1PIDFD_OPEN(2) Linux Programmer's Manual PIDFD_OPEN(2)
2
6 pidfd_open - obtain a file descriptor that refers to a process
7
9 #include <sys/types.h>
10 int pidfd_open(pid_t pid, unsigned int flags);
12
14 The pidfd_open() system call creates a file descriptor that refers to
15 the process whose PID is specified in pid. The file descriptor is
16 returned as the function result; the close-on-exec flag is set on the
17 file descriptor.
18 The flags argument is reserved for future use; currently, this argument
20 must be specified as 0.
21
23 On success, pidfd_open() returns a nonnegative file descriptor. On
24 error, -1 is returned and errno is set to indicate the cause of the
25 error.
26
28 EINVAL flags is not 0.
29 EINVAL pid is not valid.
31 EMFILE The per-process limit on the number of open file descriptors has
33 been reached (see the description of RLIMIT_NOFILE in getr‐
34 limit(2)).
35 ENFILE The system-wide limit on the total number of open files has been
37 reached.
38 ENODEV The anonymous inode filesystem is not available in this kernel.
40 ENOMEM Insufficient kernel memory was available.
42 ESRCH The process specified by pid does not exist.
44
46 pidfd_open() first appeared in Linux 5.3.
47
49 pidfd_open() is Linux specific.
50
52 Currently, there is no glibc wrapper for this system call; call it
53 using syscall(2).
54 The following code sequence can be used to obtain a file descriptor for
56 the child of fork(2):
57 pid = fork();
59 if (pid > 0) { /* If parent */
60 pidfd = pidfd_open(pid, 0);
61 ...
62 }
63 Even if the child has already terminated by the time of the
65 pidfd_open() call, its PID will not have been recycled and the returned
66 file descriptor will refer to the resulting zombie process. Note, how‐
67 ever, that this is guaranteed only if the following conditions hold
68 true:
69 * the disposition of SIGCHLD has not been explicitly set to SIG_IGN
71 (see sigaction(2));
72 * the SA_NOCLDWAIT flag was not specified while establishing a handler
74 for SIGCHLD or while setting the disposition of that signal to
75 SIG_DFL (see sigaction(2)); and
76 * the zombie process was not reaped elsewhere in the program (e.g.,
78 either by an asynchronously executed signal handler or by wait(2) or
79 similar in another thread).
80 If any of these conditions does not hold, then the child process (along
82 with a PID file descriptor that refers to it) should instead be created
83 using clone(2) with the CLONE_PIDFD flag.
84 Use cases for PID file descriptors
86 A PID file descriptor returned by pidfd_open() (of by clone(2) with the
87 CLONE_PID flag) can be used for the following purposes:
88 * The pidfd_send_signal(2) system call can be used to send a signal to
90 the process referred to by a PID file descriptor.
91 * A PID file descriptor can be monitored using poll(2), select(2), and
93 epoll(7). When the process that it refers to terminates, these
94 interfaces indicate the file descriptor as readable. Note, however,
95 that in the current implementation, nothing can be read from the
96 file descriptor (read(2) on the file descriptor fails with the error
97 EINVAL).
98 * If the PID file descriptor refers to a child of the calling process,
100 then it can be waited on using waitid(2).
101 The pidfd_open() system call is the preferred way of obtaining a PID
103 file descriptor for an already existing process. The alternative is to
104 obtain a file descriptor by opening a /proc/[pid] directory. However,
105 the latter technique is possible only if the proc(5) filesystem is
106 mounted; furthermore, the file descriptor obtained in this way is not
107 pollable and can't be waited on with waitid(2).
108
110 The program below opens a PID file descriptor for the process whose PID
111 is specified as its command-line argument. It then uses poll(2) to
112 monitor the file descriptor for process exit, as indicated by an
113 EPOLLIN event.
114 Program source
116 #define _GNU_SOURCE
118 #include <sys/types.h>
119 #include <sys/syscall.h>
120 #include <unistd.h>
121 #include <poll.h>
122 #include <stdlib.h>
123 #include <stdio.h>
124 #ifndef __NR_pidfd_open
126 #define __NR_pidfd_open 434 /* System call # on most architectures */
127 #endif
128 static int
130 pidfd_open(pid_t pid, unsigned int flags)
131 {
132 return syscall(__NR_pidfd_open, pid, flags);
133 }
134 int
136 main(int argc, char *argv[])
137 {
138 struct pollfd pollfd;
139 int pidfd, ready;
140 if (argc != 2) {
142 fprintf(stderr, "Usage: %s <pid>\n", argv[0]);
143 exit(EXIT_SUCCESS);
144 }
145 pidfd = pidfd_open(atoi(argv[1]), 0);
147 if (pidfd == -1) {
148 perror("pidfd_open");
149 exit(EXIT_FAILURE);
150 }
151 pollfd.fd = pidfd;
153 pollfd.events = POLLIN;
154 ready = poll(&pollfd, 1, -1);
156 if (ready == -1) {
157 perror("poll");
158 exit(EXIT_FAILURE);
159 }
160 printf("Events (0x%x): POLLIN is %sset\n", pollfd.revents,
162 (pollfd.revents & POLLIN) ? "" : "not ");
163 exit(EXIT_SUCCESS);
165 }
166
168 clone(2), kill(2), pidfd_send_signal(2), poll(2), select(2), waitid(2),
169 epoll(7)
170
172 This page is part of release 5.04 of the Linux man-pages project. A
173 description of the project, information about reporting bugs, and the
174 latest version of this page, can be found at
175 https://www.kernel.org/doc/man-pages/.
176Linux 2019-11-19 PIDFD_OPEN(2)