I am creating a child process, and reading its output. My code works fine when the child process creates output (cmd /c echo Hello World
), however ReadFile will hang if process does not create output (cmd /c echo Hello World > output.txt
). I am only reading after the process has terminated.
Am I doing something horribly wrong? Is there anyway to do this with synchronous mode, or do I have to use asynchronous mode? All of this is happening in a seperate thread, so I dont think asynchronous mode would offer any benefit to me, unless it is the only way to get this to work. Thanks a lot!
saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
saAttr.bInheritHandle = TRUE;
saAttr.lpSecurityDescriptor = NULL;
CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr, 0);
SetHandleInformation(g_hChildStd_OUT_Rd, HANDLE_FLAG_INHERIT, 0);
memset(&piProcInfo, 0, sizeof(PROCESS_INFORMATION));
memset(&siStartInfo, 0, sizeof(STARTUPINFO));
siStartInfo.cb = sizeof(STARTUPINFO);
siStartInfo.hStdError = g_hChildStd_OUT_Wr;
siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
siStartInfo.dwFlags |= STARTF_USESTDHANDLES;
CreateProcess(NULL, commandWideString, NULL, NULL, TRUE, 0, NULL, NULL, &siStartInfo, &piProcInfo);
while(1)
{
GetExitCodeProcess(piProcInfo.hProcess, &processExitCode);
if(processExitCode != STILL_ACTIVE)
break;
else
Sleep(1);
}
*output = (char *)calloc(32, sizeof(char));
processOutputSize = 0;
while(1)
{
bSuccess = ReadFile( g_hChildStd_OUT_Rd, processOutputTemp, 32, &dwRead, NULL);
if(!bSuccess || !dwRead)
break;
memcpy(*output + processOutputSize, processOutputTemp, dwRead);
processOutputSize += dwRead;
if(dwRead == 32)
*output = (char *)realloc(*output, processOutputSize + 32);
else
{
memset(*output + processOutputSize, 0, 1);
break;
}
}
CloseHandle(piProcInfo.hProcess);
CloseHandle(piProcInfo.hThread);
CloseHandle(g_hChildStd_OUT_Rd);
CloseHandle(g_hChildStd_OUT_Wr);
You should close the write end of the output pipe before you read from it, as @Marcus suggested in the comment.
CloseHandle(g_hChildStd_OUT_Wr);
For me this is the real answer.
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