### Question 10.6

I'm splitting up a program into multiple source files for the first time, and I'm wondering what to put in .c files and what to put in .h files. (What does ``.h'' mean, anyway?)

As a general rule, you should put these things in header (.h) files:

>     macro definitions (preprocessor #defines)
>     structure, union, and enumeration declarations
>     typedef declarations
>     external function declarations (see also question 1.11)
>     global variable declarations

It's especially important to put a declaration or definition in a header file when it will be shared between several other files. (In particular, never put external function prototypes in .c files. See also question 1.7.)

On the other hand, when a definition or declaration should remain private to one source file, it's fine to leave it there.

See also questions 1.7 and 10.7.

References: K&R2 Sec. 4.5 pp. 81-2
H&S Sec. 9.2.3 p. 267
CT&P Sec. 4.6 pp. 66-7

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