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constraints-of-delay-loading-dlls.md

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description Learn more about: Constraints of delay loading DLLs
title Constraints of delay loading DLLs
ms.date 01/19/2021
helpviewer_keywords
constraints [C++], delayed loading of DLLs
delayed loading of DLLs, constraints
DLLs [C++], constraints

Constraints of delay loading DLLs

There are several constraints on the delay loading of DLL imports.

  • Imports of data can't be supported. A workaround is to explicitly handle the data import yourself by using LoadLibrary (or by using GetModuleHandle after you know the delay-load helper has loaded the DLL) and GetProcAddress.

  • Delay loading Kernel32.dll isn't supported. This DLL must be loaded for the delay-load helper routines to work.

  • Binding of forwarded entry points isn't supported.

  • A process may have different behavior if a DLL is delay-loaded, instead of loaded on start-up. It can be seen if there are per-process initializations that occur in the entry point of the delay-loaded DLL. Other cases include static TLS (thread local storage), declared using __declspec(thread), which isn't handled when the DLL is loaded via LoadLibrary. Dynamic TLS, using TlsAlloc, TlsFree, TlsGetValue, and TlsSetValue, is still available for use in either static or delay-loaded DLLs.

  • Reinitialize static global function pointers to imported functions after the first call of each function. That's required because the first use of a function pointer points to the thunk, not the loaded function.

  • There's currently no way to delay the loading of only specific procedures from a DLL while using the normal import mechanism.

  • Custom calling conventions (such as using condition codes on x86 architectures) aren't supported. Also, the floating-point registers aren't saved on any platform. If your custom helper routine or hook routines use floating-point types, they need to save and restore the complete floating-point state on machines that use register calling conventions with floating-point parameters. Be careful about delay-loading the CRT DLL, particularly if you call CRT functions that take floating-point parameters on a numeric data processor (NDP) stack in the help function.

See also

Linker support for delay-loaded DLLs
LoadLibrary function
GetModuleHandle function
GetProcAddress function
TlsAlloc function
TlsFree function
TlsGetValue function
TlsSetValue function