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Also newer archs (riscv) don't support certain syscalls (open, fork) they are still functions you can call under your standard musl,glibc but they inernally call openat, clone.
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I see, but why were they numbered differently? Why not use the same numbers?
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As far as I know, the numbers were assigned independently by each port's developers in the 90s as they added calls in their own order. Then it became a frozen ABI nobody could ever renumber, and couldn't have been identical anyway since arches need different syscall variants for things like 32bit vs 64bit types
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