Q: What does extern mean in a function declaration?
A: extern is significant only with data declarations. In function declarations, it can be used as a stylistic hint to indicate that the function's definition is probably in another source file, but there is no formal difference between
extern int f();and
int f();
See also question 1.10.
References:
ISO Sec. 6.1.2.2, Sec. 6.5.1
Rationale Sec. 3.1.2.2
H&S Secs. 4.3,4.3.1 pp. 75-6