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coperator-precedencepointer-arithmetic

What does *--a = *--b mean in C

Suppose a and b are pointers,

My understanding is *--a = *--b means subtract 1 from a and b using pointer arithmetic, then dereference a and b and set them equal.

Is this equivalent to

--a;
--b;
*a=*b

Similarly, what is

*a++ = *b++;

equivalent to?

Solution 
  • *––a = *––b

    is logically equivalent to

    tmpa = a - 1
tmpb = b - 1
*tmpa = *tmpb
a = a - 1
b = b - 1

    with the caveat that the updates to a, b, and *tmpa can occur in any order, and those updates can even be interleaved. It’s also possible for the implementation to skip the temporaries and update the pointer values immediately:

    a = a - 1
b = b - 1
*a = *b

    but that’s not required or guaranteed. Similarly,

    *a++ = *b++

    is logically equivalent to

    tmpa = a
tmpb = b
*tmpa = *tmpb
a = a + 1
b = b + 1

    with the same caveats about the ordering of the updates to a, b, and *tmpa. Again, the implementation may skip using temporaries and evaluate it as

    *a = *b
a = a + 1
b = b + 1

    but again that’s neither required nor guaranteed.