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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