4 ms·

The main benefit is that you can implement complex logic, performance optimizations, and compatibility shims on the userspace side of the kernel/userspace divid

The main benefit is that you can implement complex logic, performance optimizations, and compatibility shims on the userspace side of the kernel/userspace divide, where it's generally much easier to do, and bugs won't result in full compromise of the kernel. There's not really a strong argument for not linking libc. Even if you want to implement your own libc, you can still do so with linker tricks and calling through to the supported syscall wrappers in the real libc.

JonChesterfield 4y ago
Your options are poor if you don't want to build on libc though.
- catiopatio 4y ago
  Outside unsupported edge cases (e.g. shell code), there's really no material difference between jumping to the address of syscall wrapper in libSystem.dylib, versus trapping to the kernel via a given syscall number — other than the latter being a supported interface, while the former is explicitly not.
  - JonChesterfield 4y ago
    Edge cases also include using a different libc, working around bugs in the supported libc, trying to minimise how much of your stack is implemented in C.
    - catiopatio 4y ago
      Alternative libc implementations just aren't a thing outside of Linux. However, if, for some reason, you really want to re-implement libc, you still can, but your library will have to link against the system-provided libc for the syscall entry points, because that's the only stable interface to the kernel. There's no reason that should be a problem for a hypothetical libc-reimplementor.
matheusmoreira 4y ago
> There's not really a strong argument for not linking libc. Sure there is. The libc sucks. Freestanding C actually turned out to be a superior language because there's not as much legacy weighing it down. There's many systems languages out there, nobody should be forced to link to C stuff.
- wongarsu 4y ago
  I think Windows solved this much better than libc by separating the concerns: there's user32.dll providing a thin abstraction over syscalls, a couple libraries providing higher-level interfaces like wintrust.dll, and thirdly the C runtime library providing all the stuff mandated by C. You can easily ditch the latter, while still keeping the benefits of user32.dll (which cosmopolitan is doing).
  - lgg 4y ago
    macOS essentially has the same distinction internally (libsystem_kernel.dylib), but that is not directly linkable and is instead re-exported via the libSystem umbrella which also exports libsystem_c.dylib. While you cannot link directly to libsystem_kernel.dylib, if you choose to ignore everything in libsystem_c.dylib and only use the syscall wrappers reexported from libsystem_kernel.dylib via libSystem it has practically the same effect on macOS (in fact, the resulting binary will be identical to what a hypothetical binary linked to just a single libsystem_kernel.dylib would be except for a single `LC_LOAD_DYLIB` command). Such a binary would have the system lib c initialized sitting in its address space, but for the rare binary that really wants its own libc that doesn't seem unreasonable.
- twic 4y ago
  What's at issue here is linking to the platform libc for syscalls. If you don't want to use the platform libc's qsort implementation, by all means, don't. But if you want to call gettimeofday, you should do that by calling the gettimeofday function in the libc, not by trying to set up a system call on your own. The former is a stable interface, the latter isn't. It's annoying that libc combines two completely different things - userspace utility functions, system calls - but unfortunately, that's where history has brought us.

JonChesterfield 4y ago

Your options are poor if you don't want to build on libc though.

catiopatio 4y ago
Outside unsupported edge cases (e.g. shell code), there's really no material difference between jumping to the address of syscall wrapper in libSystem.dylib, versus trapping to the kernel via a given syscall number — other than the latter being a supported interface, while the former is explicitly not.
- JonChesterfield 4y ago
  Edge cases also include using a different libc, working around bugs in the supported libc, trying to minimise how much of your stack is implemented in C.
  - catiopatio 4y ago
    Alternative libc implementations just aren't a thing outside of Linux. However, if, for some reason, you really want to re-implement libc, you still can, but your library will have to link against the system-provided libc for the syscall entry points, because that's the only stable interface to the kernel. There's no reason that should be a problem for a hypothetical libc-reimplementor.

matheusmoreira 4y ago

> There's not really a strong argument for not linking libc. Sure there is. The libc sucks. Freestanding C actually turned out to be a superior language because there's not as much legacy weighing it down. There's many systems languages out there, nobody should be forced to link to C stuff.

wongarsu 4y ago
I think Windows solved this much better than libc by separating the concerns: there's user32.dll providing a thin abstraction over syscalls, a couple libraries providing higher-level interfaces like wintrust.dll, and thirdly the C runtime library providing all the stuff mandated by C. You can easily ditch the latter, while still keeping the benefits of user32.dll (which cosmopolitan is doing).
- lgg 4y ago
  macOS essentially has the same distinction internally (libsystem_kernel.dylib), but that is not directly linkable and is instead re-exported via the libSystem umbrella which also exports libsystem_c.dylib. While you cannot link directly to libsystem_kernel.dylib, if you choose to ignore everything in libsystem_c.dylib and only use the syscall wrappers reexported from libsystem_kernel.dylib via libSystem it has practically the same effect on macOS (in fact, the resulting binary will be identical to what a hypothetical binary linked to just a single libsystem_kernel.dylib would be except for a single `LC_LOAD_DYLIB` command). Such a binary would have the system lib c initialized sitting in its address space, but for the rare binary that really wants its own libc that doesn't seem unreasonable.
twic 4y ago
What's at issue here is linking to the platform libc for syscalls. If you don't want to use the platform libc's qsort implementation, by all means, don't. But if you want to call gettimeofday, you should do that by calling the gettimeofday function in the libc, not by trying to set up a system call on your own. The former is a stable interface, the latter isn't. It's annoying that libc combines two completely different things - userspace utility functions, system calls - but unfortunately, that's where history has brought us.