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Merge pull request #4725 from ethereum/chriseth-patch-1

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Clarify allocation and initial value of free memoy pointer.
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Alex Beregszaszi 2018-08-07 12:48:10 +01:00 committed by GitHub
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docs/assembly.rst
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@ -609,12 +609,21 @@ first.
Solidity manages memory in a very simple way: There is a "free memory pointer" Solidity manages memory in a very simple way: There is a "free memory pointer"
at position ``0x40`` in memory. If you want to allocate memory, just use the memory at position ``0x40`` in memory. If you want to allocate memory, just use the memory
from that point on and update the pointer accordingly. starting from where this pointer points at and update it accordingly.
There is no built-in mechanism to release or free allocated memory.
Here is an assembly snippet that can be used for allocating memory::
function allocate(length) -> pos {
pos := mload(0x40)
mstore(0x40, add(pos, length))
}
The first 64 bytes of memory can be used as "scratch space" for short-term The first 64 bytes of memory can be used as "scratch space" for short-term
allocation. The 32 bytes after the free memory pointer (i.e. starting at ``0x60``) allocation. The 32 bytes after the free memory pointer (i.e. starting at ``0x60``)
is meant to be zero permanently and is used as the initial value for is meant to be zero permanently and is used as the initial value for
empty dynamic memory arrays. empty dynamic memory arrays.
This means that the allocatable memory starts at ``0x80``, which is the initial value
of the free memory pointer.
Elements in memory arrays in Solidity always occupy multiples of 32 bytes (yes, this is Elements in memory arrays in Solidity always occupy multiples of 32 bytes (yes, this is
even true for ``byte[]``, but not for ``bytes`` and ``string``). Multi-dimensional memory even true for ``byte[]``, but not for ``bytes`` and ``string``). Multi-dimensional memory
@ -698,7 +707,7 @@ We consider the runtime bytecode of the following Solidity program::
The following assembly will be generated:: The following assembly will be generated::
{ {
mstore(0x40, 0x60) // store the "free memory pointer" mstore(0x40, 0x80) // store the "free memory pointer"
// function dispatcher // function dispatcher
switch div(calldataload(0), exp(2, 226)) switch div(calldataload(0), exp(2, 226))
case 0xb3de648b { case 0xb3de648b {