I am hoping to use Nimgame2 for a project but need to update it to Nim2 first. First milestone is to just get it compiling with Nim2. Some of the code is on the edge of my knowledge so hopefully I can post the issues here for some guidance.
First issue is occurring in tar.nim. The error message I receive is: Error: expression cannot be cast to 'ptr uint8'. Some Google search results suggests changing the line to
Magic = cast[ptr UncheckedArray[uint8]]("ustar")Appreciate any advice on where to go from here.
Magic = cast[ptr uint8]("ustar")
The cast operation takes an identifier ("a thing"), which the compiler usually treats in one way, and tells the compiler to treat it as a thing of a different type. In this case, the thing you're casting is a so called string literal.
Your cast operation asks the compiler to treat this string literal as a pointer to a uint8 number. A pointer is an address in the memory space of a program, so it's just a special number. This number has a defined size ("word size"), which for modern PCs is usually 64 bits (8 bytes) or, rarely, 32 bits (4 bytes).
So what that line means as written:
"Take a string literal "ustar", which is comprised of 5 ANSI characters, each a single byte long (so 5 bytes total) and treat those 5 bytes as a single number that will be used as a pointer to a uint8 number."
This is a bit meaningless, because you can't technically treat a 5 byte number as an 8 byte address.
Let's see how the Magic constant is used in the code:
# data: ptr uint8
let magic = p + MagicOffset
for i in 0..<MagicSize:
if data[magic + i] != Magic[i]:
returnThis is the only place where the constant is used. You're just trying to compare the bytes at offset magic with a predefined sequence of bytes.
In any case, data field of the` TarFile` object should probably be properly typed as a pointer to an array of bytes/chars, not as an address to a single byte: data: ptr UncheckedArray[char] or data: ptr UncheckedArray[byte].
Next, there's a few ways to do the comparison The most obvious is:
const Magic = "ustar"
...
for i, c in Magic:
if data[magic + i] != c:
return # break is probably betterAnother option is to use openArrays, but the type of the array contents should be the same, so if data is an array of bytes (not chars) the type system won't allow the comparison. You can change the type of do a cast.
const Magic = ['u', 's', 't', 'a', 'r']
...
# data: ptr UncheckedArray[char]
let offset = p + MagicOffset
if data.toOpenArray(offset, offset+Magic.len-1) != Magic:
returnThe code is very unidiomatic to Nim and looks like a straight translation from C. It's very unsafe and does not really utilize any the conveniences and strengths Nim brings to the table (great type system, first of all).
BTW, regarding the last block, turns out there's a bunch of procs specifically to deal with byte, uint8 and char being different types that often require to be treated interchangeably: toOpenArrayByte and toOpenArrayChar
So there wouldn't be any need to change the data type or do a cast, alas, these procs don't have overloads taking ptr UncheckedArray[byte|char]. So you need to use on on the right hand side, on Magic string const.
const Magic = "ustar"
...
# data: ptr UncheckedArray[byte]
let offset = p + MagicOffset
if data.toOpenArray(offset, offset+Magic.len-1) == Magic.toOpenArrayByte(0, Magic.high)
returnThe next error message I receive references zzip.
Error message is: nimgame2/plugin/zzip.nim(63, 17) Error: type mismatch: got 'None' for 'zzipRead' but expected 'proc (context: ptr RWops, p: pointer, size: csize_t, maxnum: csize_t): csize_t{.cdecl.}'
I'm not even sure how this code works. Feel free to point me to documentation which explains what is actually happening here. Hopefully there's not too many of these curly issues.