On 2018-05-22 04:46:50 +0200, Pedro Gimeno wrote: > The C11 standard also says: > > P.47: > > 1. A pointer to void may be converted to or from a pointer to any incomplete > or object type. A pointer to any incomplete or object type may be converted > to a pointer to void and back again; the result shall compare equal to the > original pointer. > > (Definition of object, p.5 §3.14: "region of data storage in the execution > environment, the contents of which can represent values") > > [...] > > 3. An integer constant expression with the value 0, or such an expression > cast to type void *, is called a null pointer constant.55) If a null pointer > constant is converted to a pointer type, the resulting pointer, called a null > pointer, is guaranteed to compare unequal to a pointer to any object or > function. > > 4. Conversion of a null pointer to another pointer type yields a null pointer > of that type. Any two null pointers shall compare equal. > > Doesn't that allow using (void *)0 for the suggested purpose?
No, with variadic functions, a null pointer of some type is provided as an argument, and then the value is re-read as another pointer type (this is actually a simplification, because depending on the calling conventions, using incompatible types can just be meaningless). You do not have a conversion. For instance, something similar can be said between the 0 of type int and the 0 of type long: in a type conversion, the value does not change, and (int) 0 and (long) 0 compare equal. But if int is on 4 bytes and long is on 8 bytes, they are not interchangeable in a variadic function. Note that for pointers, this is even worse, since even in the case the size and alignment requirements are the same, the representation of the null pointer may still depend on the type. -- Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon) _______________________________________________ gmp-devel mailing list gmp-devel@gmplib.org https://gmplib.org/mailman/listinfo/gmp-devel
