Nim中如何向DLL外部函数传递可变长度数组？

问题背景 #

在Nim中包装OpenAL32.dll时，alcCreateContext函数的attrlist参数要求接收指向ALCint数组的指针或nil。该数组采用[标志, 数值, 标志, 数值,...]的动态格式，长度随传入标志数量变化。现有代码硬编码了数组大小，无法灵活添加ALC_FREQUENCY这类额外标志，尝试openArray容器后，希望通过类型转换实现动态传递——这也是突破OpenAL默认256声音限制的关键方法。

原代码 #

when defined(windows):
  {.push cdecl, dynlib: "OpenAL32.dll", importc.}
else:
  {.push importc.}

type 
  ALCint = cint
  ALCdevice* = pointer
  ALCcontext* = pointer

const
  ALC_MONO_SOURCES* = 0x00001010
  ALC_STEREO_SOURCES* = 0x00001011
  ALC_FREQUENCY* = 0x00001007

proc alcCreateContext*(device: ALCdevice; attrlist: ptr array[0..3, ALCint]): ALCcontext
proc alcOpenDevice*(devicename: cstring): ALCdevice

const attributes = [ALC_MONO_SOURCES.ALCint, 65536.ALCint, ALC_STEREO_SOURCES.ALCint, 65536.ALCint]

discard alcOpenDevice(nil).alcCreateContext(attributes.unsafeAddr)

解决方案 #

将alcCreateContext的attrlist参数类型替换为ptr UncheckedArray[ALCint]，调用时通过cast将数组地址转换为该类型即可，这样就能支持任意长度的属性数组。

修改后的代码 #

when defined(windows):
  {.push cdecl, dynlib: "OpenAL32.dll", importc.}
else:
  {.push importc.}

type 
  ALCint = cint
  ALCdevice* = pointer
  ALCcontext* = pointer

const
  ALC_MONO_SOURCES* = 0x00001010
  ALC_STEREO_SOURCES* = 0x00001011
  ALC_FREQUENCY* = 0x00001007

proc alcCreateContext*(device: ALCdevice; attrlist: ptr UncheckedArray[ALCint]): ALCcontext
proc alcOpenDevice*(devicename: cstring): ALCdevice

const attributes = [ALC_MONO_SOURCES.ALCint, 65536.ALCint, ALC_STEREO_SOURCES.ALCint, 65536.ALCint]

discard alcOpenDevice(nil).alcCreateContext(cast[ptr UncheckedArray[ALCint]](attributes.unsafeAddr))

内容的提问来源于stack exchange，提问作者Joshua Fenner