(message_body);
        // If we were asked to exit, do that now.
        if (got_exit_signal) break;
        // Otherwise, read the next message.
        bytes_read = fread(message_length.as_bytes, 1, 4, stdin);
    }
}