为什么ReadFile（）不返回0？ 程序试图永远从管道读取数据

Question

code:

This is from much larger MS example ( http://msdn.microsoft.com/en-us/library/ms682499(VS.85).aspx ). And of course I tried the whole example before, that my snipped below is where I stripped it down to the problem. The thing is how I'm supposed to break out of this loop if all I get is 1 . How to do signal that there is no more data to read ?

(Compiler is MiGW.)

It looks like the MS example is broken, this is one of the comments there: Parent process needs to close handles that it passes to child process after the CreateProcess() call. Not doing so will cause the parent to wait forever on ReadFile().

#include <iostream>
#include <windows.h>
using namespace std;

int main() { 
  cout << "\n->Start of parent execution.\n";
  SECURITY_ATTRIBUTES saAttr;
  saAttr.bInheritHandle = TRUE;
  saAttr.lpSecurityDescriptor = NULL;
  HANDLE g_hChildStd_OUT_Rd = NULL;
  HANDLE g_hChildStd_OUT_Wr = NULL;
  CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr, 0);

  STARTUPINFO siStartInfo;
  PROCESS_INFORMATION piProcInfo;

  ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
  siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
  siStartInfo.dwFlags |= STARTF_USESTDHANDLES;

  CreateProcess(NULL,
    (char*)"cmd /c dir",     // command line
    NULL,          // process security attributes
    NULL,          // primary thread security attributes
    TRUE,          // handles are inherited
    0,             // creation flags
    NULL,          // use parent's environment
    NULL,          // use parent's current directory
    &siStartInfo,  // STARTUPINFO pointer
    &piProcInfo);  // receives PROCESS_INFORMATION


  CHAR chBuf[4096];
  DWORD dwRead;

  // ~~~~~~this loop here~~~~~- how to break out of it ?
  while(true) {
    cout << "ReadFile: " << ReadFile( g_hChildStd_OUT_Rd, chBuf, 4096, &dwRead, NULL) << endl;
  }

  cout << "\n->End of parent execution.\n";
}

Answer 1

Did you try with all the code? MSDN has some handle inheritance stuff. Looks like the write end of the pipe is not closed, so ReadFile thinks there will be more to read if it waits.

EDIT:

"CloseHandle(g_hChildStd_OUT_Wr);" makes the read terminate once its read everything, so definitely seems that the issue is the write end never gets fully closed after the child process terminates. However the MSDN sample doesn't seem to require it, so you would need to look more carefully at why that is the case.