what is __attribute__((__bounded__(__string__,2,3)))?

Question

I was openbsd bcrypt code and I got warning of unknown attribute bounded at below code snippet:

void SHA256Update(SHA2_CTX *, const void *, size_t)
__attribute__((__bounded__(__string__,2,3)));

I tried to google the attribute bounded but no relevant result was found. I want to port that code to a different platform and if I get the meaning of bounded attribute, I want to use a similar attribute of that platform.

Any suggestion would be appreciated!

Answer 1

The __bounded__ attribute is available in the context of function declarations to enable to determine the length of the memory region pointed by one of the function arguments using the value of another of it's arguments; the first parameter slightly changes the type of the check for different styles of functions.

In this case it augments the type of the second argument to the function with the length specified by the third argument; the __string__ bound style additionally checks that the size argument doesn't come from a sizeof applied to a pointer, as you want the destination's size.

It's only available in the OpenBSD's fork of GCC (see man 1 gcc-local ); there also was a short-lived GNU C extension (somewhere between 2000 and 2003) by the same name and for the same purpose, which was a direct type qualifier instead and was usable outside function declarations too, however, AFAIK it was undocumented.