Re: restrictness of strtoi(3bsd) and strtol(3)

[Alejandro Colomar <alx@kernel.org>]

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Hello Amol,

On Sun, Dec 03, 2023 at 09:08:22PM +0530, Amol Surati wrote:
[...]

> Referring to the points you make later, removing the restrict-qualifier from
> nptr then explicitly permits *endptr and nptr to alias, as the types are now
> devoid of restrict-qualifiers.

[...]

> I think I understand. Since strtol is an external function, the compiler, when
> when compiling strtol(p, &p, 0), has enough information, in the form of the
> strtol prototype and a call to it, to warn about the fact that nptr and *endptr
> may alias in a way that triggers an undefined behaviour.

Exactly.

> 
> Based on how I understood the latest draft n3096.pdf, it is the write to a
> char through *endptr (along with a read of that char through nptr) that
> triggers the violation of the 'restrict' clause. The read and write need not
> be in a particular order. No major compiler warns, though, as evident by
> an example at https://godbolt.org/z/a4xza5xna

As you say, ISO C's formal definition of restrict permits pointers to
overlapping memory, as long as only one of the pointers is dereferenced.

> ------
> What sort of optimizations can a strtol implementation hope to achieve?
> A couple of libcs discard the restrict qualifier when calling their handlers
> for strtol. The situation with strtol doesn't seem to be similar to that with
> memcpy-memmove.
> 
> It seems that, as long as strtol does not assign a value to **endptr, it
> continues to adhere to the std.

To be pedantic, even reading a value from **endptr would cause UB.

But yeah, the point is there: the standard's definition of restrict
isn't very good.

> The historical docs point towards a decision to stamp the prototype with
> restrict under the assumption that (1) the string and the pointer to string
> are in disjoint memory locations,

This justifies the restrict on endptr.

> and (2) the implementations would
> use endptr for nothing else other than maintaining a position in the given
> string.

This is quite brittle.  The restrict on ntpr should cause the compiler
to scream.  I'll report a missing warning on bugzilla.

Cheers,
Alex

-- 
<https://www.alejandro-colomar.es/>

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