Layout in Memory¶
Solidity reserves four 32-byte slots, with specific byte ranges (inclusive of endpoints) being used as follows:
0x3f(64 bytes): scratch space for hashing methods
0x5f(32 bytes): currently allocated memory size (aka. free memory pointer)
0x7f(32 bytes): zero slot
Scratch space can be used between statements (i.e. within inline assembly). The zero slot
is used as initial value for dynamic memory arrays and should never be written to
(the free memory pointer points to
Solidity always places new objects at the free memory pointer and memory is never freed (this might change in the future).
Elements in memory arrays in Solidity always occupy multiples of 32 bytes (this
is even true for
byte, but not for
Multi-dimensional memory arrays are pointers to memory arrays. The length of a
dynamic array is stored at the first slot of the array and followed by the array
There are some operations in Solidity that need a temporary memory area larger than 64 bytes and therefore will not fit into the scratch space. They will be placed where the free memory points to, but given their short lifetime, the pointer is not updated. The memory may or may not be zeroed out. Because of this, one should not expect the free memory to point to zeroed out memory.
While it may seem like a good idea to use
msize to arrive at a
definitely zeroed out memory area, using such a pointer non-temporarily
without updating the free memory pointer can have unexpected results.