BOOL Apientry DllMain | Forex Factory

Options

Permalinks
Print Thread

Similar Threads

bool optimization 0 replies

BOOL Apientry DllMain

  • #1
  • First Post: Sep 18, 2011 1:45am Sep 18, 2011 1:45am
hi;

just a small question: I looked at a good number of MT4 dll source files and I noticed that some use: BOOL APIENTRY DllMain while others not.

(e.g. official MT4 experts/samples/DLLSample

PHP Code
//+------------------------------------------------------------------+ 
//|                                                                  | 
//+------------------------------------------------------------------+ 
BOOL APIENTRY DllMain(HANDLE hModule,DWORD ul_reason_for_call,LPVOID lpReserved) 
  { 
//---- 
   switch(ul_reason_for_call) 
     { 
      case DLL_PROCESS_ATTACH: 
      case DLL_THREAD_ATTACH: 
      case DLL_THREAD_DETACH: 
      case DLL_PROCESS_DETACH: 
         break; 
     } 
//---- 
   return(TRUE); 
  }
My question: is there any rule of thumb that such as the MTF sample should be included as default?
__Thanks__ MJ
  • #2
  • Sep 19, 2011 1:27am Sep 19, 2011 1:27am
The APIENTRY function (if one exists) is called by windows whenever a DLL is loaded/ unloaded, and can be used to allocate/free memory and run setup routines.
Since a context is supplied suitable code can be implemented. This can be useful for managing internal DLL resources when a DLL is used many times by an application EG by multiple indicators and or by multiple EAs.


Read here for more info
http://support.microsoft.com/kb/815065
  • #3
  • Sep 20, 2011 3:05am Sep 20, 2011 3:05am
Thanks rangebound.
But would you say it is adviceable to use something like the MT4 example in most of the cases. Or just leave it out as a number of other mt4 dlls do.
__Thanks__ MJ
  • #4
  • Sep 20, 2011 5:55am Sep 20, 2011 5:55am
You only need it if you need to hook into the loading/unloading etc.

Otherwise all it does is to return true (which means "success") and the same empty default function (that just returns success and does nothing else) should be installed automatically by the compiler/linker when you don't declare it yourself.
  • #5
  • Sep 20, 2011 5:59am Sep 20, 2011 5:59am
Thanks a lot 7bit. Very kind of you.
__Thanks__ MJ
  • #6
  • Sep 20, 2011 6:09am Sep 20, 2011 6:09am
hi 7bit,

a small question:

is the BOOL Apientry DllMain somewhat similar to what you did in you Pascal dll. with

PHP Code
initialization 
  PyStatus := 0; 
  LoadPyDll(); 
finalization 
  UnloadPyDll(); 
end.
would such go into the BOOL Apientry DllMain if you would write it in C++

if so how would that look like:


PHP Code
//+------------------------------------------------------------------+ 
//|                                                                  | 
//+------------------------------------------------------------------+ 
BOOL APIENTRY DllMain(HANDLE hModule,DWORD ul_reason_for_call,LPVOID lpReserved) 
  { 
//---- 
   switch(ul_reason_for_call) 
     { 
      case DLL_PROCESS_ATTACH: 
             PyStatus = 0; 
             LoadPyDll(); 

      case DLL_THREAD_ATTACH: 
      case DLL_THREAD_DETACH: 
      case DLL_PROCESS_DETACH: 
           UnloadPyDll(); 

         break; 
     } 
//---- 
   return(TRUE); 
  }
or would one put that with the DLL_THREAD_ATTACH, DLL_THREAD_DETACH
__Thanks__ MJ
  • #7
  • Sep 20, 2011 7:11am Sep 20, 2011 7:11am
Quoting M_j
hi 7bit,

a small question:

is the BOOL Apientry DllMain somewhat similar to what you did in you Pascal dll. with

PHP Code
initialization 
  PyStatus := 0; 
  LoadPyDll(); 
finalization 
  UnloadPyDll(); 
end.
would such go into the BOOL Apientry DllMain if you would write it in C++

if so how would that look like:


[php]
//+------------------------------------------------------------------+
//| |
//+------------------------------------------------------------------+
BOOL...
Loading DLLs in DllMain is a very bad idea: http://msdn.microsoft.com/en-us/library/ms682583%28v=VS.85%29.aspx
  • #8
  • Sep 20, 2011 7:28am Sep 20, 2011 7:28am
[quote=M_j;4969179]hi 7bit,

a small question:

is the BOOL Apientry DllMain somewhat similar to what you did in you Pascal dll. with

PHP Code
initialization 
  PyStatus := 0; 
  LoadPyDll(); 
finalization 
  UnloadPyDll(); 
end.
Yes, its called from within exactly that function.

The compiled binary will always automatically contain code from the runtime library that is always needed and one part of this code is a (quite complicated and flexible) DllMain function that does a lot of things automatically (initialize/deinitialize heap management, exception handling, Thread local storage, etc), things you would have to do manually in C. There are a few places where you can hook into all this in a standardized Pascal-like manner, one of these places (there exist a few more) is the initialization and finalization sections. They are called from withing the DllMain().

here at line 27 is the RTL function that will handle the bulk of the DllMain() call (its not directly called DllMain(), there is still a bit more compiler magic wrapped around it) http://svn.freepascal.org/cgi-bin/vi...nc?view=markup

initialization and finalization sections are called from somewhere within the PascalMain and FPC_Do_Exit procedures that are called there. As you can see there are a few more possible places to hook into by assigning certain procedure variables that are called there. This is all really hardcore stuff inside the runtime library that is only meant to be understood by the ones hacking on the compiler and the RTL itself.

If you download these sources and try to follow all these calls to see what is going on (or step through it with the debugger) you will find that it is actually quite complicated and a lot of things are going on under the hood to make the identical Pascal program (or library) compile and run so easily on such a wide range of architectures and operating systems.

Normal users (programmers) just use the initialization and finalization sections (if they need them or leave them away) (and maybe some of the other hook functions as they are documented) and not worry about all this and everything should just work. But its still interesting to look at this code and at least try to understand it (I have so far only understood parts of it, but I am not one of the compiler hackers, so I don't need to know everything).
  • #9
  • Sep 20, 2011 7:53am Sep 20, 2011 7:53am
Quoting Adal
Loading DLLs in DllMain is a very bad idea: http://msdn.microsoft.com/en-us/libr...=VS.85%29.aspx
"because this may create dependency loops in the DLL load order."

In this simple case (my python binding) this problem can not arise.

But it is true that caution is needed and only simple and self-contained things are done inside DllMain() because ultimately it is the operation system that is calling this function and it is not easy to rely on the exact order in which they are called in complex situations or at which times exactly they are called.
  • #10
  • Last Post: Sep 20, 2011 8:07am Sep 20, 2011 8:07am
Thanks very helpful infos.
__Thanks__ MJ
0 traders viewing now