[libc][POSIX][unistd] implement `alarm`

[nickdesaulniers]

building clang against llvm-libc fails due to the uses of alarm (unistd.h) in llvm/lib/Support/Unix/{Watchdog.inc|Program.inc}

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

building clang against llvm-libc fails due to the uses of `alarm` (unistd.h) in llvm/lib/Support/Unix/{Watchdog.inc|Program.inc}

[nickdesaulniers]

Interesting; alarm only has a syscall number for x86_64+i386, but not arm, aarch64, or riscv. Looks like bionic implements it in terms of calls to timerclear and setitimer. I could find info for setitimer in posix 2024 docs. The man page for it though says

POSIX.1-2008 marks getitimer() and
setitimer() obsolete, recommending the use of the POSIX timers API (timer_gettime(2), timer_settime(2), etc.)
instead.

https://pubs.opengroup.org/onlinepubs/9799919799/

So that's fun, POSIX says there's

• time.h
• sys/time.h
• sys/times.h

The syscall arch support table for timer_settime (and timer_settime64) is a mess, but setitimer is not, at least for architectures we currently support.

So maybe it makes sense to first implement timerclear and setitimer FIRST, then reuse bionic's implementation?

[tszhin-swe]

Are you looking for help on implementing timerclear / settimer?

[lntue]

Are you looking for help on implementing timerclear / settimer?

Feel free to create an issue for those functions and tag us to assign it to you. Thanks!