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7827 :: Accessing {.emit.}'d variables across modules?

• exelotl (orginal) [2021-04-19T18:23:33+02:00] view original

  In my game I'm using {.emit.} to achieve some things that are currently difficult in Nim.

  Unfortunately, {.emit.}'d variables are awkward... When using {.importc.} to bring them back into Nim:

  • If you don't annotate them with {.nodecl.}, it fails at the C compilation stage because the variable gets declared twice.
  • If you do annotate them with {.nodecl.}, you can't access the variable from other modules because it won't get declared.

  Here's a minimal example:

  # mymodule.nim
  
  {.emit:"""
  const char* foo = "Hello world!";
  """.}
  
  var foo* {.importc.}: cstring
  Run

  # main.nim
  
  import mymodule
  
  echo foo
  Run

  Result:

  
  /home/exelotl/nim/test/emit/nimcache/@mmymodule.nim.c:35:17: error: conflicting types for ‘foo’
     35 | extern NCSTRING foo;
        |                 ^~~
  /home/exelotl/nim/test/emit/nimcache/@mmymodule.nim.c:31:13: note: previous definition of ‘foo’ was here
     31 | const char* foo = "Hello world!";
        |             ^~~
  
  Run

  If we edit the declaration of foo:

  var foo* {.importc, nodecl.}: cstring
  Run

  Result:

  
  /home/exelotl/nim/test/emit/nimcache/@mmain.nim.c: In function ‘NimMainModule’:
  /home/exelotl/nim/test/emit/nimcache/@mmain.nim.c:152:24: error: ‘foo’ undeclared (first use in this function)
    152 |  T1_[0] = cstrToNimstr(foo);
        |                        ^~~
  Run

  Workaround:

  Use {.nodecl.} and then {.importc.} the variable in every place where it's used.

  # main.nim
  import mymodule
  var foo {.importc, nodecl.}: cstring
  echo foo
  Run

  This is what I'm doing now... It's not ideal, but it works.

  Does anybody know a solution that would allow me use an {.emit.}'d variable from another module without resorting to this?

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• juancarlospaco (orginal) [2021-04-19T18:40:45+02:00] view original

  codegendecl

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• exelotl (orginal) [2021-04-19T18:55:30+02:00] view original

  What would that look like?

  Is there a way to ensure different codegendecl output depending on whether the variable is being accessed in mymodule.nim or main.nim?

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• exelotl (orginal) [2021-04-19T23:04:56+02:00] view original

  I've settled on the following solution of using a helper module which emits the symbol to be importc'd

  (thanks to @clyybber for the idea!)

  # main.nim
  
  import mymodule
  
  echo foo   # Works!
  
  import helper  # Add helper to the build.
  Run

  # mymodule.nim
  
  # This module defines the data for foo:
  
  {.emit:"""
  const char foo_data[13] = "Hello world!";
  """.}
  
  # And it imports the pointer to foo, but the pointer itself is defined in the helper unit.
  
  let foo* {.importc.}: cstring
  Run

  # helper.nim
  
  # This module defines the pointer to foo.
  # It only needs to be added to the build.
  
  {.emit:"""
  extern const char foo_data[13];
  const char * const foo = &foo;
  """.}
  Run

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