martin@lucina.net said:
hyc@symas.com said:
My question is: Given that I'm storing C structs directly in LMDB, I need to get at least word-aligned pointers back from LMDB in order to be able to access the data safely.
I've not found anything in the documentation or the source about being able to tweak alignment of key and data values; I *think* that all I need is an option where LMDB would guarantee a minimum word (4 byte in this case) alignment of data.
How hard would this be to implement? Would you consider such a feature?
LMDB guarantees 2-byte alignment of keys, and data is generally stored contiguously with keys. If you want 4-byte alignment, it's your responsibility to pad your keys to 4-byte boundaries. If you pad appropriately, your alignment will be preserved.
In this case the keys are size_t (4 bytes on the target) and I'm using MDB_INTEGERKEY. I'm not sure what you mean by padding the keys to 4-byte boundaries given that LMDB copies my data into its memory map and I have no control over where it gets placed.
Ah. What you forgot to mention (and I tracked down in a Reddit comment somewhere) is that the (key + data) size (and not just the key size) should be a multiple of the required alignment. The problematic records in question are using VLAs to store variable length strings; if I pad the allocated space to the nearest word boundary that seems to fix the issue.
Martin