Ubuntu Manpage: kill - send signal to a process

NAME

       kill - send signal to a process

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <signal.h>

       int kill(pid_t pid, int sig);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       kill():
           _POSIX_C_SOURCE

DESCRIPTION

       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 the process with the ID specified by pid.

       If  pid  equals  0,  then sig is sent to every process in the process group of the calling
       process.

       If pid equals -1, then sig is sent to every process for  which  the  calling  process  has
       permission to send signals, except for process 1 (init), but see below.

       If pid is less than -1, then sig is sent to every process in the process group whose ID is
       -pid.

       If sig 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  the CAP_KILL capability 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 of SIGCONT, it suffices when the sending and receiving
       processes belong to the same  session.   (Historically,  the  rules  were  different;  see
       NOTES.)

RETURN VALUE

       On  success  (at  least one signal was sent), zero is returned.  On error, -1 is returned,
       and errno is set to indicate the error.

ERRORS

       EINVAL An invalid signal was specified.

       EPERM  The calling process does not have permission to send  the  signal  to  any  of  the
              target processes.

       ESRCH  The  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  been
              wait(2)ed for.

STANDARDS

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES

The only signals that can be sent to process ID 1, the init process, are those for which
init has explicitly installed signal handlers. This is done to assure the system is not
brought down accidentally.

POSIX.1 requires that kill(-1,sig) send sig 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 call kill(-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 in
sigwait(3), at least one unblocked signal must be delivered to the sending thread before
the kill() returns.

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.

BUGS

       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 error EPERM if the caller did not  have
       permission  to  send  the  signal  to  any (rather than all) 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.