4 ms·

If you overlay a struct in your (char) buffer and dereference a pointer to it you would be accessing something with a different type than its declared type(char

If you overlay a struct in your (char) buffer and dereference a pointer to it you would be accessing something with a different type than its declared type(char* as struct something *), it’s strict aliasing violation To do stay in the rules you could set up a void* to suitable region in a linkerscript

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matheusmoreira 3y ago
Welp. Linus Torvalds was right about this stuff. https://lwn.net/Articles/316126/ https://lwn.net/Articles/316126/ https://lkml.org/lkml/2003/2/26/158 https://lkml.org/lkml/2003/2/26/158
- deleted 3y ago
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- saagarjha 3y ago
  Wait until someone tells Linus about how processors do reordering. (Yes, I know he understands it. Clearly he just refuses to accept that compilers can also reorder his code and he needs to accommodate that.)
  - mtzet 3y ago
    Processors doing out-of-order execution doesn't change the semantics of the code. That's very different from the example where gcc just throws away the assignment. The idea that he just needs to accommodate the compiler people is silly. Compilers exist to serve programmers, not the other way around. It's entirely reasonable to disagree with the compiler developers and use a flag to disable behaviour your don't want.
    - saagarjha 3y ago
      It does when you have a weak memory model and multiple threads involved.
  - gpderetta 3y ago
    In fairness, CPU reordering is does not visibly affect a thread view of their own loads and stores.
  - matheusmoreira 3y ago
    > he needs to accommodate that Clearly he doesn't. He just disabled harmful features and called it a day. Stuff like strict aliasing and undefined signed integer overflow apparently do nothing but serve as an excuse for the optimizer to screw up perfectly reasonable code. https://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html https://cellperformance.beyond3d.com/articles/2006/06/unders... https://stackoverflow.com/q/2958633/512904 https://stackoverflow.com/q/2958633/512904 Looks like this is a pretty good approach to fixing the C language. Take the nonsensical undefined stuff and tell the compiler to define it. Looks like there is a may_alias type attribute to tell GCC that some types may alias: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-may_005falias-type-attribute https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.ht... I'm not sure if it's worth the effort though. Too much uncertainty.
    - saagarjha 3y ago
      That's not how they work but I doubt getting into an argument with you about how undefined behavior works with someone who is dead-set on it being harmful is going to be productive.

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matheusmoreira 3y ago

Welp. Linus Torvalds was right about this stuff. https://lwn.net/Articles/316126/ https://lwn.net/Articles/316126/ https://lkml.org/lkml/2003/2/26/158 https://lkml.org/lkml/2003/2/26/158

deleted 3y ago
[deleted]
saagarjha 3y ago
Wait until someone tells Linus about how processors do reordering. (Yes, I know he understands it. Clearly he just refuses to accept that compilers can also reorder his code and he needs to accommodate that.)
- mtzet 3y ago
  Processors doing out-of-order execution doesn't change the semantics of the code. That's very different from the example where gcc just throws away the assignment. The idea that he just needs to accommodate the compiler people is silly. Compilers exist to serve programmers, not the other way around. It's entirely reasonable to disagree with the compiler developers and use a flag to disable behaviour your don't want.
  - saagarjha 3y ago
    It does when you have a weak memory model and multiple threads involved.
- gpderetta 3y ago
  In fairness, CPU reordering is does not visibly affect a thread view of their own loads and stores.
- matheusmoreira 3y ago
  > he needs to accommodate that Clearly he doesn't. He just disabled harmful features and called it a day. Stuff like strict aliasing and undefined signed integer overflow apparently do nothing but serve as an excuse for the optimizer to screw up perfectly reasonable code. https://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html https://cellperformance.beyond3d.com/articles/2006/06/unders... https://stackoverflow.com/q/2958633/512904 https://stackoverflow.com/q/2958633/512904 Looks like this is a pretty good approach to fixing the C language. Take the nonsensical undefined stuff and tell the compiler to define it. Looks like there is a may_alias type attribute to tell GCC that some types may alias: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-may_005falias-type-attribute https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.ht... I'm not sure if it's worth the effort though. Too much uncertainty.
  - saagarjha 3y ago
    That's not how they work but I doubt getting into an argument with you about how undefined behavior works with someone who is dead-set on it being harmful is going to be productive.