How correctly call "telnet" with CreateProcess function? - winapi

Question

I want to start "telnet" through cmd.exe and write the result of this command execution into file. When I run "ipconfig" command I get all information I need, but after running of "telnet" command I get just empty file.
Here is my code:
#include "windows.h"
#include "iostream"
void SaveResult(const wchar_t *fileName, const wchar_t *commandLine)
{
SECURITY_ATTRIBUTES sa;
sa.nLength = sizeof(sa);
sa.lpSecurityDescriptor = NULL;
sa.bInheritHandle = TRUE;
HANDLE h = CreateFile(fileName, FILE_WRITE_DATA, FILE_SHARE_WRITE | FILE_SHARE_READ,
&sa, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
if (h == INVALID_HANDLE_VALUE)
return;
PROCESS_INFORMATION pi = { 0 };
STARTUPINFO si = { sizeof(si) };
si.dwFlags |= STARTF_USESTDHANDLES;
si.hStdInput = NULL;
si.hStdError = h;
si.hStdOutput = h;
wchar_t *writable_cmdline = _wcsdup(commandLine);
BOOL success = CreateProcess(NULL, writable_cmdline,
NULL, NULL, TRUE, CREATE_NO_WINDOW, NULL, NULL, &si, &pi);
bool finished = false;
//wait for 1 second
for (int i = 0; i < 10; i++)
{
if (WaitForSingleObject(pi.hProcess, 100) <= 0)
{
finished = true;
break;
}
}
if (success)
{
CloseHandle(pi.hProcess);
CloseHandle(pi.hThread);
}
CloseHandle(h);
free(writable_cmdline);
if (!finished)
printf("Process didn't finish\n");
}
int main()
{
SaveResult(L"telnet.txt", L"C:\\windows\\system32\\cmd.exe /c telnet test.com");
SaveResult(L"ipconfig.txt", L"C:\\windows\\system32\\cmd.exe /c ipconfig");
return 0;
}

Answer 1

Have you run telnet command directly in cmd console? Is there anything output? if so, The empty file may because the buffer has not been refreshed to the file. As#David pointed out,
you just wait for 1s, but if there be more than 1s to connect, then you will close the handler without refresh the buffer. Try to add the FlushFileBuffers(h) before CloseHandle(h).
For WaitForSingleObject, you could try WaitForSingleObject(pi.hProcess, INFINITE), wait until the telnet process exit.