Re: [Mingw-users] popen (was: How to create shared memory / semaphore in mingw?)

[Joerg B. <jo...@sq...>]

Hi!

Al Slater wrote:
> 
> > Aside: A colleague just stumbled over the problem that even the
> > function "popen" can not be used in Win-NT if the program is a
> > "window" application, it is restricted to "console".
> >
> 
> are you saying the "client" app must be console, or the "target" app?

She had this experience: 
- Some code calling "popen ( ... , rt )" worked in a test program, 
  which she started from a DOS box. 
- This code failed when being part of a DLL used in a program 
  which was started by a Web server. 
In your terms, this would probably be the "client" (= caller).

My interpretation is that this calling program is no "console" 
application as it does not have "stdin / stdout / stderr", 
but this is a guess. 
(Explanations of "windows program" vs. "console program" are 
welcome, especially when including the handling of "services".)

Regarding the "target": IMHO, the worst that can happen is that 
no data is read from / written to the pipe created by "popen()". 
I fail to see why this should be a problem, when the target 
terminates the pipe will be closed and any remaining data discarded.

The restriction is documented in MSDN: 
|  Note   The _popen function returns an invalid file handle, 
|  if used in a Windows program, that will cause the program 
|  to hang indefinitely. _popen works properly in a Console 
|  application. To create a Windows application that redirects 
|  input and output, read the section "Creating a Child Process 
|  with Redirected Input and Output" in the Win32 SDK. 
URL (one-liner!): 
 
http://msdn.microsoft.com/library/en-us/vccore98/HTML/_crt__popen.2c_._wpopen.asp

What surprises me most is the expression "invalid file handle" - 
I would expect a NULL if something fails (like tested in the 
application example in that man page), but certainly not a 
"hang indefinitely".

"Abandon all hope, ..."

Regards, 
Joerg

-- 
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
     (speaking only for himself)
mailto: jo...@sq...