[Swan-dev] heap allocation vs stack allocation
D. Hugh Redelmeier
hugh at mimosa.com
Thu Apr 24 23:45:07 EEST 2014
I noticed that ikev2_out_nat_v2n allocated heap space for each hash.
memory should be allocated on the stack as local variables.
Stack allocation is much better than heap allocation, if it is
- it is faster
- it does not contribute to heap fragmentation (dangerous and
insidious in a long-running daemon)
- the system takes care of the lifecycle (including freeing)
- pointers are not required
- the lifetime is trivially evident to any programmer.
When is it appropriate? Both these must be true:
- the stack lifetime is reasonable (sufficient and not excessive)
- the size is known at compile time or a reasonable bound on the
size is known at compile time
In the case of ikev2_out_nat_v2n, the hash buffers would be much better
stack-allocated than heap-allocated.
How many other places in the code have we used unnecessarily used heap
PS: in the code, the lifetime of the two hashes did not overlap, so
only one buffer is required.
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