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Document storage variable access.

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chriseth 2017-04-26 11:58:36 +02:00
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docs/assembly.rst
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@ -323,9 +323,12 @@ Access to External Variables and Functions
------------------------------------------
Solidity variables and other identifiers can be accessed by simply using their name.
For storage and memory variables, this will push the address and not the value onto the
stack. Also note that non-struct and non-array storage variable addresses occupy two slots
on the stack: One for the address and one for the byte offset inside the storage slot.
For memory variables, this will push the address and not the value onto the
stack. Storage variables are different: Values in storage might not occupy a
full storage slot, so their "address" is composed of a slot and a byte-offset
inside that slot. To retrieve the slot pointed to by the variable ``x``, you
used ``x_slot`` and to retrieve the byte-offset you used ``x_offset``.
In assignments (see below), we can even use local Solidity variables to assign to.
Functions external to inline assembly can also be accessed: The assembly will
@ -340,17 +343,13 @@ changes during the call, and thus references to local variables will be wrong.
.. code::
pragma solidity ^0.4.0;
pragma solidity ^0.4.11;
contract C {
uint b;
function f(uint x) returns (uint r) {
assembly {
b pop // remove the offset, we know it is zero
sload
x
mul
=: r // assign to return variable r
r := mul(x, sload(b_slot)) // ignore the offset, we know it is zero
}
}
}