Using Delphi, it is very easy to debug a DLL having it called from a host application. This host application can be written using Delphi or any other language.
To debug your DLL in the context for a host application, just add the host application into the corresponding field in Delphi / Menu / Run / Parameters. Then run your project as usual. Delphi will not load the DLL but the host application and intercept the DLL when it is loaded by the host application. It will then honor any breakpoint you've set.
Easy? Yes, it is easy but sometimes, there is a glitch. For some reasons, Delphi won't honor your breakpoints. Among the reasons is that you forgot to use the debug build, or you forgot to enable debugging in the DLL project options.
Sometimes, there is another reason. It made me lose a lot of time playing with all options. Sometimes, I thought that I had found the right option. But later the breakpoints were still not honored.
Finally I found the correct solution. Why it works, I don't know. But so far it worked for me. The solution is: Set option "Include remote debug symbols" to true (Delphi compiler / Linking).
This was not obvious since I didn't used the remote debugger. Anyway, it works!
When option "Include remote debug symbols" is used, Delphi creates one more file with rsm extension in the same directory as the executable. It looks like the debugger always find the symbols there.
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