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Failure to specify function prototypes results in a function being implicitly defined. Without a function prototype, the compiler will assume the the correct number and type of parameters have been supplied to a function. Calling a function with a different number of arguments then that function expects results in undefined, and perhaps unintended behavior.

C99 removed implicit function declarations from the C language [\[ref]\]. However, compilers will typically allow compilation of programs that contain implicitly defined functions, although they will issue a warning. These warnings should be resolved \[[MSC00-A|MSC00-A. Compile cleanly at high warning levels]\], but they will not prevent program compilation []. Given this, functions should be declared with the appropriate function prototype.

Non-Compliant Code Example 1

In this example, the definition of func() expects three parameters but is supplied only two. However, because there is no prototype for func(), the compiler assumes that the correct number of parameters has been supplied, using the next value on the program stack as the missing third argument.

function(1, 2);
...
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

...

int function(int,int,int); 
...

function(1,2); 
...
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

...