No, passing SIZE_MAX does not violate any precondition of strncat(), because preconditions are fixed properties chosen in advance, not something you add retrospectively when you feel like it.

The C standard does not place any upper bound on the value of strncat()'s size argument, so an strncat implementation has to work as described for all large values of the argument. It's a bug of the implementation if it doesn't, not a violated precondition.

(Reasoning about C programs has been my day job for several years. Contrary to what many think, it is possible, it only needs that the rules be set in advance and respected.)

> Also note that SIZE_MAX is an unreasonable input, one would even say it violates preconditions.

Annex K adds rsize_t, RSIZE_MAX, and friends, which restrict inputs to reasonable values. Unfortunately, it doesn't retcon the old functions like strncpy, and worse, no one actually implements it.

No one? I thought Microsoft implemented it, or at least an early version thereof.

A very early version. Among other differences the functions don't care about RSIZE_MAX (even though newer VS versions define and use rsize_t). However the Windows SDK contains <strsafe.h> which does limit the length of strings (STRSAFE_MAX_CCH).

https://msdn.microsoft.com/en-us/library/windows/desktop/ms6...