When two pointers are subtracted, both must point to elements of the same array object or just one past the last element of the array object (C Standard, 6.5.7 [ISO/IEC 9899:2024]); the result is the difference of the subscripts of the two array elements. Otherwise, the operation is undefined behavior. (See undefined behavior 4845.)
Similarly, comparing pointers using the relational operators <, <=, >=, and > gives the positions of the pointers relative to each other. Subtracting or comparing pointers that do not refer to the same array is undefined behavior. (See undefined behavior 4845 and undefined behavior 5350.)
Comparing pointers using the equality operators == and != has well-defined semantics regardless of whether or not either of the pointers is null, points into the same object, or points one past the last element of an array object or function.
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