Pointer Arithmetic: ++*ptr or *ptr++?

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These statements produce different results because of the way in which the operators bind. In particular, the prefix ++ operator has the same precedence as *, and they associate right-to-left. Thus

++*ptr

is parsed as

++(*ptr)

meaning “increment the value pointed at by ptr,”. On the other hand, the postfix ++ operator has higher precedence than the dereferrence operator *. Thefore

*ptr++

means

*(ptr++)

which means “increment ptr to go to the element after the one it points at, then dereference its old value” (since postfix ++ hands back the value the pointer used to have).

In the context you described, you probably want to write ++*ptr, which would increment x indirectly through ptr. Writing *ptr++ would be dangerous because it would march ptr forward past x, and since x isn’t part of an array the pointer would be dangling somewhere in memory (perhaps on top of itself!)

Hope this helps!

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