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Re: [PATCH][BZ 17979][BZ 17721] Fix issues with sys/cdefs.h and uchar.h when using non-gcc compiler.


On 2016-01-29 02:43, Mike Frysinger wrote:
On 28 Jan 2016 23:16, Joseph Myers wrote:
On Thu, 28 Jan 2016, Mike Frysinger wrote:
On 28 Jan 2016 22:52, Joseph Myers wrote:
On Thu, 28 Jan 2016, Dwight Guth wrote:
Okay but if so, then why put the __restrict in the header files at all
if it doesn't really matter? And why put it there only if the compiler
is gcc?

Effectively it serves as documentation of intent for people reading the
headers (much like the argument names with __ prefixes).

wouldn't it also assist automated tools like linters/static analyzers ?

If they hardcode information about particular functions, the qualifiers in
the headers are irrelevant.  If not, even having restrict qualifiers on
the parameters in the function definitions is only useful when you look at
the body of the definitions as well, unless you apply heuristics beyond
what is supported by the standard.

I would add that for an analyzer to depend on a specific implementation is a bit risky. The meaning of the restrict qualifier on the parameters of a libc function depends on the description of the function in the standard, not on any implementation.

Remember that if a function has two restricted pointer arguments (that are
restricted in the definition), this does *not* mean that they don't alias
- only that *if* a particular execution of the function modifies some
elements pointed to by one of the pointers, those elements are not also
accessed other than through that pointer.  (But it's completely valid to
have two restricted pointers to the same array, one only used to access /
modify odd-numbered elements of that array, and the other one only used to
access / modify even-numbered elements of that array.  Now, static
analyzers might reasonably consider that dubious usage that should be
diagnosed.)

i think having analyzers warn about that by default is a sane position.
i would expect that such usage is more commonly an unintended mistake
rather than the function actually has such esoteric behavior.  i don't
think any of the glibc functions are written in this manner and expect
the pointers to be distinct memory locations.

What about strncat? I think this:

  char s[10] = "abc";
  strncat(s, s, 2);

is fine according to C11.

--
Alexander Cherepanov


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