CODEWITHSUNDEEP
cpointersbitwise-operators
years_of_no_light
years_of_no_light

Reputation: 958

Meaning of applying bitwise or with a pointer address

While I was reading this code of an ART implementation, I did not understand the reason behind calling SET_LEAF to do bitwise or with pointer address

   #define SET_LEAF(x) ((void*)((uintptr_t)x | 1)) 

   *ref = (art_node*)SET_LEAF(make_leaf(key, key_len, value));

I also saw

#define IS_LEAF(x) (((uintptr_t)x & 1))

which looks at the least significant bit to check if it was a leaf node. Now the whole argument would work only if the pointer address is an even number (0x100504080).

2 | 1 // 3
3 | 1 // 3

Is it guaranteed that the pointer address would always be an even number across all the architecture?

Edit: Possible duplicate How come local types always get an even address

Question on top: Can i safely assume that the pointer would always be starting from event number no matter what kind of data I have?

Upvotes: 1

Views: 137

Answers (1)

Aplet123
Aplet123

Reputation: 35560

It is technically undefined behavior, but considering the fact that they use _mm_movemask_epi8, they probably don't care about portability. In x86 with standard glibc malloc, the returned address should always be aligned to a word (8 bytes in 64 bit), which means that it's always even.

Upvotes: 1

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