1SIGNALFD(2) Linux Programmer's Manual SIGNALFD(2)
2
6 signalfd - create a file descriptor for accepting signals
7
9 #include <sys/signalfd.h>
10 int signalfd(int fd, const sigset_t *mask, int flags);
12
14 signalfd() creates a file descriptor that can be used to accept signals
15 targeted at the caller. This provides an alternative to the use of a
16 signal handler or sigwaitinfo(2), and has the advantage that the file
17 descriptor may be monitored by select(2), poll(2), and epoll(7).
18 The mask argument specifies the set of signals that the caller wishes
20 to accept via the file descriptor. This argument is a signal set whose
21 contents can be initialized using the macros described in sigsetops(3).
22 Normally, the set of signals to be received via the file descriptor
23 should be blocked using sigprocmask(2), to prevent the signals being
24 handled according to their default dispositions. It is not possible to
25 receive SIGKILL or SIGSTOP signals via a signalfd file descriptor;
26 these signals are silently ignored if specified in mask.
27 If the fd argument is -1, then the call creates a new file descriptor
29 and associates the signal set specified in mask with that file descrip‐
30 tor. If fd is not -1, then it must specify a valid existing signalfd
31 file descriptor, and mask is used to replace the signal set associated
32 with that file descriptor.
33 Starting with Linux 2.6.27, the following values may be bitwise ORed in
35 flags to change the behavior of signalfd():
36 SFD_NONBLOCK Set the O_NONBLOCK file status flag on the open file
38 description (see open(2)) referred to by the new file
39 descriptor. Using this flag saves extra calls to
40 fcntl(2) to achieve the same result.
41 SFD_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
43 descriptor. See the description of the O_CLOEXEC flag in
44 open(2) for reasons why this may be useful.
45 In Linux up to version 2.6.26, the flags argument is unused, and must
47 be specified as zero.
48 signalfd() returns a file descriptor that supports the following opera‐
50 tions:
51 read(2)
53 If one or more of the signals specified in mask is pending for
54 the process, then the buffer supplied to read(2) is used to
55 return one or more signalfd_siginfo structures (see below) that
56 describe the signals. The read(2) returns information for as
57 many signals as are pending and will fit in the supplied buffer.
58 The buffer must be at least sizeof(struct signalfd_siginfo)
59 bytes. The return value of the read(2) is the total number of
60 bytes read.
61 As a consequence of the read(2), the signals are consumed, so
63 that they are no longer pending for the process (i.e., will not
64 be caught by signal handlers, and cannot be accepted using sig‐
65 waitinfo(2)).
66 If none of the signals in mask is pending for the process, then
68 the read(2) either blocks until one of the signals in mask is
69 generated for the process, or fails with the error EAGAIN if the
70 file descriptor has been made nonblocking.
71 poll(2), select(2) (and similar)
73 The file descriptor is readable (the select(2) readfds argument;
74 the poll(2) POLLIN flag) if one or more of the signals in mask
75 is pending for the process.
76 The signalfd file descriptor also supports the other file-
78 descriptor multiplexing APIs: pselect(2), ppoll(2), and
79 epoll(7).
80 close(2)
82 When the file descriptor is no longer required it should be
83 closed. When all file descriptors associated with the same sig‐
84 nalfd object have been closed, the resources for object are
85 freed by the kernel.
86 The signalfd_siginfo structure
88 The format of the signalfd_siginfo structure(s) returned by read(2)s
89 from a signalfd file descriptor is as follows:
90 struct signalfd_siginfo {
92 uint32_t ssi_signo; /* Signal number */
93 int32_t ssi_errno; /* Error number (unused) */
94 int32_t ssi_code; /* Signal code */
95 uint32_t ssi_pid; /* PID of sender */
96 uint32_t ssi_uid; /* Real UID of sender */
97 int32_t ssi_fd; /* File descriptor (SIGIO) */
98 uint32_t ssi_tid; /* Kernel timer ID (POSIX timers)
99 uint32_t ssi_band; /* Band event (SIGIO) */
100 uint32_t ssi_overrun; /* POSIX timer overrun count */
101 uint32_t ssi_trapno; /* Trap number that caused signal */
102 int32_t ssi_status; /* Exit status or signal (SIGCHLD) */
103 int32_t ssi_int; /* Integer sent by sigqueue(3) */
104 uint64_t ssi_ptr; /* Pointer sent by sigqueue(3) */
105 uint64_t ssi_utime; /* User CPU time consumed (SIGCHLD) */
106 uint64_t ssi_stime; /* System CPU time consumed
107 (SIGCHLD) */
108 uint64_t ssi_addr; /* Address that generated signal
109 (for hardware-generated signals) */
110 uint16_t ssi_addr_lsb; /* Least significant bit of address
111 (SIGBUS; since Linux 2.6.37)
112 uint8_t pad[X]; /* Pad size to 128 bytes (allow for
113 additional fields in the future) */
114 };
115 Each of the fields in this structure is analogous to the similarly
117 named field in the siginfo_t structure. The siginfo_t structure is
118 described in sigaction(2). Not all fields in the returned sig‐
119 nalfd_siginfo structure will be valid for a specific signal; the set of
120 valid fields can be determined from the value returned in the ssi_code
121 field. This field is the analog of the siginfo_t si_code field; see
122 sigaction(2) for details.
123 fork(2) semantics
125 After a fork(2), the child inherits a copy of the signalfd file
126 descriptor. A read(2) from the file descriptor in the child will
127 return information about signals queued to the child.
128 Semantics of file descriptor passing
130 As with other file descriptors, signalfd file descriptors can be passed
131 to another process via a UNIX domain socket (see unix(7)). In the
132 receiving process, a read(2) from the received file descriptor will
133 return information about signals queued to that process.
134 execve(2) semantics
136 Just like any other file descriptor, a signalfd file descriptor remains
137 open across an execve(2), unless it has been marked for close-on-exec
138 (see fcntl(2)). Any signals that were available for reading before the
139 execve(2) remain available to the newly loaded program. (This is anal‐
140 ogous to traditional signal semantics, where a blocked signal that is
141 pending remains pending across an execve(2).)
142 Thread semantics
144 The semantics of signalfd file descriptors in a multithreaded program
145 mirror the standard semantics for signals. In other words, when a
146 thread reads from a signalfd file descriptor, it will read the signals
147 that are directed to the thread itself and the signals that are
148 directed to the process (i.e., the entire thread group). (A thread
149 will not be able to read signals that are directed to other threads in
150 the process.)
151
153 On success, signalfd() returns a signalfd file descriptor; this is
154 either a new file descriptor (if fd was -1), or fd if fd was a valid
155 signalfd file descriptor. On error, -1 is returned and errno is set to
156 indicate the error.
157
159 EBADF The fd file descriptor is not a valid file descriptor.
160 EINVAL fd is not a valid signalfd file descriptor.
162 EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is
164 nonzero.
165 EMFILE The per-process limit on the number of open file descriptors has
167 been reached.
168 ENFILE The system-wide limit on the total number of open files has been
170 reached.
171 ENODEV Could not mount (internal) anonymous inode device.
173 ENOMEM There was insufficient memory to create a new signalfd file
175 descriptor.
176
178 signalfd() is available on Linux since kernel 2.6.22. Working support
179 is provided in glibc since version 2.8. The signalfd4() system call
180 (see NOTES) is available on Linux since kernel 2.6.27.
181
183 signalfd() and signalfd4() are Linux-specific.
184
186 A process can create multiple signalfd file descriptors. This makes it
187 possible to accept different signals on different file descriptors.
188 (This may be useful if monitoring the file descriptors using select(2),
189 poll(2), or epoll(7): the arrival of different signals will make dif‐
190 ferent file descriptors ready.) If a signal appears in the mask of
191 more than one of the file descriptors, then occurrences of that signal
192 can be read (once) from any one of the file descriptors.
193 Attempts to include SIGKILL and SIGSTOP in mask are silently ignored.
195 The signal mask employed by a signalfd file descriptor can be viewed
197 via the entry for the corresponding file descriptor in the process's
198 /proc/[pid]/fdinfo directory. See proc(5) for further details.
199 Limitations
201 The signalfd mechanism can't be used to receive signals that are syn‐
202 chronously generated, such as the SIGSEGV signal that results from
203 accessing an invalid memory address or the SIGFPE signal that results
204 from an arithmetic error. Such signals can be caught only via signal
205 handler.
206 As described above, in normal usage one blocks the signals that will be
208 accepted via signalfd(). If spawning a child process to execute a
209 helper program (that does not need the signalfd file descriptor), then,
210 after the call to fork(2), you will normally want to unblock those sig‐
211 nals before calling execve(2), so that the helper program can see any
212 signals that it expects to see. Be aware, however, that this won't be
213 possible in the case of a helper program spawned behind the scenes by
214 any library function that the program may call. In such cases, one
215 must fall back to using a traditional signal handler that writes to a
216 file descriptor monitored by select(2), poll(2), or epoll(7),
217 C library/kernel differences
219 The underlying Linux system call requires an additional argument,
220 size_t sizemask, which specifies the size of the mask argument. The
221 glibc signalfd() wrapper function does not include this argument, since
222 it provides the required value for the underlying system call.
223 There are two underlying Linux system calls: signalfd() and the more
225 recent signalfd4(). The former system call does not implement a flags
226 argument. The latter system call implements the flags values described
227 above. Starting with glibc 2.9, the signalfd() wrapper function will
228 use signalfd4() where it is available.
229
231 In kernels before 2.6.25, the ssi_ptr and ssi_int fields are not filled
232 in with the data accompanying a signal sent by sigqueue(3).
233
235 The program below accepts the signals SIGINT and SIGQUIT via a signalfd
236 file descriptor. The program terminates after accepting a SIGQUIT sig‐
237 nal. The following shell session demonstrates the use of the program:
238 $ ./signalfd_demo
240 ^C # Control-C generates SIGINT
241 Got SIGINT
242 ^C
243 Got SIGINT
244 ^\ # Control-\ generates SIGQUIT
245 Got SIGQUIT
246 $
247 Program source
249 #include <sys/signalfd.h>
251 #include <signal.h>
252 #include <unistd.h>
253 #include <stdlib.h>
254 #include <stdio.h>
255 #define handle_error(msg) \
257 do { perror(msg); exit(EXIT_FAILURE); } while (0)
258 int
260 main(int argc, char *argv[])
261 {
262 sigset_t mask;
263 int sfd;
264 struct signalfd_siginfo fdsi;
265 ssize_t s;
266 sigemptyset(&mask);
268 sigaddset(&mask, SIGINT);
269 sigaddset(&mask, SIGQUIT);
270 /* Block signals so that they aren't handled
272 according to their default dispositions */
273 if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
275 handle_error("sigprocmask");
276 sfd = signalfd(-1, &mask, 0);
278 if (sfd == -1)
279 handle_error("signalfd");
280 for (;;) {
282 s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
283 if (s != sizeof(struct signalfd_siginfo))
284 handle_error("read");
285 if (fdsi.ssi_signo == SIGINT) {
287 printf("Got SIGINT\n");
288 } else if (fdsi.ssi_signo == SIGQUIT) {
289 printf("Got SIGQUIT\n");
290 exit(EXIT_SUCCESS);
291 } else {
292 printf("Read unexpected signal\n");
293 }
294 }
295 }
296
298 eventfd(2), poll(2), read(2), select(2), sigaction(2), sigprocmask(2),
299 sigwaitinfo(2), timerfd_create(2), sigsetops(3), sigwait(3), epoll(7),
300 signal(7)
301
303 This page is part of release 5.02 of the Linux man-pages project. A
304 description of the project, information about reporting bugs, and the
305 latest version of this page, can be found at
306 https://www.kernel.org/doc/man-pages/.
307Linux 2019-03-06 SIGNALFD(2)