2:kill

From Linux Man Pages

      kill - send signal to a process
      

SYNOPSIS

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

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 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 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 -pid.
 
      If sig is 0, then no signal is sent, but error checking is still performed.
 
      For a process to have permission to send a signal it must either be privileged (under Linux:  have  the  CAP_KILL
      capability),  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.

RETURN VALUE

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

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 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.

NOTES

      The  only  signals  that  can  be sent task number one, 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.1-2001 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.
 
      POSIX.1-2001 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(), at least one unblocked signal
      must be delivered to the sending thread before the kill().

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 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.

LINUX HISTORY

      Across different kernel versions, Linux has enforced different rules for the permissions required for an unprivi-
      leged process to send a signal to another process.  In kernels 1.0 to 1.2.2, a signal could be sent if the effec-
      tive  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  effective user ID of the receiver.  The current rules, which conform to POSIX.1-2001, were
      adopted in kernel 1.3.78.

CONFORMING TO

      SVr4, 4.3BSD, POSIX.1-2001

RELATED

      _exit(2), killpg(2), signal(2), sigqueue(2), tkill(2), exit(3), capabilities(7), signal(7)

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