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`syscall()` is a very thin wrapper around calling the kernel directly by number. ntdll and libsystem aren't equivalent, they are more similar to `libc`. Neither

`syscall()` is a very thin wrapper around calling the kernel directly by number. ntdll and libsystem aren't equivalent, they are more similar to `libc`. Neither Windows nor MacOS have a stable system call interface so you have to dynamically link to the library and call through the stable library interface instead. I can't find anything that guarantees FreeBSD's system call ABI. Do you have a source? It would be very interesting if, for example, FreeBSD 10 applications that use syscalls can run on modern FreeBSD without a compatibility layer. However, if the FreeBSD project does not provide guarantees it would be folly to rely on this behaviour in the future. If it does provide such a guarantee then I stand corrected. EDIT: After some more research it seems the FreeBSD kernel needs the `COMPAT_FREEBSD10` option enabled for my hypothetical example to work. The default options for amd64 include compatibility options back to FreeBSD 4. Defaults for other platforms seems to differ (perhaps depending on when the platform was first supported). I can't find good documentation on if these provide full compatibility or if any `COMPAT_` options could be dropped in future versions.

moonchild 6y ago
> ntdll and libsystem aren't equivalent, they are more similar to `libc` I disagree. System calls on linux comprise the interface provided to applications to talk to the kernel. On windows, ntdll serves the same function--it is itself a 'very thin wrapper around calling the kernel directly by number'. (Especially important since a libc may be hard to come by on windows.) (Libsystem it seems I was mistaken about; it looks like that's just a bundle of libc, libm, libpthread, etc. Though possibly libsystem_kernel is nearer the mark? Difficult to find information on the subject, and I don't have a mac.)

moonchild 6y ago

> ntdll and libsystem aren't equivalent, they are more similar to `libc` I disagree. System calls on linux comprise the interface provided to applications to talk to the kernel. On windows, ntdll serves the same function--it is itself a 'very thin wrapper around calling the kernel directly by number'. (Especially important since a libc may be hard to come by on windows.) (Libsystem it seems I was mistaken about; it looks like that's just a bundle of libc, libm, libpthread, etc. Though possibly libsystem_kernel is nearer the mark? Difficult to find information on the subject, and I don't have a mac.)