Question 11.5

Why does the declaration

extern f(struct x *p);

give me an obscure warning message about ``struct x introduced in prototype scope''?

In a quirk of C's normal block scoping rules, a structure declared (or even mentioned) for the first time within a prototype cannot be compatible with other structures declared in the same source file (it goes out of scope at the end of the prototype).

To resolve the problem, precede the prototype with the vacuous-looking declaration

	struct x;

which places an (incomplete) declaration of struct x at file scope, so that all following declarations involving struct x can at least be sure they're referring to the same struct x.

References: ANSI Sec. 3.1.2.1, Sec. 3.1.2.6, Sec. 3.5.2.3
ISO Sec. 6.1.2.1, Sec. 6.1.2.6, Sec. 6.5.2.3

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