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yallop / ocaml-ctypes

Library for binding to C libraries using pure OCaml
MIT License
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Feature request: an "unmanaged" string view type #758

Closed vrotaru closed 3 days ago

vrotaru commented 7 months ago

Or more generally allow the user to create views types where the memory which is allocated on OCaml side is GC-ed, but the one which is passed to the C is not freed when it is not longer reachable.

Motivating example:

I'm trying to pass an OCaml callback to C which returns a string. After that the C code "takes ownership" of the returned string and frees it after use. Then when the char ptr holding a reference to string on OCaml side is garbage collected this function https://github.com/yallop/ocaml-ctypes/blob/96e1ac32a5909dbd3477ff86d6c41172b2a935fd/src/ctypes/managed_buffer_stubs.c#L19 is called and it tries to free it again.

The result of this double-free is that the program crashes.

So, if that is possible, I want an unmaged_string where the finalizer for that custom block is null or does nothing.

yallop commented 3 days ago

I think it's better to avoid string (and other views that let OCaml handle memory management) in these situations. If you want to allocate memory that some C code will later pass to free then one approach is to bind malloc and allocate the memory that way.