JEDI Windows API » DidYouKnow

Did you know? #6

Christian Wimmer — Thu, 15 May 2008 10:00:26 +0000

If you create a new process without adding the environment block (by calling CreateEnvironmentBlock) the new process may fail to call Shellexecute and some other functions. Don’t think that the enviroment variables aren’t that important.

Find out more about CreateProcess in the post about “CreateProcess in full glory“.

Did you know? #5

Wed, 14 May 2008 08:35:33 +0000

Did you know that handles are 64 bit wide in native 64 bit applications? While this doesn’t affect any Delphi developer just yet, things will change next year with the release of Delphi “Commodore” as outlined on CodeGear’s Delphi and C++Builder Roadmap page. This means, among other things, that type-casting a THandle or HANDLE from or to something like a Cardinal, Longint or DWORD is a sin.

The reason for this becomes clear if you look into the Windows SDK headers, where HANDLE is declared as:

typedef void *HANDLE;

Since the processor architecture determines the width of the pointer types, this will be 64 bit. Now Delphi on the other hand (taking Delphi 2006 as an example) declares this:

THandle = LongWord;

Which is a non-pointer type of 32 bit width. If memory serves me right, there were some changes over time in how THandle was declared and it juggled between Cardinal, LongWord and some other type. In any case, there will have to be a change in “Commodore”, unless LongWord changes its meaning from 32 to 64 bit.

Things get even crazier if you look at how the SDK declares INVALID_HANDLE_VALUE:

#define INVALID_HANDLE_VALUE ((HANDLE)(LONG_PTR)-1)

Note the explicit type-cast. This is necessary to ensure that the value will not be truncated to $FFFFFFFF instead of $FFFFFFFFFFFFFFFF.

Delphi 2006 declares it as:

const
  INVALID_HANDLE_VALUE = DWORD(-1);

… which is clearly invalid for 64 bit code. So watch out for these caveats in your code. I am sure CodeGear will do their part before the release.

Did you know? #3

Christian Wimmer — Mon, 12 May 2008 11:12:58 +0000

The WinSta0_DesktopSwitch Event allows an application to check for a desktop switch. The following examples demonstrates how we can wait for such an Event. To test it, simply lock and unlock the workstation. Both wait function will return on the two desktop switching.

uses
SysUtils
JwaWindows;

var H : THandle;
begin
H := OpenEvent(SYNCHRONIZE, false, ‘WinSta0_DesktopSwitch’);
if H = 0 then
begin
writeln(SysErrorMessage(GetLasterror));
readln;
end;

WaitForSingleObject(H,INFINITE);
writeln(‘OK1′);

WaitForSingleObject(H,INFINITE);
writeln(‘OK2′);

writeln(‘Switched’);
readln;
end.

You should know that each session (0…x) has its own event. You can’t use this mechanism directly in a service (especially not in Vista due to session isolation).

Did you know? #2

Christian Wimmer — Sat, 10 May 2008 14:26:43 +0000

If you include Forms and SvcMgr (Service classes) in the same project (in any sub unit) you may get a memory leak. It happens only if you call the Free method directly.

Forms.Application.Free;

To make it work, you have to check for a side effect that I describe here.

The memory leak results from the SvcMgr.TServiceApplication.Destroy destructor which uses the global instance Forms.Application. In detail, it creates a memory block that is maintained by a list in TApplication. If you free the TApplication instance, the internal list is also freed, but the memory block is not! As a remedy you have to free the service object first and then recreate the TApplication instance (and if you need also the TServiceApplication).

The Forms and SvcMgr units creates their objects in the initialization section of the units. So you have to play around them.

Go through your units and check if you include SvcMgr and Forms.

Did you know? #1

Christian Wimmer — Thu, 08 May 2008 20:05:00 +0000

A call to CreateProcessAsUser will fail with ERROR_INVALID_PARAMETER if your new process belongs to another session and you allowed inheritance of handles. In this case you have to set parameter bInheritHandles to FALSE.