Linking Windows-Based Applications and DLLs with /NOI

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SUMMARY

The address of a function exported by an application or DLL must be resolved by the Windows loader or a call to GetProcAddress() before the function can be called by another application or DLL. If the function is exported by ordinal number (or by name and ordinal number), its address can be resolved by the ordinal number. If no ordinal number is given for an exported function, it is said to be exported by name only and its address must be resolved by its name.

When resolving a reference by name in one application or DLL to a function exported by another application or DLL, the Windows loader or GetProcAddress() will convert the function name to uppercase and perform a case-sensitive search. If a match is not found, the Windows loader displays a dialog box with the "Call to Undefined Dynalink" message and a button to close the offending application. If a match is not found, GetProcAddress() returns NULL to signal failure. One possible cause of these errors is using the /NOIGNORECASE (/NOI) linker option.

MORE INFORMATION

The information in this article depends on the development tools that you are using. If you are not using Microsoft tools, please refer to the documentation for your development tools.

C 6.0 and C/C++ 7.0

If an application or DLL exports __cdecl functions by name only and is linked with /NOI (to preseve case), the Windows loader and GetProcAddress() won't be able to resolve the function names when performing the dynamic link, because Windows expects the names of the exported functions to use all uppercase letters, and __cdecl functions use mixed case. With the /NOI switch, only __pascal functions will be resolved, because the compiler has already converted the names to all uppercase.

Without the /NOI switch, the linker will uppercase all function names, regardless of calling convention (you can verify this in the new executable header). Therefore the Windows loader and GetProcAddress() are able to resolve the names.

Visual C++

The linker in Visual C++ always capitalizes the names of exported functions in the DLL executable header, regardless of whether or not you link with /NOI, and whether they functions are declared __cdecl or not. This was done to avoid the problems described above. One thing that can cause a problem is if you run implib on the DLL with /NOI and link your .EXE (or another DLL) to this implib with /NOI, you'll get unresolved externals because the import library has the names in all uppercase letters but the .EXE is calling them mixed-case.