Defect Report #148

Submission Date: 23 Feb 95
Submittor: BSI
Source: Clive D.W. Feather
Question
Submitted to BSI by Clive D.W. Feather [email protected].
In this Defect Report, identifiers lexically identical to those declared in standard headers refer to the identifiers declared in those standard headers, whether or not the header is explicitly mentioned.
This Defect Report has been prepared with considerable help from Mark Brader, Jutta Degener, Ronald Guilmette, and a person whose employment conditions require anonymity. However, except where stated, opinions expressed or implied should not be assumed to be those of any person other than myself.
Defect Report UK 032: Defining library functions
Subclause 7.1.7 is unclear about when it is permitted to declare a library function. Consider the following translation unit:
#include <math.h> double (sin)(double);
Subclause 7.1.7 states in part:
Any function declared in a header may be additionally implemented as a macro defined in the header, so a library function should not be declared explicitly if its header is included.
Since the wording uses the term "should", this does not appear to actually be a requirement on programs, and the code appears to be strictly conforming; in other words, the C Standard here simply uses overly restrictive wording while trying to assist readers, and does not actually forbid the above code.
Is this interpretation correct?
Note that code such as the above is useful if the #include is conditionally compiled or is within a header not under the control of the code's author.
Suggested Technical Corrigendum:
If the intent was to forbid such a declaration, then change the quoted text to:
A library function shall not be declared explicitly if its header is included.
If the intent was to allow the macros described in subclause 7.1.7 to be object-like macros (though other wording in 7.1.7 appears to forbid this), then change the quoted text to:
A library function must not be declared explicitly if its header is included, unless any macro definition of the name has been removed with #undef.
If the intent was to allow the example declaration, then change the quoted text to:
Any function declared in a header may be additionally implemented as a macro defined in the header, so one of the techniques below should be used to ensure that any explicit declaration of a library function is not affected by any such macro.
Response
The wording of the C Standard is as intended. The term "should" is intended as guidance to the programmer as is the sentence following the one cited in the C Standard.
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