C++ Socket recv call not working as expected

Question

I am using the below code to read from a non-blocking socket, but occasionally I read 1 byte less than is shown for the same socket on wireshark. For example wireshark could show that the data received has a length of 185 as it should but the recv only gets 184 bytes.

int len = 0;
unsigned char buffer[**Twice larger than could be received**]; 
while (1) {
    int rc = recv(sock, buffer+len, sizeof(buffer) - len, 0);
    if (rc < 0) {
        if (errno == EWOULDBLOCK){
            continue;
        }
        break;
     }
     if (rc == 0) {
         break;
     }
     len += rc;
}