pidfd_send_signal(2)

1PIDFD_SEND_SIGNAL(2)       Linux Programmer's Manual      PIDFD_SEND_SIGNAL(2)
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NAME

6       pidfd_send_signal  -  send  a  signal  to a process specified by a file
7       descriptor
8

SYNOPSIS

10       #include <signal.h>
11
12       int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
13                             unsigned int flags);
14

DESCRIPTION

16       The pidfd_send_signal() system call sends the signal sig to the  target
17       process  referred  to  by pidfd, a PID file descriptor that refers to a
18       process.
19
20       If the info argument points to a siginfo_t buffer, that  buffer  should
21       be populated as described in rt_sigqueueinfo(2).
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23       If  the info argument is a NULL pointer, this is equivalent to specify‐
24       ing a pointer to a siginfo_t buffer whose fields match the values  that
25       are implicitly supplied when a signal is sent using kill(2):
26
27       *  si_signo is set to the signal number;
28       *  si_errno is set to 0;
29       *  si_code is set to SI_USER;
30       *  si_pid is set to the caller's PID; and
31       *  si_uid is set to the caller's real user ID.
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33       The  calling  process  must  either be in the same PID namespace as the
34       process referred to by pidfd, or be in an ancestor of that namespace.
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36       The flags argument is reserved for future use; currently, this argument
37       must be specified as 0.
38

RETURN VALUE

40       On  success,  pidfd_send_signal()  returns 0.  On error, -1 is returned
41       and errno is set to indicate the cause of the error.
42

ERRORS

44       EBADF  pidfd is not a valid PID file descriptor.
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46       EINVAL sig is not a valid signal.
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48       EINVAL The calling process is not in a PID namespace from which it  can
49              send a signal to the target process.
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51       EINVAL flags is not 0.
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53       EPERM  The  calling process does not have permission to send the signal
54              to the target process.
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56       EPERM  pidfd doesn't refer to the calling process, and info.si_code  is
57              invalid (see rt_sigqueueinfo(2)).
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59       ESRCH  The  target  process does not exist (i.e., it has terminated and
60              been waited on).
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VERSIONS

63       pidfd_send_signal() first appeared in Linux 5.1.
64

CONFORMING TO

66       pidfd_send_signal() is Linux specific.
67

NOTES

69       Currently, there is no glibc wrapper for  this  system  call;  call  it
70       using syscall(2).
71
72   PID file descriptors
73       The  pidfd  argument  is  a PID file descriptor, a file descriptor that
74       refers to  process.  Such a file descriptor can be obtained in  any  of
75       the following ways:
76
77       *  by opening a /proc/[pid] directory;
78
79       *  using pidfd_open(2); or
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81       *  via  the  PID file descriptor that is returned by a call to clone(2)
82          or clone3(2) that specifies the CLONE_PIDFD flag.
83
84       The pidfd_send_signal() system call allows the avoidance of race condi‐
85       tions that occur when using traditional interfaces (such as kill(2)) to
86       signal a process.  The problem is that the traditional interfaces spec‐
87       ify the target process via a process ID (PID), with the result that the
88       sender may accidentally send a signal to the wrong process if the orig‐
89       inally  intended  target  process  has  terminated and its PID has been
90       recycled for another process.  By contrast, a PID file descriptor is  a
91       stable  reference  to  a  specific process; if that process terminates,
92       pidfd_send_signal() fails with the error ESRCH.
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EXAMPLES

95       #define _GNU_SOURCE
96       #include <limits.h>
97       #include <signal.h>
98       #include <fcntl.h>
99       #include <stdio.h>
100       #include <string.h>
101       #include <stdlib.h>
102       #include <unistd.h>
103       #include <sys/syscall.h>
104
105       #ifndef __NR_pidfd_send_signal
106       #define __NR_pidfd_send_signal 424
107       #endif
108
109       static int
110       pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
111               unsigned int flags)
112       {
113           return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
114       }
115
116       int
117       main(int argc, char *argv[])
118       {
119           siginfo_t info;
120           char path[PATH_MAX];
121           int pidfd, sig;
122
123           if (argc != 3) {
124               fprintf(stderr, "Usage: %s <pid> <signal>\n", argv[0]);
125               exit(EXIT_FAILURE);
126           }
127
128           sig = atoi(argv[2]);
129
130           /* Obtain a PID file descriptor by opening the /proc/PID directory
131              of the target process */
132
133           snprintf(path, sizeof(path), "/proc/%s", argv[1]);
134
135           pidfd = open(path, O_RDONLY);
136           if (pidfd == -1) {
137               perror("open");
138               exit(EXIT_FAILURE);
139           }
140
141           /* Populate a 'siginfo_t' structure for use with
142              pidfd_send_signal() */
143
144           memset(&info, 0, sizeof(info));
145           info.si_code = SI_QUEUE;
146           info.si_signo = sig;
147           info.si_errno = 0;
148           info.si_uid = getuid();
149           info.si_pid = getpid();
150           info.si_value.sival_int = 1234;
151
152           /* Send the signal */
153
154           if (pidfd_send_signal(pidfd, sig, &info, 0) == -1) {
155               perror("pidfd_send_signal");
156               exit(EXIT_FAILURE);
157           }
158
159           exit(EXIT_SUCCESS);
160       }
161

COLOPHON

167       This  page  is  part of release 5.07 of the Linux man-pages project.  A
168       description of the project, information about reporting bugs,  and  the
169       latest     version     of     this    page,    can    be    found    at
170       https://www.kernel.org/doc/man-pages/.
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174Linux                             2020-06-09              PIDFD_SEND_SIGNAL(2)