@Araq, how do you know it's a callback?
When you create a proc to be used in C (callback):
* Don't use exceptions
* Don't use string or sequences
* Deactivate stacktraces
* Use `int32` or `cint` for `int` use `uint` or `csize_t` for `size_t`
* tag `{.cdecl, gcsafe.}` your proc to detect at compile-time the issue
Also one thing I've noticed is that Nim `endOfFile()` is quitting to early
compared to `c_eof` from the `system/ansi_c` module (see my h264 encoder test
case
[https://github.com/mratsim/trace-of-radiance/blob/99f7d85d/trace_of_radiance/io/h264.nim#L278](https://github.com/mratsim/trace-of-radiance/blob/99f7d85d/trace_of_radiance/io/h264.nim#L278))
Example to pass a Nim callback to C code for MP4 muxing:
[https://github.com/mratsim/trace-of-radiance/blob/99f7d85d/trace_of_radiance/io/mp4.nim#L115-L126](https://github.com/mratsim/trace-of-radiance/blob/99f7d85d/trace_of_radiance/io/mp4.nim#L115-L126)
proc MP4E_open(
sequential_mod_flag: cint,
enable_fragmentation: cint,
token: pointer,
write_callback: proc(
offset: int64,
buffer: pointer,
size: csize_t,
token: pointer
): cint{.cdecl, gcsafe.}
): ptr MP4E_mux_t {.importc, header: "minimp4.h".}
## The C code
{.push stackTrace: off.}
proc writeToFile(
offset: int64,
buffer: pointer,
size: csize_t,
token: pointer
): cint {.cdecl, gcsafe.} =
## The callback
let file = cast[File](token)
file.setFilePos(offset)
let bytesWritten = file.writeBuffer(buffer, size)
return cint(bytesWritten.csize_t != size)
{.pop.}
Run
