Q38729: Mixing Old Declarations with Prototypes Causes Passing Errors

Article: Q38729
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# G881107-5854 | mspl13_c
Last Modified: 12-DEC-1988

Question:

It is my understanding that both of the following functions should
produce identical results:

float AAA(float x1, float x2)
{
    x1 = x2;
}

float BBB(x1,x2)
float x1,x2;
{
    x1 = x2;
}

However, they don't. Looking at the code, I see that in BBB, the two
parameters are treated as double instead of float. When I compile with
the /Zg switch, which generates function prototypes, the following
prototypes are generated:

extern  float AAA(float x1, float x2);
extern  float BBB(double x1, double x2);

Why is this behavior occuring?

Response:

The following is from the May 5, 1988 ANSI draft, Section 3.3.2.2:

"If the expression that denotes the called function has a type that
does not include a prototype...arguments that have type float are
promoted to double.

"If the expression that denotes the called function has a type that
includes a prototype, the arguments are implicitly converted, as if
by assignment, to the types of the corresponding parameters."

For AAA, you're using the new function definition style. Note that if
you call this function (perhaps in another module) without a prototype
in scope, you'll have problems because you'll pass doubles rather than
floats (see first paragraph above).

BBB uses the old style of definition as described in K & R. Because
K & R specified that the floats are to be widened to doubles when
they're passed to functions (and in a variety of other situations
as well), the old style declarations maintain the old semantics.

Therefore, the /Zg switch is correctly generating the function
prototypes.

Your program wouldn't run when you declared the following prototype
before calling BBB because the prototype that was in scope when you
CALLED BBB did not match the implicit prototype generated when the
function was defined:

   void BBB(float, float);

As a result, you passed floats to a function that was expecting
doubles.

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.