CreateProcess with pipes, reading from process stdout.

windows-create-process-pipes.cpp

#include <Windows.h>
#include <algorithm>
#include <iostream>
#include <vector>
#include <array>

auto main(int argc, char** argv) -> int {
  using pipe_handle = void*;

  auto stdout_read = pipe_handle(); // not needed
  auto stdout_write = pipe_handle(); // neede to obtain stdout of the child process

  auto security_attrs = SECURITY_ATTRIBUTES {
    .nLength = sizeof(SECURITY_ATTRIBUTES), // needed for god knows for
    .lpSecurityDescriptor = nullptr, // default security options
    .bInheritHandle = TRUE // child processes inherit this handle
  };

  CreatePipe(
    &stdout_read, 
    &stdout_write, 
    &security_attrs,
    0 // default buffer size for a pipe
  );
  
  SetHandleInformation(stdout_read, HANDLE_FLAG_INHERIT, 0); // child processes inherit the handle

  auto start_info = STARTUPINFO {
    .cb = sizeof(STARTUPINFOA), // needed for god knows for
    .dwFlags = STARTF_USESTDHANDLES, // uses the pipe handles passed to it
    .hStdOutput = stdout_write
    // there's also stderr and stdin if you want to read/write from/to them
  };


  auto proc_info = PROCESS_INFORMATION();
  const auto exe_name = TEXT("C://Windows//System32//cmd.exe");
#ifdef UNICODE
  auto args = const_cast<wchar_t*>(LR"(/c "dir")"); // needed because CreateProcess needs char*
#else
  auto args = const_cast<char*>(R"(/c "dir")");
#endif

  CreateProcess(
    exe_name, // executable name relative to the parent path, ignores env
    args, // arguments in a string form
    nullptr, // security attributes for the child process
    nullptr, // security attributes for the child process' thread
    TRUE, // should the process inherit the handles, yes you have to set this information in 123123 places for some reason
    0, // idk some flags
    nullptr, // the child process uses the parent processes enviroment variables
    nullptr, // the child process uses the parent processes current directory
    &start_info,
    &proc_info // proc_info receives the information about the new process, out argument
  );

  CloseHandle(stdout_write); // closing it sothat it doesn't block while using ReadFile; not needed
  std::cout << "Child process PID and TID: " << proc_info.dwProcessId <<  ", " << proc_info.dwThreadId << "\n\n";

  // reading from the process
  auto output = std::vector<char>(4096);
  auto bytes_read = DWORD(0);

  while(true) {
    auto bytes_avaiable = DWORD(0);

    const auto success =
      ReadFile(
        stdout_read, // device handle, here a pipe
        output.data() + bytes_read, // pointer to the buffer
        4096 - bytes_read, // buffer size
        &bytes_avaiable, // the number of bytes read, out argument
        nullptr // not needed
      );

    if((not success) or (bytes_avaiable == 0)) break;

    bytes_read += bytes_avaiable;

    std::cout << "Bytes read: " << bytes_avaiable << '\n';
  }

  std::cout << "The whole process output (" << bytes_read << " total bytes read)" << ": \n[\n";
  std::ranges::copy(output, std::ostream_iterator<char>(std::cout));
  std::cout << "]\n";
 
  WaitForSingleObject(proc_info.hProcess, INFINITY); // waits till the child process exits
  CloseHandle(stdout_read); // closes the handle
}