Failing to identify a root would cause the GC to free too much memory. Memory leaks happen when too little memory is freed. Boehm GC is called conservative because it can't distinguish pointers from other memory content, so it will determine some allocations to be reachable even if there's no pointer to them, because some random integer looks like a pointer into that allocation.

I see, that's what you meant. You are right. You need to use boehmgc in precise mode, which is not the default. You need to tag your pointers. Also use incremental mode for shorter pause times.

Conservative collection is unfortunately terrible when you're dealing with data that looks random, like cryptography. It generates many false "pointer live" positives, which ends up retaining a lot more memory which looks like it's leaking.

True if you don't cooperate at all with the collector, but easily avoided in many cases. E.g., when using Boehm, try using gc_malloc_atomic() for data that you know cannot contain pointers, like your encryption buffers.