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POWhy would fclose hang / deadlock? (Windows)
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copied!<p>I have a directory change monitor process that reads updates from files within a set of directories. I have another process that performs small writes to a lot of files to those directories (test program). Figure about 100 directories with 10 files in each, and about 500 files being modified per second.</p> <p>After running for a while, the directory monitor process hangs on a call to <code>fclose()</code> in a method that is basically tailing the file. In this method, I <code>fopen()</code> the file, check that the handle is valid, do a few seeks and reads, and then call <code>fclose()</code>. These reads are all performed by the same thread in the process. After the hang, the thread never progresses.</p> <p>I couldn't find any good information on why <code>fclose()</code> might deadlock instead of returning some kind of error code. The documentation does mention <code>_fclose_nolock()</code>, but it doesn't seem to be available to me (Visual Studio 2003).</p> <p>The hang occurs for both debug and release builds. In a debug build, I can see that <code>fclose()</code> calls <code>_free_base()</code>, which hangs before returning. Some kind of call into kernel32.dll => ntdll.dll => KernelBase.dll => ntdll.dll is spinning. Here's the assembly from ntdll.dll that loops indefinitely:</p> <pre><code>77CEB83F cmp dword ptr [edi+4Ch],0 77CEB843 lea esi,[ebx-8] 77CEB846 je 77CEB85E 77CEB848 mov eax,dword ptr [edi+50h] 77CEB84B xor dword ptr [esi],eax 77CEB84D mov al,byte ptr [esi+2] 77CEB850 xor al,byte ptr [esi+1] 77CEB853 xor al,byte ptr [esi] 77CEB855 cmp byte ptr [esi+3],al 77CEB858 jne 77D19A0B 77CEB85E mov eax,200h 77CEB863 cmp word ptr [esi],ax 77CEB866 ja 77CEB815 77CEB868 cmp dword ptr [edi+4Ch],0 77CEB86C je 77CEB87E 77CEB86E mov al,byte ptr [esi+2] 77CEB871 xor al,byte ptr [esi+1] 77CEB874 xor al,byte ptr [esi] 77CEB876 mov byte ptr [esi+3],al 77CEB879 mov eax,dword ptr [edi+50h] 77CEB87C xor dword ptr [esi],eax 77CEB87E mov ebx,dword ptr [ebx+4] 77CEB881 lea eax,[edi+0C4h] 77CEB887 cmp ebx,eax 77CEB889 jne 77CEB83F </code></pre> <p>Any ideas what might be happening here?</p>

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