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KILL(2) Linux Programmer's Manual KILL(2)

NAME

     kill - send signal to a process

SYNOPSIS

     #include <sys/types.h>
     #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 call-
     ing  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 appropriately.

ERRORS

     EINVAL An invalid signal was specified.

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

     ESRCH  The process or process group  does  not  exist.   Note  that  an
            existing  process  might  be a zombie, a process that has termi-
            nated execution, but has not yet been wait(2)ed for.

CONFORMING TO

     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 implemen-
     tation-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 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 effective user ID
     of the target, or the real user ID of the sender matched the real  user
     ID  of  the  target.  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
     effective  user  ID of the target.  The current rules, which conform to
     POSIX.1, were adopted in kernel 1.3.78.

BUGS

     In 2.6 kernels up to and including 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.  Notwith-
     standing this error return, the signal was still delivered  to  all  of
     the processes for which the caller had permission to signal.