Question

I have a question about default argument promotions in C99 standard. In the book “C Programming – A Modern Approach, 2nd Edition” I’ve read that:

Argument Conversions:

[…]

1) The compiler has encountered a prototype prior to the call. […]

2) The compiler has not encountered a prototype prior to the call. The compiler performs the default argument promotions: (1)floatarguments are converted todouble. (2) The integral promotions are performed, causingcharandshortarguments to be converted toint. (In C99, the integer promotions are performed.)

A few lines further is shown an example in which there is no function prototype or definition before calling it. It is commented as follows:

Of course, a much better solution is to provide a prototype forsquarebefore calling it. In C99, callingsquarewithout first providing a declaration or definition of the function is an error.

Aren’t those two cursive sentences kind of contrary with each other? I mean, if C99 forbids calling functions without previous declaration/definition how can it determine promotions in that kind of function call?

Answer 1

No they are not contradictory.

A declaration is not necessarily a prototype:

int f();

declares the function f but isn’t a prototype since nothing is known about the argument types.

int (a)
 in a;
{
 ...
}

is a definition but isn’t a prototype either.