Convert string to unsigned char array

[James Graves]

We can't convert a Lua string to an array of unsigned characters (or other varieties of 8-bit unsigned types like uint8_t).

local ffi = require "ffi"
ffi.cdef[[
int strncmp(const char *s1, const unsigned char *s2, size_t n);
]]
print (ffi.C.strncmp("two", "three", 3))

returns:

lua: unable to convert argument 2 from lua<string> to cdata<const unsigned char*>
stack traceback:
        [C]: in function 'strncmp'
        string_ffi.lua:7: in main chunk
        [C]: in ?

This runs with LuaJIT2, but not with luaffi. I initially stumbled upon this problem when going through the FFI tutorial.

[James McKaskill]

Thanks for the report. Hrrm this is one of those cases where most compilers will balk at you if you try in C. However I'll add a check for it.

[James Graves]

I think I was unclear. In the FFI tutorial, they are using compress2() and uncompress() which normally take arguments of the form const uint8_t *. However luaffi won't convert a Lua string to any unsigned character array.

I just used strncmp() because it made for a short example, even though it normally takes char *.

[James McKaskill]

No its me being unclear. The issue is that the pointer cast rules are stricter in luaffi than in luajit at the moment. So string -> const char* (https://github.com/jmckaskill/luaffi/blob/master/ffi.c#L398) which requires an explicit cast to const uint8_t* at the moment (this is because it requires that ct->type matches (https://github.com/jmckaskill/luaffi/blob/master/ffi.c#L498 call to is_same_type). The change I need to make is relax some of the checks in check_typed_pointer to match C rules.

Using an explicit compresse(ffi.cast('uint8_t*', 'my string'),...) should work though.