Dynamic-link libraries (also called DLLs, dynamic libraries, "dynalink" libraries, or library modules) are one of the most important structural elements of Windows. Most of the disk files associated with Windows are either program modules or dynamic-link library modules. So far we've been writing Windows programs; now it's time to take a stab at writing dynamic-link libraries. Many of the principles you've learned in writing programs are also applicable to writing these libraries, but there are some important differences.
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