1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
|
.\" Copyright (c) 2019 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pidfd_send_signal 2 2023-03-30 "Linux man-pages 6.04"
.SH NAME
pidfd_send_signal \- send a signal to a process specified by a file descriptor
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/signal.h>" " /* Definition of " SIG* " constants */"
.BR "#include <signal.h>" " /* Definition of " SI_* " constants */"
.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "int syscall(SYS_pidfd_send_signal, int " pidfd ", int " sig ,
.BI " siginfo_t *_Nullable " info ", unsigned int " flags );
.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR pidfd_send_signal (),
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
The
.BR pidfd_send_signal ()
system call sends the signal
.I sig
to the target process referred to by
.IR pidfd ,
a PID file descriptor that refers to a process.
.\" See the very detailed commit message for kernel commit
.\" 3eb39f47934f9d5a3027fe00d906a45fe3a15fad
.PP
If the
.I info
argument points to a
.I siginfo_t
buffer, that buffer should be populated as described in
.BR rt_sigqueueinfo (2).
.PP
If the
.I info
argument is a NULL pointer,
this is equivalent to specifying a pointer to a
.I siginfo_t
buffer whose fields match the values that are
implicitly supplied when a signal is sent using
.BR kill (2):
.PP
.PD 0
.IP \[bu] 3
.I si_signo
is set to the signal number;
.IP \[bu]
.I si_errno
is set to 0;
.IP \[bu]
.I si_code
is set to
.BR SI_USER ;
.IP \[bu]
.I si_pid
is set to the caller's PID; and
.IP \[bu]
.I si_uid
is set to the caller's real user ID.
.PD
.PP
The calling process must either be in the same PID namespace as the
process referred to by
.IR pidfd ,
or be in an ancestor of that namespace.
.PP
The
.I flags
argument is reserved for future use;
currently, this argument must be specified as 0.
.SH RETURN VALUE
On success,
.BR pidfd_send_signal ()
returns 0.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I pidfd
is not a valid PID file descriptor.
.TP
.B EINVAL
.I sig
is not a valid signal.
.TP
.B EINVAL
The calling process is not in a PID namespace from which it can
send a signal to the target process.
.TP
.B EINVAL
.I flags
is not 0.
.TP
.B EPERM
The calling process does not have permission to send the signal
to the target process.
.TP
.B EPERM
.I pidfd
doesn't refer to the calling process, and
.I info.si_code
is invalid (see
.BR rt_sigqueueinfo (2)).
.TP
.B ESRCH
The target process does not exist
(i.e., it has terminated and been waited on).
.SH STANDARDS
Linux.
.SH HISTORY
Linux 5.1.
.SH NOTES
.SS PID file descriptors
The
.I pidfd
argument is a PID file descriptor,
a file descriptor that refers to process.
Such a file descriptor can be obtained in any of the following ways:
.IP \[bu] 3
by opening a
.IR /proc/ pid
directory;
.IP \[bu]
using
.BR pidfd_open (2);
or
.IP \[bu]
via the PID file descriptor that is returned by a call to
.BR clone (2)
or
.BR clone3 (2)
that specifies the
.B CLONE_PIDFD
flag.
.PP
The
.BR pidfd_send_signal ()
system call allows the avoidance of race conditions that occur
when using traditional interfaces (such as
.BR kill (2))
to signal a process.
The problem is that the traditional interfaces specify the target process
via a process ID (PID),
with the result that the sender may accidentally send a signal to
the wrong process if the originally intended target process
has terminated and its PID has been recycled for another process.
By contrast,
a PID file descriptor is a stable reference to a specific process;
if that process terminates,
.BR pidfd_send_signal ()
fails with the error
.BR ESRCH .
.SH EXAMPLES
.\" SRC BEGIN (pidfd_send_signal.c)
.EX
#define _GNU_SOURCE
#include <fcntl.h>
#include <limits.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
static int
pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
unsigned int flags)
{
return syscall(SYS_pidfd_send_signal, pidfd, sig, info, flags);
}
int
main(int argc, char *argv[])
{
int pidfd, sig;
char path[PATH_MAX];
siginfo_t info;
if (argc != 3) {
fprintf(stderr, "Usage: %s <pid> <signal>\en", argv[0]);
exit(EXIT_FAILURE);
}
sig = atoi(argv[2]);
/* Obtain a PID file descriptor by opening the /proc/PID directory
of the target process. */
snprintf(path, sizeof(path), "/proc/%s", argv[1]);
pidfd = open(path, O_RDONLY);
if (pidfd == \-1) {
perror("open");
exit(EXIT_FAILURE);
}
/* Populate a \[aq]siginfo_t\[aq] structure for use with
pidfd_send_signal(). */
memset(&info, 0, sizeof(info));
info.si_code = SI_QUEUE;
info.si_signo = sig;
info.si_errno = 0;
info.si_uid = getuid();
info.si_pid = getpid();
info.si_value.sival_int = 1234;
/* Send the signal. */
if (pidfd_send_signal(pidfd, sig, &info, 0) == \-1) {
perror("pidfd_send_signal");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR clone (2),
.BR kill (2),
.BR pidfd_open (2),
.BR rt_sigqueueinfo (2),
.BR sigaction (2),
.BR pid_namespaces (7),
.BR signal (7)
|
