While it is generally prohibited to define a C-style variadic function, such a function may still be defined when that function has external, C language linkage (see DCL50-CPP. Do not define a C-style variadic function for details). Under these circumstances, care must be taken when invoking the va_start() macro. The C standard library macro va_start() defines several semantic restrictions on the type of the value of its second parameter. The C Standard, subclause 7.16.1.4, paragraph 4 [ISO/IEC 9899:2011], states:
The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the
...). If the parameter parmN is declared with theregisterstorage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behavior is undefined.
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