How to read output from cmd.exe using CreateProcess() and CreatePipe()

Question

How to read output from cmd.exe using CreateProcess() and CreatePipe()

I have been trying to create a child process executing c

Answer 1

Ian Boyd's answer had this gem: Once you've launched your child process: be sure to close those ends of the pipe you no longer need.

I've produced another version of the CreatePipe + CreateProcess solution which, I hope, is more clear:

int main()
{
    BOOL ok = TRUE;
    HANDLE hStdInPipeRead = NULL;
    HANDLE hStdInPipeWrite = NULL;
    HANDLE hStdOutPipeRead = NULL;
    HANDLE hStdOutPipeWrite = NULL;

    // Create two pipes.
    SECURITY_ATTRIBUTES sa = { sizeof(SECURITY_ATTRIBUTES), NULL, TRUE };
    ok = CreatePipe(&hStdInPipeRead, &hStdInPipeWrite, &sa, 0);
    if (ok == FALSE) return -1;
    ok = CreatePipe(&hStdOutPipeRead, &hStdOutPipeWrite, &sa, 0);
    if (ok == FALSE) return -1;

    // Create the process.
    STARTUPINFO si = { };
    si.cb = sizeof(STARTUPINFO);
    si.dwFlags = STARTF_USESTDHANDLES;
    si.hStdError = hStdOutPipeWrite;
    si.hStdOutput = hStdOutPipeWrite;
    si.hStdInput = hStdInPipeRead;
    PROCESS_INFORMATION pi = { };
    LPCWSTR lpApplicationName = L"C:\\Windows\\System32\\cmd.exe";
    LPWSTR lpCommandLine = (LPWSTR)L"C:\\Windows\\System32\\cmd.exe /c dir";
    LPSECURITY_ATTRIBUTES lpProcessAttributes = NULL;
    LPSECURITY_ATTRIBUTES lpThreadAttribute = NULL;
    BOOL bInheritHandles = TRUE;
    DWORD dwCreationFlags = 0;
    LPVOID lpEnvironment = NULL;
    LPCWSTR lpCurrentDirectory = NULL;
    ok = CreateProcess(
        lpApplicationName,
        lpCommandLine,
        lpProcessAttributes,
        lpThreadAttribute,
        bInheritHandles,
        dwCreationFlags,
        lpEnvironment,
        lpCurrentDirectory,
        &si,
        &pi);
    if (ok == FALSE) return -1;

    // Close pipes we do not need.
    CloseHandle(hStdOutPipeWrite);
    CloseHandle(hStdInPipeRead);

    // The main loop for reading output from the DIR command.
    char buf[1024 + 1] = { };
    DWORD dwRead = 0;
    DWORD dwAvail = 0;
    ok = ReadFile(hStdOutPipeRead, buf, 1024, &dwRead, NULL);
    while (ok == TRUE)
    {
        buf[dwRead] = '\0';
        OutputDebugStringA(buf);
        puts(buf);
        ok = ReadFile(hStdOutPipeRead, buf, 1024, &dwRead, NULL);
    }

    // Clean up and exit.
    CloseHandle(hStdOutPipeRead);
    CloseHandle(hStdInPipeWrite);
    DWORD dwExitCode = 0;
    GetExitCodeProcess(pi.hProcess, &dwExitCode);
    return dwExitCode;
}

Some notes:

• The pipe for StdIn is not really required:
  • This is because the DIR command doesn't require user input (but, I left it in the code, since it is a good template for running other commands)
  • everyting to do with hStdInPipeRead & hStdInPipeWrite can be omitted
  • setting si.hStdInput can be omitted
• Replace hardcoded L"C:\\Windows\\System32\\cmd.exe" with reading the COMSPEC environment variable.
• Replace LPWSTR with LPTSTR if we wish to compile for non-UNICODE.
• Replace cmd.exe /k DIR with cmd.exe /c DIR since when the DIR command finishes we don't really want the cmd.exe to stick around.