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POhow does putty detect if xfer is backing up?
 

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    <p>So I'm trying to understand how PuTTY is implementing SCP and have a few questions.</p> <p>First, here's a copy of an older and (imho) easier to read version of PuTTY's SCP implementation:</p> <p><a href="https://github.com/Yasushi/putty/blob/4eeae4a39bc9faf539b8f819c12b1d1b9f22fc86/scp.c" rel="nofollow">https://github.com/Yasushi/putty/blob/4eeae4a39bc9faf539b8f819c12b1d1b9f22fc86/scp.c</a></p> <p>Here's the function that sends data:</p> <pre><code>int scp_send_filedata(char *data, int len) { int bufsize = back-&gt;send(data, len); /* * If the network transfer is backing up - that is, the remote * site is not accepting data as fast as we can produce it - * then we must loop on network events until we have space in * the buffer again. */ while (bufsize &gt; MAX_SCP_BUFSIZE) { if (!scp_process_network_event()) return 1; bufsize = back-&gt;sendbuffer(); } return 0; } </code></pre> <p>Here's scp_process_network_event:</p> <pre><code>static int scp_process_network_event(void) { fd_set readfds; FD_ZERO(&amp;readfds); FD_SET(scp_ssh_socket, &amp;readfds); if (select(1, &amp;readfds, NULL, NULL, NULL) &lt; 0) return 0; /* doom */ select_result((WPARAM) scp_ssh_socket, (LPARAM) FD_READ); return 1; } </code></pre> <p>So scp_process_network_event does a select() system call, blocking until a write operation wouldn't block?</p> <p>I'm thinking back->sendbuffer and back->send correspond to this:</p> <p><a href="https://github.com/Yasushi/putty/blob/4eeae4a39bc9faf539b8f819c12b1d1b9f22fc86/ssh.c" rel="nofollow">https://github.com/Yasushi/putty/blob/4eeae4a39bc9faf539b8f819c12b1d1b9f22fc86/ssh.c</a></p> <pre><code>/* * Called to send data down the Telnet connection. */ static int ssh_send(char *buf, int len) { if (s == NULL || ssh_protocol == NULL) return 0; ssh_protocol(buf, len, 0); return ssh_sendbuffer(); } /* * Called to query the current amount of buffered stdin data. */ static int ssh_sendbuffer(void) { int override_value; if (s == NULL || ssh_protocol == NULL) return 0; /* * If the SSH socket itself has backed up, add the total backup * size on that to any individual buffer on the stdin channel. */ override_value = 0; if (ssh_throttled_all) override_value = ssh_overall_bufsize; if (ssh_version == 1) { return override_value; } else if (ssh_version == 2) { if (!mainchan || mainchan-&gt;closes &gt; 0) return override_value; else return override_value + bufchain_size(&amp;mainchan-&gt;v.v2.outbuffer); } return 0; } </code></pre> <p>I'm not completely sure on that though and tbh I'm not very good at C either :(</p> <p>Anyway it's not entirely clear to me what bufsize is supposed to be.</p> <p>Any ideas?</p> <p>Thanks!</p>
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