Win32 Programming Lesson 19: Introduction to DLLs

Where are we?  Memory allocation: sorted  Processes and Threads: Done  But… how does Windows structure its executables – what are DLLs and how can we use them to our advantage?

DLLs  Understanding DLLs is essential to understanding Windows  Three most important: Kernel32: Memory, processes and threads User32: User interface tasks GDI32: Graphics  However, Windows contains lots of DLLs  The entire Win32 API is in DLLs

DLL Features  Using DLLs: Extends the features of an application Language-independent (that is, you can write DLLs in many different languages) Simplify project management (modular development and versioning) Conserve memory (remember, DLLs share memory) Resource sharing (shared dialogue boxes, for example) Helpful for localization (different DLLs for different locales) Platform independence (loading on older versions, for example) Special Functions (like hooking into existing applications :: evil grin :: )

DLLs and the Process  To use a DLL a Process needs to load it into its address space  Once this occurs, the DLL looks, for all intents and purposes, like the part of the process, but the DLL is shared between multiple processes How? Remember PAGE_WRITECOPY?

Implications  Because of the close relationship of the DLL to the process objects created by the DLL are owned by the process If the DLL calls VirtualAlloc, for example, that memory counts against the process’ usage If the DLL is later unmapped this memory is not automatically freed

Code Problem  The temptation is to always free memory allocated by the DLL directly – that is, if the DLL uses malloc to provide some memory, the process should call free when it’s done  But this is WRONG!  What if the underlying libraries are different?  If a DLL allocated memory, it should provide a call to free it. Ignore this at your own peril.

How DLLs work  For the sake of this discussion, we’ll talk about “executables” (programs) and DLLs (modules) even though DLLs are executable themselves  We’ll look at the lifecycle of a DLL and executable

First  Before we can use a DLL you need to build it  This means we tell Visual Studio that we’re building a DLL  Create a header file that details the exported types  Linker produces a.lib file if at least one function is exported  Linker produces a.dll file

Second  Build an application using the DLL  Compiler uses DLL header file to build.obj files  The.lib file is used to resolve imported functions

Headers  One of the most important parts of the DLL is the header file, as this will be used when compiling programs which use the DLL  Best to use the same header file for building the DLL as for building files which use the DLL  You can look at exports and imports using dumpbin (from VS 2013)

Running the Executable  The OS looks for the needed imports  Where? The directory containing the executable image file The process’s current directory The Windows system directory The Windows directory The directories listed in the PATH environment variable

Assignment  Write your own DLL  Create a.h file which can be used by programs which call your DLL  Recast your linked list program (remember that first assignment!) as a linked-list DLL which exports the functions I specified  Make sure you work out how to free memory correctly in your DLL  Write a program which compiles and runs using your linked list DLL