some GC languages pause garbage collection while calling out to native code. under this scheme it's no problem to call C to do something like multiply matrices

some GC languages pause garbage collection while calling out to native code. under this scheme it's no problem to call C to do something like multiply matrices or compute hash functions. the problems come when you want the C code to call back into GC code, for example an event loop or a huge computation that wants to report its progress. it's hard. mixing manual and automatic memory management is tricky.