Implementation of C11 Bounds-checking interfaces

[Ulrich Bayer]

> I definitely like the idea of keeping it separate, but I question the
> amount of code duplication. For example, tmpfile_s does not need to
> reimplement tmpfile in terms of mkstemp; in fact, such an
> implementation does not conform to ISO C since it references POSIX
> symbols outside the ISO C namespace. A much simpler implementation
> would just be:
>
> errno_t tmpfile_s(FILE * restrict * restrict streamptr)
> {
> 	if (!streamptr) {
> 		RUNTIME_CONSTRAINT_HANDLER();
> 		return EINVAL;
> 	}
> 	*streamptr = tmpfile();
> 	return *streamptr ? 0 : errno;
> }

You raise a good point. tmpfile_s is a special case.  
1) I was under the impression that tmpfile fails to provide exclusive access. I did not check whether that is true for glibc.
2) We decided to extend the ISO functionality of tmpfile_s by adding functions that allow setting and retrieving 
the temporary directory where tmpfile_s and tmpnam_s create files. If you set this temporary directory to a non-shared
directory even the use of tmpnam becomes okay imho because no one else can write files there. We were mainly following:
https://www.securecoding.cert.org/confluence/display/seccode/FIO43-C.+Do+not+create+temporary+files+in+shared+directories
Whether this is useful can be discussed of course.

Ulrich