Custom libc does not call to system libc.
“Custom” and “system” aren’t the terms I use; I say user libc and you libc (because they are basically the same libc - either both are musl or both are glibc).
User libc calls to the Fil-C runtime, which filters syscalls, and those filtered syscalls are made via yolo libc.
Hence, a Fil-C program is memory safe down to the syscalls and syscalls cannot be used to escape the protections (unless you do weird stuff with /proc)
The way I see it, as far as memory safety is concerned that's just framing. Both Fil-C and Rust have a boundary below which the unsafe lives. If Fil-C is only safer than rust when you don't pass the `-F unsafe_code` argument to rustc then IMO "safer" in this case is somewhat of an empty claim.
I think you should watch the linked presentation, it will show how it is "safer" than rust. Unfortunately there are performance implications, but fil-c seems fast enough to go into production for many workloads and surprisingly needs very little code changes to existing C/C++ projects which is a huge benefit.
I think Fil-C might make a lot of sense for running legacy C or C++ codebases. But for new code, it seems like it’s trying to compete with other GC languages. Take away C’s performance advantages and I don’t know why anyone would use it.
Fil-C: Combining the ergonomics of C with the performance of Python!