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1654 :: Returing a Pointer to an Array of Arbitrary Length from C

• darrint (orginal) [2015-09-20T19:32:43+02:00] view original

  I've been having trouble with calling a c function in jackd. I managed to fix the problem but I don't understand why the solution works:

  type
    jack_nframes_t* = uint32
    jack_default_audio_sample_t* = cfloat
    Sample* = jack_default_audio_sample_t
    Samples*{.unchecked.} = array[0..0, Sample]
  
  proc jack_port_get_buffer*(port: ptr jack_port_t; a3: jack_nframes_t): ptr Samples {.
    importc: "jack_port_get_buffer", dynlib: "libjack.so.0.1.0".}
  
  # C interface:
  # void * jack_port_get_buffer (jack_port_t *port, jack_nframes_t) JACK_OPTIONAL_WEAK_EXPORT;
  Run

  What I'm confused about is the need for the ptr in the return type. My intuition was that type Samples itself represented a pointer to the array.

  Furthermore, when I was returning just type Samples and trying to populate the array (in a callback from a c function) my loop tended to terminate after 10 to 20 assignments. But I didn't crash or get a segfault. Wrapping the assignment in an exception handler didn't catch anything.

  So when I was handing the return type as plain Samples and trying to populate it, what was actually happening? I'm willing imagine a universe where I was somehow expecting to return the array by value. But without a bound, what would that even mean?

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• Stefan_Salewski (orginal) [2015-09-20T20:07:44+02:00] view original

  My intuition was that type Samples itself represented a pointer to the array.

  It is an array, not a pointer. See

  https://en.wikipedia.org/wiki/Array#In_computer_science

  Note that the * in Nim denotes export of that symbol, and does not indicate pointers as in C language.

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• darrint (orginal) [2015-09-20T21:02:00+02:00] view original

  I understand about the asterisk.

  So leaving the ptr off the function and then calling it resulted in returning an unchecked array of zero length by value?

  And since it was unchecked I was bashing the stack by trying to assign to it?

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• Stefan_Salewski (orginal) [2015-09-20T22:29:33+02:00] view original

  Note, the array has not length zero. 0..0 indicates the index range, so 0 is the only valid index, and length is one (element) in this notation. I guess unchecked indicates that you can use arbitrary indices. (And I guess that an array of length 0 does not really exist in Nim.) Most often, when the C function returns a pointer, the Nim wrapper should return a pointer too. For pointer arguments it may be possible to use a var parameter. Generally, for your wrapped function, I wonder if ptr jack_port_t for the first parameter is really the best solution.

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• darrint (orginal) [2015-09-21T01:17:46+02:00] view original

  This is just what c2nim spit out except that I changed the return type to make it useful.

  I'm sure you are right that this isn't idiomatic Nim. I'm too new to have any sense of that.

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