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Linux man pages : kill (2)
KILL(2)			   Linux Programmer's Manual		       KILL(2)


kill - send signal to a process


#include <sys/types.h> #include <signal.h> int kill(pid_t pid, int sig);


The kill system call can be used to send any signal to any process group or process. If pid is positive, then signal sig is sent to pid. If pid equals 0, then sig is sent to every process in the process group of the current process. If pid equals -1, then sig is sent to every process except for process 1 (init), but see below. If pid is less than -1, then sig is sent to every process in the pro- cess group -pid. If sig is 0, then no signal is sent, but error checking is still per- formed.


On success, zero is returned. On error, -1 is returned, and errno is set appropriately.


EINVAL An invalid signal was specified. ESRCH The pid or process group does not exist. Note that an existing process might be a zombie, a process which already committed termination, but has not yet been wait()ed for. EPERM The process does not have permission to send the signal to any of the receiving processes. For a process to have permission to send a signal to process pid it must either have root privi- leges, or the real or effective user ID of the sending process must equal the real or saved set-user-ID of the receiving pro- cess. In the case of SIGCONT it suffices when the sending and receiving processes belong to the same session.


It is impossible to send a signal to task number one, the init process, for which it has not installed a signal handler. This is done to assure the system is not brought down accidentally. POSIX 1003.1-2001 requires that kill(-1,sig) send sig to all processes that the current process may send signals to, except possibly for some implementation-defined system processes. Linux allows a process to signal itself, but on Linux the call kill(-1,sig) does not signal the current process.


Across different kernel versions, Linux has enforced different rules for the permissions required for an unprivileged process to send a sig- nal to another process. In kernels 1.0 to 1.2.2, a signal could be sent if the effective user ID of the sender matched that of the receiver, or the real user ID of the sender matched that of the receiver. From kernel 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 effec- tive user ID of the receiver. The current rules, which conform to POSIX 1003.1-2001, were adopted in kernel 1.3.78.


SVr4, SVID, POSIX.1, X/OPEN, BSD 4.3, POSIX 1003.1-2001


_exit(2), exit(3), signal(2), signal(7) Linux 2.5.0 2001-12-18 KILL(2)