从 c++ 向铬扩展 [Native Messaging] 发送消息时的奇怪行为 - 没有发送数据

Question

Hi and hope everyone is doing good!

I started working on a chrome extension which talks to a cpp exe. At the moment I am just trying to read some data from the extension which works as expected using the following code

unsigned int length = 0;

{
    unsigned int read_char = getchar();
    length = length | (read_char << index*8);
}

std::string message = "";
for (int index = 0; index < length; index++)
{
    message += getchar();
}

When trying to send the same information from the cpp -> extension using the following code

unsigned int len = message.length();


printf("%c%c%c%c",  (char) (len & 0xff),
                    (char) (len << 8 & 0xff),
                    (char) (len << 16 & 0xff),
                    (char) (len << 24 & 0xff));


printf("%s", message.c_str());

it works as expected Screenshot of the working scenario

However, when i try to send back another string rather than the same string that we send it won't work. Example:

    unsigned int length = 0;


for (int index = 0; index < 4; index++)
{
    unsigned int read_char = getchar();
    cerr << read_char << endl;
    length = length | (read_char << index*8);
}

std::string message = "";
for (int index = 0; index < length; index++)
{
    message += getchar();
}

std::string outputMessage = "output";

unsigned int len = outputMessage.length();


printf("%c%c%c%c",  (char) (len & 0xff),
                    (char) (len << 8 & 0xff),
                    (char) (len << 16 & 0xff),
                    (char) (len << 24 & 0xff));


printf("%s", outputMessage.c_str());



return 0;

Here is also the screenshot of the scenario that is failing. Failed scenario

---

I am not sure what I missed here, consider that I am just creating a new string and also using it's length for information that the extension needs.

Anyway, it would be great if anyone could look at this one. Thanks in advance:)

UPDATE: After trying different ways of returning messages I found this on the web and it works as expected

message_t encode_message(json content) {
    string encoded_content = content.dump();
    message_t m;
    m.content = encoded_content;
    m.length = (uint32_t)encoded_content.length();
    return m;
}

// Send an encoded message to stdout.
void send_message(message_t encoded_message) {
    char* raw_length = reinterpret_cast<char*>(&encoded_message.length);
    fwrite(raw_length, 4, sizeof(char), stdout);
    fwrite(encoded_message.content.c_str(), encoded_message.length, sizeof(char), stdout);
    fflush(stdout);
}

Answer 1

You have to explicitly flush the buffer, if those data are to be sent to the native messaging host ASAP as shown below:

std::string outputMessage = "output";

unsigned int len = outputMessage.length();

printf("%c%c%c%c", (char) (len & 0xff),
                   (char) ((len >> 8) & 0xff),
                   (char) ((len >> 16) & 0xff),
                   (char) ((len >> 24) & 0xff));


printf("%s", outputMessage.c_str());
fflush(stdout); // This will flush the buffer