Implicit function declarations in C

Question

What is meant by the term \"implicit declaration of a function\"? A call to a standard library function without including the appropriate header file produces a warning as i

Answer 1

To complete the picture, since -Werror might considered too "invasive",
for gcc (and llvm) a more precise solution is to transform just this warning in an error, using the option:

-Werror=implicit-function-declaration

See Make one gcc warning an error?

Regarding general use of -Werror: Of course, having warningless code is recommendable, but in some stage of development it might slow down the prototyping.