C++ Websocket recv 2 bytes - c++

Question

Helo. I'm working on a C++ Websocket library. All was ok until one strange problem appeared.
int n = 0, n_add = 0;
char *buf = (char*)malloc(BUFLEN);
char new_buffer[4096];
while ((n = recv(client_id, buf, BUFLEN, 0)) > 0) {
strcat(new_buffer, buf);
int new_buffer_length = strlen(new_buffer);
int buf_length = strlen(buf);
n_add+= n;
// debug
cout << "buf: '" << buf << "'" << endl;
cout << "new_buffer_length: '" << new_buffer_length << "'" << endl;
cout << "buf_length: '" << buf_length << "'" << endl;
cout << "n: '" << n << "'" << endl;
cout << "n_add: '" << n_add << "'" << endl;
memset(buf, '\0', BUFLEN);
if (n_add == new_buffer_length && n < BUFLEN) {
cout << "new_buffer: '" << new_buffer << "'" << endl;
// if client is already connected
if (ws_clients[client_id][2] == WS_READY_STATE_OPEN) {
this->ws_client_message(client_id, new_buffer, new_buffer_length);
}
// if client needs a handshake
if (ws_clients[client_id][2] == WS_READY_STATE_CONNECTING) {
this->ws_client_handshake(client_id, new_buffer);
}
memset(&new_buffer, '\0', 4096);
n_add = 0;
FD_ZERO(&this->tmp_fds);
}
}
The handshake is done perfectly aswell as any payload that is less than 126. When that happends i get this:
buf: 'þ'
new_buffer_length: '2'
buf_length: '2'
n: '128'
n_add: '128'
buf: '½HÆA¯J'
new_buffer_length: '8'
buf_length: '6'
n: '6'
n_add: '134'
The n says i recived 128 bytes but it's actually only 2, the second time gives me 6 bytes, and those are ok. If i change my BUFLEN which is limited to 128 and put to 2, i'm getting everything ok, except for the last loop, it never gets to 134, the actual payload length.
Ok so if everybody has any idea, i'm using http://www.websocket.org/echo.html for testing, i tried everything. please give me some hints

Answer 1

The data framing section of RFC6455 shows that websocket messages are not plain text and are not null-terminated strings. You can't use C string handling functions like strlen or strcat on messages you read.
If you want to keep using C strings, use n to determine the number of bytes you have read and strncpy to add that to a buffer. Since you're writing a C++ server though, you'll probably find things easier if you switch to using std::string instead.
The difference in handling messages around 128 chars probably comes from the fact that the number of bytes used to indicate the length of the message increases for payloads longer than 125 bytes. For messages between 126 and 255 bytes long, there will be a zero byte in the extended payload length. Your code will (incorrectly) interpret this as the terminator of a C string.

Answer 2

A char* is not necessarily a C string, It has to be properly terminated.
while ((n = recv(client_id, buf, BUFLEN, 0)) > 0) {
strcat(new_buffer, buf);
....
This spells problem, whatever received in buf, may not be '\0' terminated.
and when you call strcat, it wouldn't work.
checkout the manual of strcat and strncat function, make sure you use these function correctly.