undefined | Better HN

0 pointsCJefferson9y ago0 comments

No, that's exactly wrong (in this case).

For variadic C functions, you MUST use 0 (or NULL, which is the same as 0). Passing nullptr will case abort() to be called (as is the case whenever you pass a C++ class into a variadic function).

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CJeffersonOP9y ago· 2 in thread

Reply (to myself, based on a correction)

Apologises, I mis-remembered the rule -- it is only C++ types which have a non-trivial destructor which cause an abort when you pass them to a variadic funciton.

HOWEVER. This is still undefined behaviour. nullptr is an object of type std::nullptr_t. When you pass it to execlp, it just gets pushed through as a bit-pattern (as with any type passed to a variadic C function), and there is no guarantee what comes out looks like a "true" (void*)0 C-style null pointer.

ben0x5399y ago

There isn't a guarantee that NULL has a bit pattern that's the same as a true (void)0 C-style null pointer either.
I didn't look it up directly but apparently NULL is allowed to be nullptr instead of 0, and nullptr converts to void
in varargs arguments.

I think C/C++ also allow actual null pointers to have different bit patterns than the integer zero, so in C++03 passing NULL to terminate a varargs argument list would technically be incorrect, or maybe implementation-defined.

CJeffersonOP9y ago

Damn, there is a special exception in the definition of vararg functions, just for nullptr! throws things in the air I give up having discussions about C++.

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