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2797 :: FFI to C, what is the equivalent to const u8 *

• enthus1ast (orginal) [2017-02-14T22:43:36+01:00] view original

  Hi,

  i've got a c declaration that i like to wrap:

  
          void crypto_ed25519_sign(u8        signature[64],
                                   const u8  secret_key[32],
                                   const u8 *message,
                                   size_t    message_size)
  Run

  c2nim produces this:

  proc crypto_ed25519_sign*(signature: array[64, uint8]
                                   secret_key: array[32, uint8] message: ptr uint8
                                   message_size: csize)
  Run

  to actually use it i had to change it like this:

  proc crypto_ed25519_sign*(signature: array[64, uint8];
                                   secret_key: array[32, uint8]; message: ptr openarray[uint8];
                                   message_size: csize) {.cdecl, importc: "crypto_ed25519_sign".}
  Run

  the questions are:

  • what is the correct way to give arbitrary length data to message?

    (so what ist the eqvivalent to const u8 *message)

  • Atm i work around my limitation by declareing

    message: array[1024, uint8]

    and give the actual msg size in message_size.

    It seems system.nim declares a dummy 100_000/(big enough) sized array to avoid bound checks so i guess this is a somewhat convenient way of doing it?

  greetings

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• cheatfate (orginal) [2017-02-14T23:15:05+01:00] view original

  Does it helps?

  proc crypto_ed25519_sign(signature: array[64, uint8];
                           secret_key: array[32, uint8];
                           message: ptr uint8;
                           message_size: csize) {.cdecl, importc: "crypto_ed25519_sign".}
  
  var message = newSeq[uint8]()
  crypto_ed25519_sign(signature, key, cast[ptr uint8](addr message[0]), len(message))
  
  Run

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• enthus1ast (orginal) [2017-02-14T23:35:47+01:00] view original

  Thanks, works!

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• lucian (orginal) [2017-02-15T01:12:54+01:00] view original

  1. C lang: passing (void *) as an parameter (typed *) seems legit for the gcc compiler (apparently not so for g++)
  2. the Nim to C backend might actually consider the type of the message variable and create the proper/compatible parameter for the C call

  both work in your favor and this simpler form also works (basically save some typing and let the C compiler do the pointer casting):

  proc crypto_ed25519_sign(signature: array[64, uint8];
              secret_key: array[32, uint8];
              message: ptr;
              message_size: csize) {.cdecl, importc: "crypto_ed25519_sign".}
  
  var msg = @[0xFE.uint8, 0x32, 0xd5, 0x5C, 0x77, 0x2A, 0x49, 0x11]
  crypto_ed25519_sign(sig, key, addr msg[0], len(msg))
  Run

  post edit: assumption 2. not true :-), var msg = @[0xFE.uint16,...

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