How to check a DLL if a function exists? - delphi

Question

I'm working on something which dynamically loads specially formulated DLL's. I need to be able to check the DLL and make sure all the expected functions exist before I consider using this DLL. If it's missing some certain functions, I should not try to load it. I know I could attempt to call one of the functions and see if there's an exception, but I would see errors in debug mode.
How should I go about checking a DLL if a function exists? I'd like to check it before I load it (using LoadLibrary) but I guess it's OK if I have to load it to perform this check too.
UPDATE
I've accepted David's answer below, and thought I'd post my final code to show the whole process. I turned it into a function returning a Bool, whether it succeeded or not, cleaned the code a bit, and added another function at the bottom which uses this one to check each name one by one.
I decided to use this method instead of reading GetProcAddress because it will help me in the future with other things.
type
  PIMAGE_NT_HEADERS = ^IMAGE_NT_HEADERS;
  PIMAGE_EXPORT_DIRECTORY = ^IMAGE_EXPORT_DIRECTORY;
function ImageNtHeader(Base: Pointer): PIMAGE_NT_HEADERS; stdcall;
external 'dbghelp.dll';
function ImageRvaToVa(NtHeaders: Pointer; Base: Pointer; Rva: ULONG;
LastRvaSection: Pointer): Pointer; stdcall; external 'dbghelp.dll';
function ExportedFunctionNames(const ImageName: string; NamesList: TStrings): Bool;
var
i: Integer;
FileHandle: THandle;
ImageHandle: THandle;
ImagePointer: Pointer;
Header: PIMAGE_NT_HEADERS;
ExportTable: PIMAGE_EXPORT_DIRECTORY;
NamesPointer: Pointer;
Names: PAnsiChar;
NamesDataLeft: Integer;
begin
Result:= False;
NamesList.Clear;
FileHandle:= CreateFile(PChar(ImageName), GENERIC_READ, FILE_SHARE_READ,
nil, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
if FileHandle = INVALID_HANDLE_VALUE then Exit;
try
ImageHandle:= CreateFileMapping(FileHandle, nil, PAGE_READONLY, 0, 0, nil);
if ImageHandle = 0 then Exit;
try
ImagePointer:= MapViewOfFile(ImageHandle, FILE_MAP_READ, 0, 0, 0);
if not Assigned(ImagePointer) then Exit;
try
Header:= ImageNtHeader(ImagePointer);
if not Assigned(Header) then Exit;
if Header.Signature <> $00004550 then Exit; // "PE\0\0" as a DWORD.
ExportTable:= ImageRvaToVa(Header, ImagePointer,
Header.OptionalHeader.DataDirectory[0].VirtualAddress, nil);
if not Assigned(ExportTable) then Exit;
NamesPointer:= ImageRvaToVa(Header, ImagePointer,
Cardinal(ExportTable.AddressOfNames), nil);
if not Assigned(NamesPointer) then Exit;
Names:= ImageRvaToVa(Header, ImagePointer, Cardinal(NamesPointer^), nil);
if not Assigned(Names) then Exit;
NamesDataLeft:= Header.OptionalHeader.DataDirectory[0].Size;
for i:= 0 to ExportTable.NumberOfNames - 1 do begin
NamesList.Add(Names);
while (Names^ <> chr(0)) and (NamesDataLeft > 0) do begin
Inc(Names);
Dec(NamesDataLeft);
end;
Inc(Names);
end;
Result:= True;
finally
UnmapViewOfFile(ImagePointer);
end;
finally
CloseHandle(ImageHandle);
end;
finally
CloseHandle(FileHandle);
end;
end;
function IsMyDLL(const Filename: String): Bool;
var
H: THandle;
L: TStringList;
function InList(const Func: String): Bool;
begin
Result:= L.IndexOf(Func) >= 0;
end;
begin
Result:= False;
L:= TStringList.Create;
try
if ExportedFunctionNames(Filename, L) then begin
Result:=//Names of functions which need to exist
InList('GetName') and
InList('GetDescription') and
InList('GetVersion') and
InList('Start') and
InList('Stop');
end;
finally
L.Free;
end;
end;

Answer 1

You have to use LoadLibrary, and then use GetProcAddress for each function you want to check existence for. There's really no other reasonable choice (unless there are specific reasons you need to avoid`LoadLibrary). Since your intent seems to be just to check to see if the functions are present and nothing more, LoadLibrary and GetProcAddress are the simplest means to do so; you can do all of the work in very few lines of code, and error checking is extremely simple and straightforward.

Answer 2

If you are in control of the DLLs and you don't want to load them in order to check capability, then you could use the version resource to indicate capability. This would require the host app to have knowledge of what was the minimum supported version for each optional DLL feature. You can read the version resource cheaply without loading the DLL.
It is perfectly possible, and rather simple, to obtain the list of functions exported by a DLL with loading it into your process with LoadLibrary. The dbghelp.dll system library provides services to do that. However, I suspect that is overkill for your situation.
If it is not a problem to load and unload the DLL then GetProcAddress is probably the preferred solution. If there is some reason why you need to avoid loading the DLL in order to check capability, use the version resource to infer capability. If you need to do this with legacy DLLs that do not have a meaningful version resource then use dbghelp.dll to find the exported functions.
For the sake of completeness, here is some code to read all the exported symbols from a DLL, without loading it with LoadLibrary.
type
PIMAGE_NT_HEADERS = ^IMAGE_NT_HEADERS;
PIMAGE_EXPORT_DIRECTORY = ^IMAGE_EXPORT_DIRECTORY;
function ImageNtHeader(Base: Pointer): PIMAGE_NT_HEADERS; stdcall; external 'dbghelp.dll';
function ImageRvaToVa(NtHeaders: Pointer; Base: Pointer; Rva: ULONG; LastRvaSection: Pointer): Pointer; stdcall; external 'dbghelp.dll';
procedure ImageExportedFunctionNames(const ImageName: string; NamesList: TStrings);
var
i: Integer;
FileHandle: THandle;
ImageHandle: THandle;
ImagePointer: Pointer;
Header: PIMAGE_NT_HEADERS;
ExportTable: PIMAGE_EXPORT_DIRECTORY;
NamesPointer: Pointer;
Names: PAnsiChar;
NamesDataLeft: Integer;
begin
//NOTE: our policy in this procedure is to exit upon any failure and return an empty list
NamesList.Clear;
FileHandle := CreateFile(
PChar(ImageName),
GENERIC_READ,
FILE_SHARE_READ,
nil,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
0
);
if FileHandle=INVALID_HANDLE_VALUE then begin
exit;
end;
Try
ImageHandle := CreateFileMapping(FileHandle, nil, PAGE_READONLY, 0, 0, nil);
if ImageHandle=0 then begin
exit;
end;
Try
ImagePointer := MapViewOfFile(ImageHandle, FILE_MAP_READ, 0, 0, 0);
if not Assigned(ImagePointer) then begin
exit;
end;
Try
Header := ImageNtHeader(ImagePointer);
if not Assigned(Header) then begin
exit;
end;
if Header.Signature<>$00004550 then begin // "PE\0\0" as a DWORD.
exit;
end;
ExportTable := ImageRvaToVa(Header, ImagePointer, Header.OptionalHeader.DataDirectory[0].VirtualAddress, nil);
if not Assigned(ExportTable) then begin
exit;
end;
NamesPointer := ImageRvaToVa(Header, ImagePointer, Cardinal(ExportTable.AddressOfNames), nil);
if not Assigned(NamesPointer) then begin
exit;
end;
Names := ImageRvaToVa(Header, ImagePointer, Cardinal(NamesPointer^), nil);
if not Assigned(Names) then begin
exit;
end;
NamesDataLeft := Header.OptionalHeader.DataDirectory[0].Size;
for i := 0 to ExportTable.NumberOfNames-1 do begin
NamesList.Add(Names);
// Locate the next name
while (Names^<>chr(0)) and (NamesDataLeft>0) do begin
inc(Names);
dec(NamesDataLeft);
end;
inc(Names);
end;
Finally
UnmapViewOfFile(ImagePointer); // Ignore error as there is not much we could do.
End;
Finally
CloseHandle(ImageHandle);
End;
Finally
CloseHandle(FileHandle);
End;
end;