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RuntimeWrapping

This project demonstrates how to wrap a C function -- specifically, printf(3).

Here's how it works:

  1. WrappedLibc exports a printf function whose prototype matches the one declared in stdio.h.

  2. WrappedLibc's printf calls dlsym with the module handle RTLD_NEXT to find the original printf at runtime and invoke it with its own arguments. It can't invoke it with the original arguments because printf is variadic. So it creates a va_list and calls vprintf instead.

  3. WrappedLibc's OTHER_LDFLAGS includes -sub_library libSystem, which makes libWrappedLibc.dylib reexport all the symbols from libSystem (to which libc is an alias on OS X).

  4. WrapperTester links against libWrappedLibc.dylib. It does not link against libSystem --- LINK_WITH_STANDARD_LIBRARIES is set to NO. This causes it to look up all standard library functions as indirect symbols via libWrappedLibc.