cerc-io/solidity
16
0
Fork 0
mirror of https://github.com/ethereum/solidity synced 2023-10-03 13:03:40 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

Clarify allocation and initial value of free memoy pointer.

Browse Source
This commit is contained in:
chriseth 2018-08-07 12:26:28 +02:00
parent cda3fbda33
commit 5d5626e6d7
1 changed files with 11 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
docs/assembly.rst
View File

@ -609,12 +609,21 @@ first.
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
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
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
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
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::
{
mstore(0x40, 0x60) // store the "free memory pointer"
mstore(0x40, 0x80) // store the "free memory pointer"
// function dispatcher
switch div(calldataload(0), exp(2, 226))
case 0xb3de648b {