KILL(2)

💖 KILL(2) 💖

okay bestie, so we're doing the kill page today — and no, it's not what it sounds like, it's just how you poke a process and tell it something. she's giving "send signal to a process" and honestly? iconic. this lives in the System Calls Manual, section 2, iykyk.

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📚 LIBRARY

Standard C Library (libc, -lc) — that's where she hangs out.

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✨ SYNOPSIS ✨

#include <sys/types.h>
#include <signal.h>

int
kill(pid_t pid, int sig);

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🌸 DESCRIPTION 🌸

The kill() system call sends the signal given by sig to pid, which can be a process or a whole group of processes. cute, right? 🎀

The sig argument may be one of the signals specified in sigaction(2) — or it may be 0, in which case error checking happens but no signal is actually sent. that's the sneaky vibe check: you use it to check the validity of pid without actually yeeting anything at anyone. 🫶

now, permissions, because of course there's a gatekeeper:

• the user must be the super-user, or
• the real or saved user ID of the receiving process must match the real or effective user ID of the sending process.

one single exception: the signal SIGCONT, which may always be sent to any process with the same session ID as the sender. special little princess behaviour, deserved. 💅

also, if the security.bsd.conservative_signals sysctl(9) is set to 1, and the user is not a super-user, and the receiver is set-uid — then only job control and terminal control signals may be sent. in particular, only SIGKILL, SIGINT, SIGTERM, SIGALRM, SIGSTOP, SIGTTIN, SIGTTOU, SIGTSTP, SIGHUP, SIGUSR1, SIGUSR2. that's the whole shortlist, no substitutions bestie 😭

so like, what does pid actually mean? depends on the vibe:

• If pid is greater than zero: The sig signal is sent to the process whose ID is equal to pid.
• If pid is zero: The sig signal is sent to all processes whose group ID is equal to the process group ID of the sender, and for which the process has permission; this is a variant of killpg(2).
• If pid is -1: If the user has super-user privileges, the signal is sent to all processes excluding system processes (with P_SYSTEM flag set), process with ID 1 (usually init(8)), and the process sending the signal. If the user is not the super user, the signal is sent to all processes which the caller has permissions to, excluding the process sending the signal. No error is returned if any process could be signaled.
• If the process number is negative but not -1, the signal is sent to all processes whose process group ID is equal to the absolute value of the process number. this is a variant of killpg(2).

lowkey terrifying that -1 exists, but she's documented and she's legal, so. 🔥

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🦋 RETURN VALUES 🦋

The kill() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. classic.

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💀 ERRORS 💀

The kill() system call will fail and no signal will be sent if:

• [EINVAL] — The sig argument is not a valid signal number. bestie that's not a real signal 😭
• [ESRCH] — No process or process group can be found corresponding to that specified by pid. she's not here, girl, nobody's home 🙈
• [EPERM] — The sending process does not have permission to send sig to any receiving process. permission denied, the audacity 💅

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👑 SEE ALSO 👑

getpgrp(2), getpid(2), killpg(2), sigaction(2), sigqueue(2), raise(3), init(8)

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🌈 STANDARDS 🌈

The kill() system call is expected to conform to ISO/IEC 9945-1:1990 ("POSIX.1"). she's certified, periodt. ⭐

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🧚 HISTORY 🧚

A version of the kill() function appeared in Version 3 AT&T UNIX. The signal number was added to the kill() function in Version 4 AT&T UNIX. she's been around since before your parents were cool, and she's still slaying. 💜

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no cap, that's the whole page. go forth and signal responsibly bestie 🍓

mwah 💖 this was the Debian build of the page, dated December 1, 2019 — thanks for reading, love u, bye!! ✨