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copied!<p>The method that Beej is alluding to, and AlastairG mentions, works something like this:</p> <p>For each concurrent connection, you maintain a buffer of read-but-not-yet-processed data. (This is the buffer that Beej suggests sizing to twice the maximum packet length). Obviously, the buffer starts off empty:</p> <pre><code>unsigned char recv_buffer[BUF_SIZE]; size_t recv_len = 0; </code></pre> <p>Whenever your socket is readable, read into the remaining space in the buffer, then immediately try and process what you have:</p> <pre><code>result = recv(sock, recv_buffer + recv_len, BUF_SIZE - recv_len, 0); if (result > 0) { recv_len += result; process_buffer(recv_buffer, &recv_len); } </code></pre> <p>The <code>process_buffer()</code> will <em>try</em> and process the data in the buffer as a packet. If the buffer doesn't contain a full packet yet, it just returns - otherwise, it processes the data and removes it from the buffer. So for you example protocol, it would look something like:</p> <pre><code>void process_buffer(unsigned char *buffer, size_t *len) { while (*len >= 3) { /* We have at least 3 bytes, so we have the payload length */ unsigned payload_len = buffer[2]; if (*len < 3 + payload_len) { /* Too short - haven't recieved whole payload yet */ break; } /* OK - execute command */ do_command(buffer[0], buffer[1], payload_len, &buffer[3]); /* Now shuffle the remaining data in the buffer back to the start */ *len -= 3 + payload_len; if (*len > 0) memmove(buffer, buffer + 3 + payload_len, *len); } } </code></pre> <p>(The <code>do_command()</code> function would check for a valid header and command byte).</p> <p>This kind of technique ends up being necessary, because <em>any</em> <code>recv()</code> can return a short length - with your proposed method, what happens if your payload length is 500, but the next <code>recv()</code> only returns you 400 bytes? You'll have to save those 400 bytes until the next time the socket becomes readable anyway.</p> <p>When you handle multiple concurrent clients, you simply have one <code>recv_buffer</code> and <code>recv_len</code> per client, and stuff them into a per-client struct (which likely contains other things too - like the client's socket, perhaps their source address, current state etc.).</p>

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