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One cool thing I like about Sortix is how it handles threading. TLS is set up by the kernel, so errno can be a thread local variable.

jeffreyrogers 9y ago
I think errno is thread local in most modern unix variants.
- sortie 9y ago
  Author here. POSIX requires errno to be thread local, but with few requirements. Commonly libc's #define errno (*__get_errno_location()). Why? It's because the thread local storage isn't online very early on. The libc literally has to open its own program executable and locate the the thread local storage and mmap a copy of it for each thread. Thread level storage comes online late in Linux, which bloats statically linked executables with code to parse their own executable. In Sortix, I just have the kernel set up the TLS for statically linked executables and just declare errno as: __thread int errno; That means errno is online from the very first instruction in _start in statically linked executables, without the need to parse the program file in early libc. There's no error conditions between _start and main. This is the kind of implementation quality I want to have in Sortix and part of why I develop Sortix.
  - jeffreyrogers 9y ago
    Ah, that's very neat. Thanks for explaining.
jchw 9y ago
Hm. I thought libc traditionally handled errno, not the kernel, and the syscall return code was simply translated to errno by libc.
- sortie 9y ago
  Author here. Like in Linux, libc provides errno on Sortix. The difference is that glibc does "#define errno (*__get_errno_location())" while Sortix libc does "__thread int errno;". linux syscalls return a negative value containing the negated errno, while Sortix system calls use two output registers: The return value register and the errno register. That allows Sortix system calls to legitimately return -42 if they want, which is not possible on Linux. Sortix libc assigns the errno register to the thread local errno if the errno register is non-zero.

jeffreyrogers 9y ago

I think errno is thread local in most modern unix variants.

sortie 9y ago
Author here. POSIX requires errno to be thread local, but with few requirements. Commonly libc's #define errno (*__get_errno_location()). Why? It's because the thread local storage isn't online very early on. The libc literally has to open its own program executable and locate the the thread local storage and mmap a copy of it for each thread. Thread level storage comes online late in Linux, which bloats statically linked executables with code to parse their own executable. In Sortix, I just have the kernel set up the TLS for statically linked executables and just declare errno as: __thread int errno; That means errno is online from the very first instruction in _start in statically linked executables, without the need to parse the program file in early libc. There's no error conditions between _start and main. This is the kind of implementation quality I want to have in Sortix and part of why I develop Sortix.
- jeffreyrogers 9y ago
  Ah, that's very neat. Thanks for explaining.

jchw 9y ago

Hm. I thought libc traditionally handled errno, not the kernel, and the syscall return code was simply translated to errno by libc.

sortie 9y ago
Author here. Like in Linux, libc provides errno on Sortix. The difference is that glibc does "#define errno (*__get_errno_location())" while Sortix libc does "__thread int errno;". linux syscalls return a negative value containing the negated errno, while Sortix system calls use two output registers: The return value register and the errno register. That allows Sortix system calls to legitimately return -42 if they want, which is not possible on Linux. Sortix libc assigns the errno register to the thread local errno if the errno register is non-zero.