Executing a program from PHP hangs APACHE

Question

Hello and thank you in advance for your interest.

During the past two weeks I've been struggling with something that is driving me nuts. I have APACHE (2.2.22) and PHP (5.4.3) installed on my Windows box and I'm trying to call a program from a PHP script which calls another program at the same time. Both programs are written in C/C++ and compiled with MINGW32. Regarding to Windows version, I've tested Windows 2003 Server and Windows 7 Professional and both give me the same problems.

Let me introduce these two programs:

1) mytask.exe: this is a program that is to be executed in background and that periodically populates its status to a file.

2) job.exe: this is the program I want to call from the PHP script. Its goal is to spawn mytask.exe as an independent process (not as a thread).

If I run from a Console window the command below, then job.exe immediately returns and leaves mytask.exe running on the background until it terminates.

> job.exe spawn mytask.exe
jobid=18874111458879FED

Note that job.exe dumps an identifier which is used to manage mytask.exe. For example:

> job.exe status 18874111458879FED
RUNNING

I've checked that if I run the first command from a PHP script, the PHP script randomly blocks forever. If I look to the Windows's task manager, I can see that job.exe is there in a zombie-like state. I can assert that job.exe effectively reaches the usual return 0; statement in its main() routine, so it seems to be something under the wood, in the C runtime. Furthermore, if I write a simple mytask.exe that simply sleeps for 10 seconds then the PHP script blocks for 10 seconds too (or blocks forever following the random behavior I've just mentioned). In other words, I have no way to make job.exe spawning a process without waiting for it to end, when I call job.exe from a PHP script.

So: there's something I'm doing wrong when spawning mytask.exe and, now, here comes the second part of this digression.

I use the WINAPI function CreateProcess() to spawn the tasks from job.exe. As of the MSDN documentation, I call CreateProcess with bInheritHandles = FALSE, to avoid the child process to yield I/O deadlocks with the PHP script. I also close the process handles returned by CreateProcess() in the PROCESS_INFORMATION structure. The only thing I don't do is waiting for the process to end. On the other hand, regarding to the PHP side, I've tried both exec() and proc_open() PHP functions to call job.exe with no success.

My last observations, though, seem to be in the right way, yet they do not convince me because I don't understand why they work somehow. The fact is that if mytask.exe does fclose(stdout) before sleeping, then the PHP script returns immediately. BUT, HOW??? I told CreateProcess() to not inherit handles, so why am I getting these results? Anyway, I cannot stick with this patch because programs launched by job.exe may not know about who is calling them, so closing stdout from those programs is not a good solution. In UNIX, things are so simple... One just calls fork(), closes standard streams and then calls execve to call the program. In Windows, I've also tried to create a wrapper thread with CreateThread() (to emulate fork()) and then call CreateProcess() from that thread after closing standard streams... but that closed the streams of job.exe too!

All this question could be synthesized in a single one: how can I execute from PHP a program that creates other processes?

I hope somebody could shed some light on this issue... Thank you very much!

Answer 1

I think I've nailed the solution, which is divided in two parts:

1) Regarding the fact the main process stops until the child process ends.

As of MSDN documentation, this is the definition of CreateProcess():

BOOL WINAPI CreateProcess(
  _In_opt_     LPCTSTR lpApplicationName,
  _Inout_opt_  LPTSTR lpCommandLine,
  _In_opt_     LPSECURITY_ATTRIBUTES lpProcessAttributes,
  _In_opt_     LPSECURITY_ATTRIBUTES lpThreadAttributes,
  _In_         BOOL bInheritHandles,
  _In_         DWORD dwCreationFlags,
  _In_opt_     LPVOID lpEnvironment,
  _In_opt_     LPCTSTR lpCurrentDirectory,
  _In_         LPSTARTUPINFO lpStartupInfo,
  _Out_        LPPROCESS_INFORMATION lpProcessInformation
);

As I said in my question, I pass FALSE to bInheritHandles, but I was also passing 0 to dwCreationFlags. After a little bit of more research, I found that there's a flag called DETACHED_PROCESS, for which MSDN says:

For console processes, the new process does not inherit its parent's console (the default). The new process can call the AllocConsole function at a later time to create a console. For more information, see Creation of a Console.

Now, job.exe returns immediately despite the fact the child process continues its execution.

2) Regarding the fact the PHP script randomly hangs when calling exec()

It seems to be a bug of PHP. Running exec() family functions in the context of a PHP session may make APACHE to randomly hang, being necessary to restart the server. I found a thread in the Internet in which a user noticed that closing the session (thru session_write_close()) before calling exec() would prevent the script from hanging. The same applies for the proc_open/proc_close functions. So, my script now looks like this:

session_write_close();  //Close the session before proc_open()
$proc = proc_open($cmd,$pipedesc,$pipes);
//do stuff with pipes... 
//... and close pipes
$retval = proc_close($proc);
session_start(); //restore session

Hope this helps.