Why do functions need to be declared before they are used?

Question

When reading through some answers to this question, I started wondering why the compiler actually does need to know about a function when it first encounters it. Wo

Answer 1

One of the biggest reasons why this was made mandatory even in C99 (compared to C89, where you could have implicitly-declared functions) is that implicit declarations are very error-prone. Consider the following code:

First file:

#include 
void doSomething(double x, double y)
{
    printf("%g %g\n",x,y);
}

Second file:

int main()
{
    doSomething(12345,67890);
    return 0;
}

This program is a syntactically valid* C89 program. You can compile it with GCC using this command (assuming the source files are named test.c and test0.c):

gcc -std=c89 -pedantic-errors test.c test0.c -o test

Why does it print something strange (at least on linux-x86 and linux-amd64)? Can you spot the problem in the code at a glance? Now try replacing c89 with c99 in the command line — and you'll be immediately notified about your mistake by the compiler.

Same with C++. But in C++ there're actually other important reasons why function declarations are needed, they are discussed in other answers.

_{* But has undefined behavior}