sys_tgkill(9)

💖 sys_tgkill(9) — like, the thread-targeted signal syscall, bestie 💖

okay so this is a page from the Kernel API Manual, and it's all about sys_tgkill — the syscall that slides into one specific thread's DMs with a signal 🎯✨

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🌸 NAME 🌸

sys_tgkill — send a signal to one specific thread, not the whole group, no cap 💅

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✨ SYNOPSIS ✨

long sys_tgkill (pid_t tgid , pid_t pid , int sig );

literally three args, that's it, that's the whole vibe 🫶

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🎀 ARGUMENTS 🎀

• tgid — the thread group ID of the thread 👑 (yes it wants the group too, we'll get to why)
• pid — the PID of the thread 🦋 the specific one, the main character
• sig — the signal to be sent 🔥 the actual message

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💜 DESCRIPTION 💜

so here's the iconic part: this syscall also checks the tgid, and it returns -ESRCH even if the PID exists — but that PID isn't belonging to the target process anymore. 😭

and like… that's not a bug, that's the whole point. it solves the problem of threads exiting and PIDs getting reused. because bestie, PIDs get recycled constantly and if you weren't double-checking you'd be out here firing signals at some totally random new process that just happened to inherit the number 💀🙈

no cap, the tgid check is the safety rail here. it's giving "confirm before you send" energy and honestly? we stan a defensive syscall 🧚

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🦋 SEE ALSO 🦋

the kernel file ./kernel/signal.c — go read it, it's where all of this lives 📂

and then the whole friend group, in the order the original rolls with:

• task_set_jobctl_pending(9)
• task_clear_jobctl_trapping(9)
• task_clear_jobctl_pending(9)
• task_participate_group_stop(9)
• ptrace_trap_notify(9)
• force_sig_seccomp(9)
• do_notify_parent_cldstop(9)
• do_signal_stop(9)
• do_jobctl_trap(9)
• do_freezer_trap(9)
• signal_delivered(9)
• sys_restart_syscall(9)
• set_current_blocked(9)
• sys_rt_sigprocmask(9)
• sys_rt_sigpending(9)
• copy_siginfo_to_external32(9)
• do_sigtimedwait(9)
• sys_rt_sigtimedwait(9)
• sys_kill(9)
• sys_pidfd_send_signal(9)
• sys_tkill(9)
• sys_rt_sigqueueinfo(9)
• sys_sigpending(9)
• sys_sigprocmask(9)
• sys_rt_sigaction(9)
• sys_rt_sigsuspend(9)

like, that's the whole signal-handling squad right there, iykyk 🌈⭐

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aaaaand that's the page!! 💖 this one's from ./kernel, dated October 2026, and it's sys_tgkill(9) — mwah, go send a signal to exactly one thread and be normal about it 😌💅🎀