MT4: catch dll import error | Forex Factory

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MT4: catch dll import error

  • #1
  • First Post: Sep 17, 2011 11:41pm Sep 17, 2011 11:41pm
hi,

just a small question: maybe one of you knows.

if I have something like in a mqh header file.

PHP Code
#import "xxx.dll"
   void yyy(); 
#import
and the xxx.dll can not be found I get an error in the log:

cannot load library 'xxx.dll' (error 126)

Anyone knows an easy way to display such error with an Alert or MessageBox instead of only writting it to the log file.


Thanks
__Thanks__ MJ
  • #2
  • Sep 19, 2011 1:11am Sep 19, 2011 1:11am
Not without a little work of your own ... try

LoadLibrary & FreeLibrary found in Kernel32.dll


so you would do something like this in Init()


int hmod = LoadLibrary("c:MT4LibraryDirxxx.dll");
if (hmod != 0 ) FreeLibrary(hmod);
else
Alert("xxx.DLL cannot be loaded");


Note we should free our loading of the DLL because MT4 will have already loaded it and windows maintains a counter of the number of times the DLL is loaded by an application instance. If you dont Free our call to loadlibrary then MT4 wont let the DLL Go when the EA ends. Although I must point out that MT4 cant be relied upon to free all Dll handles when an EA completes anyway but we should at least do what we should!


Of course this only ensures that the DLL file exists and that it is a valid DLL it does not ensure that the functions defined in your import statement actually exist. For that you should use the function GetProcAddress()

See the MSDN docs here

http://msdn.microsoft.com/en-us/libr...=VS.85%29.aspx
  • #3
  • Sep 19, 2011 1:57am Sep 19, 2011 1:57am
Quoting rangebound
Not without a little work of your own ... try

LoadLibrary & FreeLibrary found in Kernel32.dll


so you would do something like this in Init()


int hmod = LoadLibrary("c:MT4LibraryDirxxx.dll");
if (hmod != 0 ) FreeLibrary(hmod);
else
Alert("xxx.DLL cannot be loaded");


Note we should free our loading of the DLL because MT4 will have already loaded it and windows maintains a counter of the number of times the DLL is loaded by an application instance. If you dont Free our call to loadlibrary then MT4 wont let the DLL Go when...
Thank you rangebound!
  • #4
  • Sep 19, 2011 10:45am Sep 19, 2011 10:45am
No Problem.

I'm not sure what will happen if the "Allow DLL Calls" box is unchecked. I imagine that a call to LoadLibrary() will return 0 even if the DLL file exists.

If this is the case then you could use WinAPI functions FileOpen() or FindFirst() to find out if the DLL file is present, Allowing you to 'Alert' either of the two cases :

1) DLL exists but cannot be opened - suggesting "Allow DLLs" is unchecked
2) DLL file is missing.
  • #5
  • Sep 19, 2011 11:17am Sep 19, 2011 11:17am
Doh!

Of course the IsDllsAllowed() function covers that little problem!

  • #6
  • Sep 20, 2011 3:03am Sep 20, 2011 3:03am
Thanks a lot: I will give this a try.
__Thanks__ MJ
  • #7
  • Sep 20, 2011 5:39am Sep 20, 2011 5:39am
Quoting rangebound
Although I must point out that MT4 cant be relied upon to free all Dll handles when an EA completes anyway
This bug happens only when running an EA in the strategy tester. There it forgets to FreeLibrary() the dll that has been loaded with #import after the strategy test is ended. Each run of the tester will leave the reference count increased by one. It might be possible to workaround this in deinit() with IsTesting() and manually FreeLibrary() one more time.

During normal operation (on a live chart) it will always free what has been loaded (with #import) correctly.

All the above is only referring to #import. If you have additional LoadLibrary() in your code then of course you still need an equal amount of calls to FreeLibrary().
  • #8
  • Sep 26, 2016 4:27am Sep 26, 2016 4:27am
I`m having the same problem in Demo Account with EA Indicator. When i check the experts i get the error cannot load xxxx .dll ( 126 )
Does anybody think they can fix this for a small fee?
I have access to source code of the EA & Indicator as file is mql4 but not dll.
  • #9
  • Last Post: Sep 26, 2016 4:58am Sep 26, 2016 4:58am
Quoting fx9
I`m having the same problem in Demo Account with EA Indicator. When i check the experts i get the error cannot load xxxx .dll ( 126 ) Does anybody think they can fix this for a small fee? I have access to source code of the EA & Indicator as file is mql4 but not dll.
I have experience in fixing it and have pm you the contact.
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