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99: Mainline

The C compiler ordinarily requires C programs to be ordered such that each function's definition appears before calls to that function appear. Thus, doubling_time must be defined before main is defined, because main contains a call to doubling_time.

The reason for requiring programs to be so ordered is that such ordering simplifies the development of efficient compilers.

Later, in the hardcopy version of this book, you learn that function prototypes make it possible to write programs in which function calls do appear before definitions. Until then, be sure that each function's definition appears before calls to that function appear.