To be clearer: safety is no longer compositional in this system. To safely use the Boehm garbage collector, it's not sufficient that my own code doesn't use "disguised" pointers: I have to verify that my libraries don't either. That's impractical and contrary to good software engineering at best, impossible at worst.
No, you have to verify that your libraries don't keep disguised pointers to objects you allocated via the GC, that you don't yourself keep pointers to. It's quite unusual in C to give a reference to an object to another piece of code then drop all the pointers to it that you hold, because the usual discipline is that the module that allocates an object frees it.
Not if you build the GC with --enable-redirect-malloc to use with existing libraries/code bases. Which should be fine because, after all, this is a "conservative" garbage collector. Right?
