Compile a DLL in C C then call it from another program

Question

I want to make a simple  simple DLL which exports one or two functions  then try to call it from another program    Everywhere I ve looked so far  is for complicated matters  different ways of linking things together  weird problems that I haven t even begun to realize exist yet    I just want to get started  by doing something like so   Make a DLL which exports some functions  like   int add2 int num      return num   2     int mult int num1  int num2      int product     product   num1   num2     return product      I m compiling with MinGW  I d like to do this in C  but if there s any real differences doing it in C    I d like to know those also  I want to know how to load that DLL into another C  and C    program  and then call those functions from it  My goal here  after playing around with DLLs for a bit  is to make a VB front-end for C     code  by loading DLLs into visual basic  I have visual studio 6  I just want to make some forms and events for the objects on those forms  which call the DLL    I need to know how to call gcc   g    to make it create a DLL  but also how to write   generate  an exports file    and what I can cannot do in a DLL  like  can I take arguments by pointer reference from the VB front-end  Can the DLL call a theoretical function in the front-end  Or have a function take a  function pointer   I don t even know if that s possible  from VB and call it   I m fairly certain I can t pass a variant to the DLL   but that s all I know really   update again  Okay  I figured out how to compile it with gcc  to make the dll I ran  gcc -c -DBUILD DLL dll c gcc -shared -o mydll dll dll o -Wl --out-implib libmessage a   and then I had another program load it and test the functions  and it worked great  thanks so much for the advice  but I tried loading it with VB6  like this  Public Declare Function add2 Lib  C  c dll mydll dll   num As Integer  As Integer   then I just called add2 text1 text  from a form  but it gave me a runtime error    Can t find DLL entry point add2 in C  c dll mydll dll   this is the code I compiled for the DLL    ifdef BUILD DLL  define EXPORT   declspec dllexport   else  define EXPORT   declspec dllimport   endif  EXPORT int   stdcall add2 int num     return num   2     EXPORT int   stdcall mul int num1  int num2     return num1   num2      calling it from the C program like this worked  though    include lt stdio h gt   include lt windows h gt   int main       HANDLE ldll    int   add2  int     int   mul  int int      ldll   LoadLibrary  mydll dll      if ldll gt  void  HINSTANCE ERROR       add2   GetProcAddress ldll   add2        mul   GetProcAddress ldll   mul        printf  add2 3    d nmul 4 5    d   add2 3   mul 4 5        else       printf  ERROR              any ideas   solved it  To solve the previous problem  I just had to compile it like so   gcc -c -DBUILD DLL dll c gcc -shared -o mydll dll dll o -Wl --add-stdcall-alias   and use this API call in VB6  Public Declare Function add2 Lib  C  c dll mydll         ByVal num As Integer  As Integer   I learned not to forget to specify ByVal or ByRef explicitly--I was just getting back the address of the argument I passed  it looked like  -3048

User · Answer

For VB6   You need to declare your C functions as   stdcall  otherwise you get  invalid calling convention  type errors  About other your questions      can I take arguments by pointer reference from the VB front-end     Yes  use ByRef ByVal modifiers      Can the DLL call a theoretical function in the front-end     Yes  use AddressOf statement  You need to pass function pointer to dll before      Or have a function take a  function pointer   I don t even know if that s possible  from VB and call it      Yes  use AddressOf statement   update  more questions appeared          to load it into VB  do I just do the usual method  what I would do to load winsock ocx or some other runtime  but find my DLL instead  or do I put an API call into a module     You need to decaler API function in VB6 code  like next   Private Declare Function SHGetSpecialFolderLocation Lib  shell32        ByVal hwndOwner As Long        ByVal nFolder As Long        ByRef pidl As Long  As Long

User · Answer

Regarding building a DLL using MinGW  here are some very brief instructions   First  you need to mark your functions for export  so they can be used by callers of the DLL  To do this  modify them so they look like  for example     declspec  dllexport   int add2 int num      return num   2      then  assuming your functions are in a file called funcs c  you can compile them   gcc -shared -o mylib dll funcs c   The -shared flag tells gcc to create a DLL   To check if the DLL has actually exported the functions  get hold of the free Dependency Walker tool and use it to examine the DLL   For a free IDE which will automate all the flags etc  needed to build DLLs  take a look at the excellent Code  Blocks  which works very well with MinGW   Edit  For more details on this subject  see the article Creating a MinGW DLL for Use with Visual Basic on the MinGW Wiki

User · Answer

Here is how you do it   In  h   ifdef BUILD DLL  define EXPORT   declspec dllexport   else  define EXPORT   declspec dllimport   endif  extern  C     Only if you are using C   rather than C         EXPORT int   stdcall add2 int num     EXPORT int   stdcall mult int num1  int num2       in  cpp  extern  C     Only if you are using C   rather than C       EXPORT int   stdcall add2 int num      return num   2      EXPORT int   stdcall mult int num1  int num2      int product    product   num1   num2    return product        The macro tells your module  i e your  cpp files  that they are providing the dll stuff to the outside world  People who incude your  h file want to import the same functions  so they sell EXPORT as telling the linker to import  You need to add BUILD DLL to the project compile options  and you might want to rename it to something obviously specific to your project  in case a dll uses your dll    You might also need to create a  def file to rename the functions and de-obfuscate the names  C C   mangles those names   This blog entry might be an interesting launching off point about that   Loading your own custom dlls is just like loading system dlls  Just ensure that the DLL is on your system path  C  windows  or the working dir of your application are an easy place to put your dll

User · Answer

There is but one difference  You have to take care or name mangling win C    But on windows you have to take care about  1  decrating the functions to be exported from the DLL 2  write a so called  def file which lists all the exported symbols    In Windows while compiling a DLL have have to use     declspec dllexport    but while using it you have to write   declspec dllimport   So the usual way of doing that is something like   ifdef BUILD DLL  define EXPORT   declspec dllexport   else  define EXPORT   declspec dllimport   endif   The naming is a bit confusing  because it is often named EXPORT   But that s what you ll find in most of the headers somwhere  So in your case you d write  with the above  define   int DLL EXPORT add     int DLL EXPORT mult     Remember that you have to add the Preprocessor directive BUILD DLL during building the shared library   Regards Friedrich

User · Answer

The thing to watch out for when writing C   dlls is name mangling  If you want interoperability between C and C    you d be better off by exporting non-mangled C-style functions from within the dll    You have two options to use a dll   Either use a lib file to link the symbols -- compile time dynamic linking Use LoadLibrary   or some suitable function to load the library  retrieve a function pointer  GetProcAddress  and call it -- runtime dynamic linking   Exporting classes will not work if you follow the second method though

[c++] Compile a DLL in C/C++, then call it from another program

Examples related to c++

Examples related to c

Examples related to dll

Examples related to gcc

Examples related to vb6