4 ms·

Read the gcc mailing list. There is alot of user hostile "fix your broken program" from language lawyers. I think aliasing rules messes up memcpy if you are do

Read the gcc mailing list. There is alot of user hostile "fix your broken program" from language lawyers. I think aliasing rules messes up memcpy if you are dogmatic?

ilyt 4y ago
To be fair it is extremely easy to write broken C/C++ program
maccard 4y ago
Theres a big difference between "the GCC mailing list is full of language lawyers" and "memcpy doesn't work because UB and the compiler Devs fixed it by adding an exception for it". > I think aliasing rules messes up memcpy if you are dogmatic? I'm _not_ dogmatic or a language lawyer and aliasing is a total pain in the ass, but that's a whole different topic to compilers getting fundamentals wrong because of language lawyering.
anonymoushn 4y ago
I think memcpy is fine because it operates on a special type that is allowed to alias anything. Strict aliasing has other fun issues like the impossibility of implementing malloc-like functions or the lack of any requirement for access to non-char members of structs to work at all[0]. [0]: https://stackoverflow.com/questions/49298704/how-reason-about-strict-aliasing-for-malloc-like-functions#comment87025368_49988951 https://stackoverflow.com/questions/49298704/how-reason-abou...
- olliej 4y ago
  memcpy is UB because it reads padding bytes in structs. memmove fails as it requires pointer comparisons between separate allocations, which is UB.
  - anonymoushn 4y ago
    Sounds like we shouldn't use memcpy on structs containing padding!
    - maccard 4y ago
      No, memcpy is fine because it's a language level function. What you _can't_ do is implement your own memcpy alternative safely.
  - nayuki 4y ago
    memcpy() and memmove() are language-level functions. They are allowed to do things that cannot be implemented in user code. The C standard library's implementation of these functions have tight integration with the compiler to ensure that it can implement all the required semantics without triggering undefined behavior.
  - leni536 4y ago
    reading padding bytes is not UB. Comparing unrelated pointers is unspecified, not undefined. It's also an implementation detail of memmove. It can use a magical total order comparison of pointers that is otherwise unavailable to users of the language.

Read the gcc mailing list. There is alot of user hostile "fix your broken program" from language lawyers. I think aliasing rules messes up memcpy if you are dogmatic?

ilyt 4y ago

To be fair it is extremely easy to write broken C/C++ program

maccard 4y ago

Theres a big difference between "the GCC mailing list is full of language lawyers" and "memcpy doesn't work because UB and the compiler Devs fixed it by adding an exception for it". > I think aliasing rules messes up memcpy if you are dogmatic? I'm _not_ dogmatic or a language lawyer and aliasing is a total pain in the ass, but that's a whole different topic to compilers getting fundamentals wrong because of language lawyering.

anonymoushn 4y ago

I think memcpy is fine because it operates on a special type that is allowed to alias anything. Strict aliasing has other fun issues like the impossibility of implementing malloc-like functions or the lack of any requirement for access to non-char members of structs to work at all[0]. [0]: https://stackoverflow.com/questions/49298704/how-reason-about-strict-aliasing-for-malloc-like-functions#comment87025368_49988951 https://stackoverflow.com/questions/49298704/how-reason-abou...

olliej 4y ago
memcpy is UB because it reads padding bytes in structs. memmove fails as it requires pointer comparisons between separate allocations, which is UB.
- anonymoushn 4y ago
  Sounds like we shouldn't use memcpy on structs containing padding!
  - maccard 4y ago
    No, memcpy is fine because it's a language level function. What you _can't_ do is implement your own memcpy alternative safely.
- nayuki 4y ago
  memcpy() and memmove() are language-level functions. They are allowed to do things that cannot be implemented in user code. The C standard library's implementation of these functions have tight integration with the compiler to ensure that it can implement all the required semantics without triggering undefined behavior.
- leni536 4y ago
  reading padding bytes is not UB. Comparing unrelated pointers is unspecified, not undefined. It's also an implementation detail of memmove. It can use a magical total order comparison of pointers that is otherwise unavailable to users of the language.