Undocumented alignment of secp256k1_context

[Kixunil]

secp256k1_context doesn't have alignment documented but looking at the code it's clearly bigger than 1 since it contains pointers and int. Possibly becuase of lacking documentation Rust bindings used array of bytes (u8 AKA uint8_t) which is very wrong.

[elichai]

I assume you mean the buffer to the preallocated context and not the context itself?
if so, the documentation says that it should be aligned to hold "suitably aligned to hold an object of any type.":

secp256k1/include/secp256k1_preallocated.h

Lines 42 to 43 in ac83be3

	* of size at least secp256k1_context_preallocated_size(flags) bytes, suitably
	* aligned to hold an object of any type.

Also, see this discussion: rust-bitcoin/rust-secp256k1#141 (comment)

[Kixunil]

Ah, OK, I expected to find this at type not a function. Thanks!

[elichai]

Note that the secp256k1_context type is opaque and can only be created via function calls to libsecp (either context_create/clone or context_preallocated_create/clone), so either libsecp allocates the memory and takes care of the alignment or the user supplies a buffer to the preallocated API and then they need to make sure it has the alignment that the preallocated API requires.