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

Implicit declarations are not valid in C.

C99 removed this feature (present in C89).

gcc chooses to only issue a warning by default with -std=c99 but a compiler has the right to refuse to translate such a program.