Demo of delayload with Visual Studio

• mep is is the application
• mep links with vart
• vart links with xrt
• vart delayloads xrt
• vart calls AddDllDirectory during static global initialization
• xrt is loaded when xrt::xrt construtor is first called from vart
• xrt prints the path from where it was loaded
• xrt_legacy doesn't have the xrt::run API.

Build and run

% cmake -B build/latest
% cmake --build build/latest --verbose --config Release --target install

% cd build/latest/bin

% mep.exe
vart.cpp:dllpath::dllpath()
vart::vart()
vart::run()
X::X()
C:\Users\stsoe\tmp\MEP2\build\bin\xrt.dll
xrt::xrt()
xrt::run()
xrt::~xrt()
vart::~vart()

% mv xrt.dll c:\users\stsoe\tmp\dllrepo

% mep.exe
vart.cpp:dllpath::dllpath()
vart::vart()
vart::run()
X::X()
c:\users\stsoe\tmp\dllrepo\xrt.dll
xrt::xrt()
xrt::run()
xrt::~xrt()
vart::~vart()

Demo of missing API feature

The demo xrt class implements xrt::version() to indicate to application vart that the function xrt::run() is present or not.

vart is built against a version of xrt that contains the run() API, but if we deploy the legacy version of xrt.dll, then the function won't be found at runtime.

With delay loading of xrt.dll the application vart will still run, but if it tries to call xrt::run() a delay hook is called to indicate GetProc failure.

In the vart example, the call to xrt::run() is protected against a version check (xrt::version()) so if at runtime the xrt.dll is the legacy one, then vart will still run successfully, only not call xrt::run(). This means that the delay hook is unused in this example.

% cmake -B build/latest
% cmake --build build/latest --verbose --config Release --target install
% cmake -B build/legacy
% cmake --build build/legacy --verbose --config Release --target install

% cd build/latest/bin
% mep.exe
vart.cpp:dllpath::dllpath()
vart::vart()
vart::run()
X::X()
C:\<path>\bin\xrt.dll
xrt::xrt()
xrt::run()
xrt::~xrt()
vart::~vart()

% copy ..\legacy\bin\xrt.dll .
% mep.exe
vart.cpp:dllpath::dllpath()
vart::vart()
vart::run()
X::X()
C:\Users\stsoe\git\stsoe\delayload\build\latest\bin\xrt.dll
xrt::xrt()
xrt::~xrt()
vart::~vart()

To demo the delay hook, uncomment the conditional version check in vart.cpp.