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POLinux CentOS 5: non-blocking socket send hangs indefenitely
 

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  1. POLinux CentOS 5: non-blocking socket send hangs indefenitely
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    <p>I have the following C++ code in linux:</p> <pre><code>if (epoll_wait(hEvent,&amp;netEvents,1,0)) { // check FIRST for disconnection to avoid send() to a closed socket (halts on centos on my server!) if ((netEvents.events &amp; EPOLLERR)||(netEvents.events &amp; EPOLLHUP)||(netEvents.events &amp; EPOLLRDHUP)) { save_log("&gt; client terminated connection"); goto connection_ended; // ---[ if its a CLOSE event .. close :) } if (netEvents.events &amp; EPOLLOUT) // ---[ if socket is available for write { if (send_len) { result = send(s,buffer,send_len,MSG_NOSIGNAL); save_slogf("1112:send (s=%d,len=%d,ret=%d,errno=%d,epoll=%d,events=%d)",s,send_len,result,errno,hEvent,netEvents.events); if (result &gt; 0) { send_len = 0; current_stage = CL_STAGE_USE_LINK_BRIDGE; if (close_after_send_response) { save_log("&gt; destination machine closed connection"); close_after_send_response = false; goto connection_ended; } } else { if (errno == EAGAIN) return; else if (errno == EWOULDBLOCK) return; else { save_log("&gt; unexpected error on socket, terminating"); connection_ended: close_client(); reset(); return; } } } } } } </code></pre> <p>hEvent: epoll created to listen to EPOLLIN,EPOLLOUT,EPOLLERR,EPOLLHUP,EPOLLRDHUP</p> <p>s: NON-BLOCKING (!!!) socket created from an accept on a nonblocking listening socket</p> <p>Basically this code is attempting to send a packet back to a connected user that connected to a server. It usually works ok but on RANDOM occasions (perhaps when some wierd network event happends) the program hangs indefinitely on the "result = send(s,buffer,send_len,MSG_NOSIGNAL) line.</p> <p>I have no idea what may be the cause for this, I have tried to monitor the socket operations and nothing seemed to give me a hint of a clue as to why it happends. I have to assume this is either a KERNEL bug or something very wierd because I have the same program written under Windows and it works perfect there.</p>
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    1. USMiki Berkovich
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    1. COYou have to explicitly make 's' non-blocking if you haven't done so, it doesn't inherit non-blocking-ness from the listening socket.
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      1. POLinux CentOS 5: non-blocking socket send hangs indefenitely
      1. USAmbroz Bizjak
    2. COI have tried doing it by adding a "fcntl(socket, F_SETFL, flags | O_NONBLOCK);" on the accepted socket right after it is created by accept() .. but now the information that is sent back to the client seems to be corrupted (I am sending a FLV file stream.. it worked fine before I added this fcntl)... why can it be?
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      1. POLinux CentOS 5: non-blocking socket send hangs indefenitely
      1. USMiki Berkovich
    3. COIn non-blocking mode, it is normal behavior that send() manages to send less data than you asked it to, and that recv() manages to receive less data than you asked it to. You have to use buffers to get sending/receiving right. For all you know, recv() could be giving data to you one byte at a time.
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      1. POLinux CentOS 5: non-blocking socket send hangs indefenitely
      1. USAmbroz Bizjak
 

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