How to find size of an array in C without sizeof
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How to find size of an array in C without sizeof
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Re: How to find size of an array in C without sizeof
#2Given how many bugs & errors stem from simple fails in range checks etc, I would much rather go with the tried and true way rather than use something "clever".
Quoting http://stackoverflow.com/a/16019052/1470607
Note that this trick will only work in places where `sizeof` would have worked anyway.Re: How to find size of an array in C without sizeof
#3Despite the argument at the end, this is undefined behavior in the latest C specification. The code dereferences a pointer one past the last element.
C11 6.5.6/8:
If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated
Re: How to find size of an array in C without sizeof
#4How is this better than the sizeof method? This looks like a clever way to access sizeof information without explicitly using the sizeof operator.
Re: How to find size of an array in C without sizeof
#5The result you get with this trick is signed, while the result you get with sizeof is unsigned.
Edit: Just to clarify, what you get is ptrdiff_t instead of size_t. So if array size is greater than PTRDIFF_MAX, you get undefined behavior [1].
Re: How to find size of an array in C without sizeof
#6I haven't written C in a while, but I think this is pretty stupid. sizeof() is a compile-time thing in C, so it's substituted with a number by the time you get an executable. See:
http://stackoverflow.com/questions/671790/how-does-sizeofarr...
I think this is effectively doing the same thing, but in a non-standard way; ie. I think `int n = (&arr)[1] - arr;` is substituted with the actual the number by the compiler the same way sizeof() would be, only noone will know wtf is going on.
Disclaimer: I didn't look at the generated code to confirm; I guess it could even be compiler/runtime dependent.
Re: How to find size of an array in C without sizeof
#7Given how many bugs & errors stem from simple fails in range checks etc, I would much rather go with the tried and true way rather than use something "clever". Quoting http://stackoverflow.com/a/16019052/1470607 Note that this trick will only work in places where `sizeof` would have worked anyway.
Yes. This only works for arrays on the stack, at best. It assumes that arrays are placed on the stack in the order of declaration, which is not a requirement of the C standard and may differ between compilers.
Unless you're writing a buffer overflow exploit, in which case you need to know exactly what's on the stack and where, this isn't a good way to program.
Update: misread the article; thought he was differencing with the beginning of the next array.
Re: How to find size of an array in C without sizeof
#8Despite the argument at the end, this is undefined behavior in the latest C specification. The code dereferences a pointer one past the last element. C11 6.5.6/8: If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated
The snipper only calculates the pointer, and does not dereference it. Should be fine.
Re: How to find size of an array in C without sizeof
#9I haven't written C in a while, but I think this is pretty stupid. sizeof() is a compile-time thing in C, so it's substituted with a number by the time you get an executable. See: http://stackoverflow.com/questions/671790/how-does-sizeofarr... I think this is effectively doing the same thing, but in a non-standard way; ie. I think `int n = (&arr)[1] - arr;` is substituted with the actual the number by the compiler th…
I don't think anyone is proposing that people use this. I read it as an exercise to stretch our understanding of other bits of the language.
Re: How to find size of an array in C without sizeof
#10I'm surprised at all of the comments calling this stupid or pointless. The point is not that you should this trick in lieu of sizeof; the point is to shed light on a subtly of C arrays.