4 ms·

I guess one could still be evil and pass a pointer to a non-int type to the function. That example is really bad, you don't need a void pointer to show this ub

by vrfcodf 10y ago

I guess one could still be evil and pass a pointer to a non-int type to the function. That example is really bad, you don't need a void pointer to show this ub 'optimization'.

PeCaN 10y ago
That'd still be defined though. It'd just be defined to set the first sizeof(int) bytes of `* p` to 1. ;) void * and (char * ) can alias to anything and the compiler has to make it work. Of course if you pass a pointer to a type smaller than sizeof(int)-1 bytes then it accesses uninitialized memory which is UB, but that's a different issue. EDIT: Aaaand I wasn't thinking. ̶E̶D̶I̶T̶ ̶E̶D̶I̶T̶:̶ ̶N̶o̶t̶ ̶o̶n̶l̶y̶ ̶d̶i̶d̶ ̶I̶ ̶m̶i̶s̶-̶r̶e̶a̶d̶,̶ ̶b̶u̶t̶ ̶v̶o̶i̶d̶ ̶p̶o̶i̶n̶t̶e̶r̶ ̶c̶a̶n̶'̶t̶ ̶a̶l̶i̶a̶s̶ ̶a̶n̶y̶t̶h̶i̶n̶g̶ ̶i̶n̶ ̶s̶t̶a̶n̶d̶a̶r̶d̶ ̶C̶ ̶a̶n̶y̶w̶a̶y̶ ̶(̶t̶h̶o̶u̶g̶h̶ ̶I̶I̶R̶C̶ ̶G̶C̶C̶ ̶t̶r̶e̶a̶t̶s̶ ̶i̶t̶ ̶l̶i̶k̶e̶ ̶c̶h̶a̶r̶ ̶p̶o̶i̶n̶t̶e̶r̶ ̶w̶r̶t̶ ̶a̶l̶i̶a̶s̶i̶n̶g̶)̶. EDIT EDIT EDIT: Actually maybe it can and I shouldn't be reading technical docs at 2am.
- vrfcodf 10y ago
  That is not correct. The type used to write to the memory is int, not void or char. If the object, whose address is passed to the function is not compatible with int, the behavior is undefined. You should read about strict aliasing, you will be surprised.
  - PeCaN 10y ago
    You're right, I misread and misinterpreted that! Thanks. Somehow I wasn't thinking and didn't realize that `t` aliases whatever was passed as `p`, and dereferencing a non int or char pointer would be undefined there.
- orf 10y ago
  On Firefox your edit strikethrough makes the page very very wide.
- quotemstr 10y ago
  Maybe you should familiarize yourself with the standard before preaching about how easy it is to comply with it.

by vrfcodf 10y ago

I guess one could still be evil and pass a pointer to a non-int type to the function. That example is really bad, you don't need a void pointer to show this ub 'optimization'.

PeCaN 10y ago

That'd still be defined though. It'd just be defined to set the first sizeof(int) bytes of `* p` to 1. ;) void * and (char * ) can alias to anything and the compiler has to make it work. Of course if you pass a pointer to a type smaller than sizeof(int)-1 bytes then it accesses uninitialized memory which is UB, but that's a different issue. EDIT: Aaaand I wasn't thinking. ̶E̶D̶I̶T̶ ̶E̶D̶I̶T̶:̶ ̶N̶o̶t̶ ̶o̶n̶l̶y̶ ̶d̶i̶d̶ ̶I̶ ̶m̶i̶s̶-̶r̶e̶a̶d̶,̶ ̶b̶u̶t̶ ̶v̶o̶i̶d̶ ̶p̶o̶i̶n̶t̶e̶r̶ ̶c̶a̶n̶'̶t̶ ̶a̶l̶i̶a̶s̶ ̶a̶n̶y̶t̶h̶i̶n̶g̶ ̶i̶n̶ ̶s̶t̶a̶n̶d̶a̶r̶d̶ ̶C̶ ̶a̶n̶y̶w̶a̶y̶ ̶(̶t̶h̶o̶u̶g̶h̶ ̶I̶I̶R̶C̶ ̶G̶C̶C̶ ̶t̶r̶e̶a̶t̶s̶ ̶i̶t̶ ̶l̶i̶k̶e̶ ̶c̶h̶a̶r̶ ̶p̶o̶i̶n̶t̶e̶r̶ ̶w̶r̶t̶ ̶a̶l̶i̶a̶s̶i̶n̶g̶)̶. EDIT EDIT EDIT: Actually maybe it can and I shouldn't be reading technical docs at 2am.

vrfcodf 10y ago
That is not correct. The type used to write to the memory is int, not void or char. If the object, whose address is passed to the function is not compatible with int, the behavior is undefined. You should read about strict aliasing, you will be surprised.
- PeCaN 10y ago
  You're right, I misread and misinterpreted that! Thanks. Somehow I wasn't thinking and didn't realize that `t` aliases whatever was passed as `p`, and dereferencing a non int or char pointer would be undefined there.
orf 10y ago
On Firefox your edit strikethrough makes the page very very wide.
quotemstr 10y ago
Maybe you should familiarize yourself with the standard before preaching about how easy it is to comply with it.