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A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends1 if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.
A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.
When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in C99 6.5.6/8.
The value is as specified in the standard and the type is determined by the ABI.
[1] Future versions of GCC may zero-extend, or use
a target-defined ptr_extend pattern. Do not rely on sign extension.