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POHow can I read a child process's output?
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  1. POHow can I read a child process's output?
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    copied!<p>I have written a function that attempts to read a child process's command line output via a pipe. This should be a simple subset of <a href="http://msdn.microsoft.com/en-us/library/ms682499%28VS.85%29.aspx" rel="noreferrer">the MSDN Creating a Child Process with Redirected Input and Output article</a>, but I am clearly making an error of some sort. </p> <p>The ReadFile(...) call below blocks forever no matter if I place it before or after the WaitForSingleObject(...) call that should signal the end of the child process. </p> <p>I have read all the answers that suggest "Use asynchronous ReadFile" and I am open to that suggestion if someone could give me some idea how that is accomplished on a pipe. Although I don't understand why asynchronous I/O should be needed for this case.</p> <pre><code>#include "stdafx.h" #include &lt;string&gt; #include &lt;windows.h&gt; unsigned int launch( const std::string &amp; cmdline ); int _tmain(int argc, _TCHAR* argv[]) { launch( std::string("C:/windows/system32/help.exe") ); return 0; } void print_error( unsigned int err ) { char* msg = NULL; FormatMessageA( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, err, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&amp;msg, 0, NULL ); std::cout &lt;&lt; "------ Begin Error Msg ------" &lt;&lt; std::endl; std::cout &lt;&lt; msg &lt;&lt; std::endl; std::cout &lt;&lt; "------ End Error Msg ------" &lt;&lt; std::endl; LocalFree( msg ); } unsigned int launch( const std::string &amp; cmdline ) { TCHAR cl[_MAX_PATH*sizeof(TCHAR)]; memset( cl, 0, sizeof(cl) ); cmdline.copy( cl, (_MAX_PATH*sizeof(TCHAR)) - 1); HANDLE stdoutReadHandle = NULL; HANDLE stdoutWriteHandle = NULL; SECURITY_ATTRIBUTES saAttr; memset( &amp;saAttr, 0, sizeof(saAttr) ); saAttr.nLength = sizeof(SECURITY_ATTRIBUTES); saAttr.bInheritHandle = TRUE; saAttr.lpSecurityDescriptor = NULL; // Create a pipe for the child process's STDOUT. if ( ! CreatePipe(&amp;stdoutReadHandle, &amp;stdoutWriteHandle, &amp;saAttr, 5000) ) throw std::runtime_error( "StdoutRd CreatePipe" ); // Ensure the read handle to the pipe for STDOUT is not inherited. if ( ! SetHandleInformation(stdoutReadHandle, HANDLE_FLAG_INHERIT, 0) ) throw std::runtime_error( "Stdout SetHandleInformation" ); STARTUPINFO startupInfo; memset( &amp;startupInfo, 0, sizeof(startupInfo) ); startupInfo.cb = sizeof(startupInfo); startupInfo.hStdError = stdoutWriteHandle; startupInfo.hStdOutput = stdoutWriteHandle; startupInfo.hStdInput = GetStdHandle(STD_INPUT_HANDLE); startupInfo.dwFlags |= STARTF_USESTDHANDLES; char* rawEnvVars = GetEnvironmentStringsA(); //__asm _emit 0xcc; PROCESS_INFORMATION processInfo; memset( &amp;processInfo, 0, sizeof(processInfo) ); std::cout &lt;&lt; "Start [" &lt;&lt; cmdline &lt;&lt; "]" &lt;&lt; std::endl; if ( CreateProcessA( 0, &amp;cl[0], 0, 0, false, CREATE_NO_WINDOW | CREATE_UNICODE_ENVIRONMENT, rawEnvVars, 0, &amp;startupInfo, &amp;processInfo ) ) { //CloseHandle( stdoutWriteHandle ); DWORD wordsRead; char tBuf[257] = {'\0'}; bool success = true; std::string outBuf(""); unsigned int t; while(success) { //__asm _emit 0xcc; std::cout &lt;&lt; "Just before ReadFile(...)" &lt;&lt; std::endl; success = ReadFile( stdoutReadHandle, tBuf, 256, &amp;wordsRead, NULL); (t=GetLastError())?print_error(t):t=t; std::cout &lt;&lt; "Just after ReadFile(...) | read " &lt;&lt; wordsRead&lt;&lt; std::endl; std::cout &lt;&lt; "."; if( success == false ) break; outBuf += tBuf; tBuf[0] = '\0'; } std::cout &lt;&lt; "output = [" &lt;&lt; outBuf &lt;&lt; "]" &lt;&lt; std::endl; if ( WaitForSingleObject( processInfo.hProcess, INFINITE ) == WAIT_OBJECT_0 ) { unsigned int exitcode = 0; GetExitCodeProcess( processInfo.hProcess, (LPDWORD)&amp;exitcode ); std::cout &lt;&lt; "exitcode = [" &lt;&lt; exitcode &lt;&lt; "]" &lt;&lt; std::endl; //__asm _emit 0xcc; CloseHandle( processInfo.hProcess ); CloseHandle( processInfo.hThread ); return exitcode; } } else { DWORD procErr = GetLastError(); std::cout &lt;&lt; "FAILED TO CREATE PROCESS!" &lt;&lt; std::endl; print_error( procErr ); } return -1; } // end launch() </code></pre>
 

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