Alexey Rodriguez wrote: > Dear all, > > We are experiencing crashes in Caml-calling C code. This happens if > garbage collection runs after Caml code has raised an exception. We now > understand why this happens but we are puzzled as to why the "Interfacing > C with Ocaml" chapter of the Ocaml manual doesn't warn about this > situation. > > Suppose you have C code that calls Caml code as follows: > > ... > CAMLparam2(v1,v2); > CAMLlocal2(...,res); > res = callback2_exn(...,v1,v2); > foobar(); > ... > > We have found that this code will crash with "Fatal error: out of memory." > if the following two things happen: > * the function called by [callback2_exn] raises an exception, and > * [foobar] triggers a garbage collection through the allocation of values > in the Caml heap. (just calling [caml_gc_full_major] is enough to cause > the crash). > > The reason for this crash is that [res] will contain an invalid pointer if > an exception is thrown. The GC follows this bogus pointer ([res] is > registered as a root by [CAMLlocal2]) which ultimately causes a crash in > the GC code. Why does [res] contain a bogus pointer? > It's not really a bogus pointer, but the lower bits are tagged in order to > denote a thrown exception. These bits are usually tested/cleared by > [Is_exception_result] and [Extract_exception].
This is already in the manual, but I agree that the requirement to do so could be stated more clearly. Section 18.7.1[1], last paragraph states "The return v of the caml_callback*_exn function **must** be tested with the macro Is_exception_result(v)". It also clearly indicates that v is only a valid [value] if Is_exception_result(v) returns false so storing the return of caml_callback*_exn in a local root and allowing the Gc to run before you update that root with the result of Extract_exception is "obviously" a Gc violation. David [1] http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html#htoc245 -- Caml-list mailing list. Subscription management and archives: https://sympa-roc.inria.fr/wws/info/caml-list Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
