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7268 :: Is this a supported usecase: update global variables from a dynamic library?

• gcao (orginal) [2020-12-18T17:22:02+01:00] view original

  Hi,

  Is this a supported usecase: update global variables from a dynamic library? I understand in this simplified example, I can probably pass in As to add(). However in more complex applications, if the global variable can not be passed in, is there an alternative way to make this work?

  types.nim

  
  type
    A* = ref object
      name*: string
    Add* = proc(a: A) {.nimcall.}
  
  var As*: seq[A] = @[]
  Run

  extension.nim

  
  {.push dynlib exportc.}
  
  import ./types
  
  proc add*(a: A) =
    As.add(a)
  
  {.pop.}
  Run

  main.nim

  
  import dynlib
  import ./types
  
  var lib = load_lib("build/libextension.dylib")
  var add = cast[Add](lib.sym_addr("add"))
  add(A(name: "first"))
  add(A(name: "second"))
  
  assert As.len == 2    # This assertion fails
  Run

  The code above can be cloned and run from https://github.com/gcao/test.nim

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• tommo (orginal) [2020-12-19T04:14:47+01:00] view original

  "main" and "extension" modules will have their own "As" global var. You need to pass the context of "main" to "extension".

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• gcao (orginal) [2020-12-19T17:55:05+01:00] view original

  Thank you. Any suggestion on how to pass the context? Like below?

  extension.nim

  
  proc init*(<global variables>) =
    ...
  Run

  main.nim

  
  # load extension
  var init = cast[Init](lib.sym_addr("init"))
  init(<global variables>)
  Run

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• coffeepot (orginal) [2020-12-21T15:51:17+01:00] view original

  You can use the export statement to forward imported symbols: https://nim-lang.org/docs/manual.html#modules-export-statement

  For example if you want extension.nim to forward types.nim to main.nim:

  extension.nim:

  {.push dynlib exportc.}
  
  import ./types
  export types # Importing `extension.nim` gives access to symbols in `types.nim`.
  # You can also export individual symbols, for instance: `export As`.
  
  proc add*(a: A) =
    As.add(a)
  
  {.pop.}
  Run

  main.nim:

  import dynlib, extension
  # `types.nim` is now accessible through `extension.nim`.
  
  var lib = load_lib("build/libextension.dylib")
  var add = cast[Add](lib.sym_addr("add"))
  echo add.repr
  add(A(name: "first"))
  add(A(name: "second"))
  
  assert As.len == 2 # Asserts correctly.
  Run

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