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Epsilon User's Manual and Reference
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Epsilon User's Manual and Reference > Primitives and EEL Subroutines > Operating System Primitives >

Calling Windows DLLs

int call_dll(char *dll_name, char *func_name,
             char *ftype, char *args, ...)

The call_dll( ) primitive calls a function in a Windows DLL. Epsilon can only call 32-bit DLLs. The dll_name parameter specifies the DLL file name. The func_name parameter specifies the name of the particular function you want to call.

The ftype parameter specifies the routine's calling convention. The character C specifies the C calling convention, while P specifies the Pascal calling convention. Most Windows DLLs use the Pascal calling convention, but any function that accepts a variable number of parameters must use the C calling convention.

The args parameter specifies the type of each remaining parameter. Each letter in args specifies the type of one parameter, according to the following table.

 Character  Description  
 L  unsigned long  DWORD
 I  int  INT, UINT, HWND, most other handles
 S  far char *  LPSTR
 P  far void *  LPVOID
 R  far void **  LPVOID *

The I character represents a 32-bit parameter, and is equivalent to L in this version. L, S, P, and R always represent 32-bit parameters.

S represents a null-terminated string being sent to the DLL. P is passed similarly, but Epsilon will not check the string for null termination. It's useful when the string is an output parameter of the DLL, and may not be null-terminated before the call, or when passing structure pointers to a DLL.

R indicates that a DLL function returns a pointer by reference. Epsilon will pass the pointer you supply (if any) and retrieve the result. Use this for DLL functions that require a pointer to a pointer, and pass the address of any EEL variable whose type is "pointer to ..." (other than "pointer to function").

Here's an example, using call_dll( ) to determine the main Windows directory:

#define GetWindowsDirectory(dir, size)  (is_gui == IS_WIN31 \
    ? call_dll("kernel.dll", "GetWindowsDirectory", \
               "p", "pi", dir, size) \
    : call_dll("kernel32.dll", "GetWindowsDirectoryA", \
               "p", "pi", dir, size))

    char dir[FNAMELEN];

    GetWindowsDirectory(dir, FNAMELEN);
    say("The Windows directory is %s", dir);

A DLL function that exists in both 16-bit and 32-bit environments will usually be in different .dll files, and will often go by a different name. Its parameters will often be different as well. In particular, remember that a structure that includes int members will be a different size in the two environments. To write an EEL interface to a DLL function that takes a pointer to such a structure, you'll need to declare two different versions of the structure, and pass the correct one to the DLL function, if you want your EEL interface to work in both 16-bit and 32-bit environments.

After you call a function in a DLL, Epsilon keeps the DLL loaded to make future calls fast. You can unload a DLL loaded by call_dll( ) by including just the name of the DLL, and omitting the name of any function or parameters. For example, call_dll("extras.dll"); unloads a DLL named extras.dll.

char *make_pointer(int value)

The make_pointer( ) primitive can be useful when interacting with system DLLs. It takes a machine address as a number, and returns an EEL pointer that may be used to access memory at that address. No error checking will be done on the validity of the pointer.



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