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Merge pull request #4725 from ethereum/chriseth-patch-1
Clarify allocation and initial value of free memoy pointer.
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@ -609,12 +609,21 @@ first.
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Solidity manages memory in a very simple way: There is a "free memory pointer"
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at position ``0x40`` in memory. If you want to allocate memory, just use the memory
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from that point on and update the pointer accordingly.
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starting from where this pointer points at and update it accordingly.
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There is no built-in mechanism to release or free allocated memory.
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Here is an assembly snippet that can be used for allocating memory::
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function allocate(length) -> pos {
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pos := mload(0x40)
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mstore(0x40, add(pos, length))
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}
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The first 64 bytes of memory can be used as "scratch space" for short-term
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allocation. The 32 bytes after the free memory pointer (i.e. starting at ``0x60``)
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is meant to be zero permanently and is used as the initial value for
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empty dynamic memory arrays.
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This means that the allocatable memory starts at ``0x80``, which is the initial value
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of the free memory pointer.
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Elements in memory arrays in Solidity always occupy multiples of 32 bytes (yes, this is
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even true for ``byte[]``, but not for ``bytes`` and ``string``). Multi-dimensional memory
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@ -698,7 +707,7 @@ We consider the runtime bytecode of the following Solidity program::
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The following assembly will be generated::
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{
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mstore(0x40, 0x60) // store the "free memory pointer"
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mstore(0x40, 0x80) // store the "free memory pointer"
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// function dispatcher
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switch div(calldataload(0), exp(2, 226))
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case 0xb3de648b {
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