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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |NOTES |BUGS |SEE ALSO |COLOPHON | |
kill(2) System Calls Manualkill(2)kill - send signal to a process
Standard C library (libc,-lc)
#include <signal.h>int kill(pid_tpid, intsig); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):kill(): _POSIX_C_SOURCE
Thekill() system call can be used to send any signal to any process group or process. Ifpid is positive, then signalsig is sent to the process with the ID specified bypid. Ifpid equals 0, thensig is sent to every process in the process group of the calling process. Ifpid equals -1, thensig is sent to every process for which the calling process has permission to send signals, except for process 1 (init), but see below. Ifpid is less than -1, thensig is sent to every process in the process group whose ID is-pid. Ifsig is 0, then no signal is sent, but existence and permission checks are still performed; this can be used to check for the existence of a process ID or process group ID that the caller is permitted to signal. For a process to have permission to send a signal, it must either be privileged (under Linux: have theCAP_KILLcapability in the user namespace of the target process), or the real or effective user ID of the sending process must equal the real or saved set- user-ID of the target process. In the case ofSIGCONT, it suffices when the sending and receiving processes belong to the same session. (Historically, the rules were different; see HISTORY.)
On success, zero is returned. If signals were sent to a process group, success means that at least one signal was delivered. On error, -1 is returned, anderrno is set to indicate the error.
EINVALAn invalid signal was specified.EPERMThe calling process does not have permission to send the signal to any of the target processes.ESRCHThe target process or process group does not exist. Note that an existing process might be a zombie, a process that has terminated execution, but has not yet beenwait(2)ed for.
POSIX.1-2008.
POSIX.1-2001, SVr4, 4.3BSD.Linux notes Across different kernel versions, Linux has enforced different rules for the permissions required for an unprivileged process to send a signal to another process. In Linux 1.0 to 1.2.2, a signal could be sent if the effective user ID of the sender matched effective user ID of the target, or the real user ID of the sender matched the real user ID of the target. From Linux 1.2.3 until 1.3.77, a signal could be sent if the effective user ID of the sender matched either the real or effective user ID of the target. The current rules, which conform to POSIX.1, were adopted in Linux 1.3.78.
The only signals that can be sent to process ID 1, theinit process, are those for whichinit has explicitly installed signal handlers. This is done to assure the system is not brought down accidentally. POSIX.1 requires thatkill(-1,sig) sendsig to all processes that the calling process may send signals to, except possibly for some implementation-defined system processes. Linux allows a process to signal itself, but on Linux the callkill(-1,sig) does not signal the calling process. POSIX.1 requires that if a process sends a signal to itself, and the sending thread does not have the signal blocked, and no other thread has it unblocked or is waiting for it insigwait(3), at least one unblocked signal must be delivered to the sending thread before thekill() returns.
In Linux 2.6 up to and including Linux 2.6.7, there was a bug that meant that when sending signals to a process group,kill() failed with the errorEPERMif the caller did not have permission to send the signal toany (rather thanall) of the members of the process group. Notwithstanding this error return, the signal was still delivered to all of the processes for which the caller had permission to signal.
kill(1),_exit(2),pidfd_send_signal(2),signal(2),tkill(2),exit(3),killpg(3),sigqueue(3),capabilities(7),credentials(7),signal(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17kill(2)Pages that refer to this page:capsh(1), fuser(1), kill(1@@coreutils), kill(1), kill(1@@procps-ng), killall(1), pgrep(1), skill(1), strace(1), clone(2), _exit(2), F_GETSIG(2const), getpid(2), getrlimit(2), pause(2), pidfd_open(2), pidfd_send_signal(2), ptrace(2), rt_sigqueueinfo(2), setfsgid(2), setfsuid(2), sigaction(2), signal(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwaitinfo(2), syscalls(2), tkill(2), wait(2), gsignal(3), id_t(3type), killpg(3), psignal(3), pthread_kill(3), raise(3), sd_event_add_child(3), sigpause(3), sigqueue(3), sigset(3), sigvec(3), systemd.exec(5), systemd.kill(5), capabilities(7), cpuset(7), credentials(7), pid_namespaces(7), pthreads(7), signal(7), signal-safety(7), systemd-coredump(8)
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