1KILL(2) Linux Programmer's Manual KILL(2)
2
6 kill - send signal to a process
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9 #include <sys/types.h>
10 #include <signal.h>
11 int kill(pid_t pid, int sig);
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15 The kill() system call can be used to send any signal to any process
16 group or process.
17 If pid is positive, then signal sig is sent to pid.
19 If pid equals 0, then sig is sent to every process in the process group
21 of the current process.
22 If pid equals -1, then sig is sent to every process for which the call‐
24 ing process has permission to send signals, except for process 1
25 (init), but see below.
26 If pid is less than -1, then sig is sent to every process in the
28 process group -pid.
29 If sig is 0, then no signal is sent, but error checking is still per‐
31 formed.
32 For a process to have permission to send a signal it must either be
34 privileged (under Linux: have the CAP_KILL capability), or the real or
35 effective user ID of the sending process must equal the real or saved
36 set-user-ID of the target process. In the case of SIGCONT it suffices
37 when the sending and receiving processes belong to the same session.
38
40 On success (at least one signal was sent), zero is returned. On error,
41 -1 is returned, and errno is set appropriately.
42
44 EINVAL An invalid signal was specified.
45 EPERM The process does not have permission to send the signal to any
47 of the target processes.
48 ESRCH The pid or process group does not exist. Note that an existing
50 process might be a zombie, a process which already committed
51 termination, but has not yet been wait()ed for.
52
54 The only signals that can be sent process ID 1, the init process, are
55 those for which init has explicitly installed signal handlers. This is
56 done to assure the system is not brought down accidentally.
57 POSIX.1-2001 requires that kill(-1,sig) send sig to all processes that
59 the current process may send signals to, except possibly for some
60 implementation-defined system processes. Linux allows a process to
61 signal itself, but on Linux the call kill(-1,sig) does not signal the
62 current process.
63 POSIX.1-2001 requires that if a process sends a signal to itself, and
65 the sending thread does not have the signal blocked, and no other
66 thread has it unblocked or is waiting for it in sigwait(), at least one
67 unblocked signal must be delivered to the sending thread before the
68 kill().
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71 In 2.6 kernels up to and including 2.6.7, there was a bug that meant
72 that when sending signals to a process group, kill() failed with the
73 error EPERM if the caller did have permission to send the signal to any
74 (rather than all) of the members of the process group. Notwithstanding
75 this error return, the signal was still delivered to all of the pro‐
76 cesses for which the caller had permission to signal.
77
79 Across different kernel versions, Linux has enforced different rules
80 for the permissions required for an unprivileged process to send a sig‐
81 nal to another process. In kernels 1.0 to 1.2.2, a signal could be
82 sent if the effective user ID of the sender matched that of the
83 receiver, or the real user ID of the sender matched that of the
84 receiver. From kernel 1.2.3 until 1.3.77, a signal could be sent if
85 the effective user ID of the sender matched either the real or effec‐
86 tive user ID of the receiver. The current rules, which conform to
87 POSIX.1-2001, were adopted in kernel 1.3.78.
88
90 SVr4, 4.3BSD, POSIX.1-2001
91
93 _exit(2), killpg(2), signal(2), sigqueue(2), tkill(2), exit(3), capa‐
94 bilities(7), signal(7)
95Linux 2.6.7 2004-06-24 KILL(2)