1SIGNALFD(2) Linux Programmer's Manual SIGNALFD(2)
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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 descriptor.
30 If fd is not -1, then it must specify a valid existing signalfd file
31 descriptor, and mask is used to replace the signal set associated with
32 that descriptor.
33 Starting with Linux 2.6.27, the following values may be bitwise ORed in
35 flags to change the behaviour of signalfd():
36 SFD_NONBLOCK Set the O_NONBLOCK file status flag on the new open file
38 description. Using this flag saves extra calls to
39 fcntl(2) to achieve the same result.
40 SFD_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
42 descriptor. See the description of the O_CLOEXEC flag in
43 open(2) for reasons why this may be useful.
44 In Linux up to version 2.6.26, the flags argument is unused, and must
46 be specified as zero.
47 signalfd() returns a file descriptor that supports the following opera‐
49 tions:
50 read(2)
52 If one or more of the signals specified in mask is pending for
53 the process, then the buffer supplied to read(2) is used to
54 return one or more signalfd_siginfo structures (see below) that
55 describe the signals. The read(2) returns information for as
56 many signals as are pending and will fit in the supplied buffer.
57 The buffer must be at least sizeof(struct signalfd_siginfo)
58 bytes. The return value of the read(2) is the total number of
59 bytes read.
60 As a consequence of the read(2), the signals are consumed, so
62 that they are no longer pending for the process (i.e., will not
63 be caught by signal handlers, and cannot be accepted using sig‐
64 waitinfo(2)).
65 If none of the signals in mask is pending for the process, then
67 the read(2) either blocks until one of the signals in mask is
68 generated for the process, or fails with the error EAGAIN if the
69 file descriptor has been made nonblocking.
70 poll(2), select(2) (and similar)
72 The file descriptor is readable (the select(2) readfds argument;
73 the poll(2) POLLIN flag) if one or more of the signals in mask
74 is pending for the process.
75 The signalfd file descriptor also supports the other file-
77 descriptor multiplexing APIs: pselect(2), ppoll(2), and
78 epoll(7).
79 close(2)
81 When the file descriptor is no longer required it should be
82 closed. When all file descriptors associated with the same sig‐
83 nalfd object have been closed, the resources for object are
84 freed by the kernel.
85 The signalfd_siginfo structure
87 The format of the signalfd_siginfo structure(s) returned by read(2)s
88 from a signalfd file descriptor is as follows:
89 struct signalfd_siginfo {
91 uint32_t ssi_signo; /* Signal number */
92 int32_t ssi_errno; /* Error number (unused) */
93 int32_t ssi_code; /* Signal code */
94 uint32_t ssi_pid; /* PID of sender */
95 uint32_t ssi_uid; /* Real UID of sender */
96 int32_t ssi_fd; /* File descriptor (SIGIO) */
97 uint32_t ssi_tid; /* Kernel timer ID (POSIX timers)
98 uint32_t ssi_band; /* Band event (SIGIO) */
99 uint32_t ssi_overrun; /* POSIX timer overrun count */
100 uint32_t ssi_trapno; /* Trap number that caused signal */
101 int32_t ssi_status; /* Exit status or signal (SIGCHLD) */
102 int32_t ssi_int; /* Integer sent by sigqueue(2) */
103 uint64_t ssi_ptr; /* Pointer sent by sigqueue(2) */
104 uint64_t ssi_utime; /* User CPU time consumed (SIGCHLD) */
105 uint64_t ssi_stime; /* System CPU time consumed (SIGCHLD) */
106 uint64_t ssi_addr; /* Address that generated signal
107 (for hardware-generated signals) */
108 uint8_t pad[X]; /* Pad size to 128 bytes (allow for
109 additional fields in the future) */
110 };
111 Each of the fields in this structure is analogous to the similarly
113 named field in the siginfo_t structure. The siginfo_t structure is
114 described in sigaction(2). Not all fields in the returned sig‐
115 nalfd_siginfo structure will be valid for a specific signal; the set of
116 valid fields can be determined from the value returned in the ssi_code
117 field. This field is the analog of the siginfo_t si_code field; see
118 sigaction(2) for details.
119 fork(2) semantics
121 After a fork(2), the child inherits a copy of the signalfd file
122 descriptor. A read(2) from the file descriptor in the child will
123 return information about signals queued to the child.
124 execve(2) semantics
126 Just like any other file descriptor, a signalfd file descriptor remains
127 open across an execve(2), unless it has been marked for close-on-exec
128 (see fcntl(2)). Any signals that were available for reading before the
129 execve(2) remain available to the newly loaded program. (This is anal‐
130 ogous to traditional signal semantics, where a blocked signal that is
131 pending remains pending across an execve(2).)
132 Thread semantics
134 The semantics of signalfd file descriptors in a multithreaded program
135 mirror the standard semantics for signals. In other words, when a
136 thread reads from a signalfd file descriptor, it will read the signals
137 that are directed to the thread itself and the signals that are
138 directed to the process (i.e., the entire thread group). (A thread
139 will not be able to read signals that are directed to other threads in
140 the process.)
141
143 On success, signalfd() returns a signalfd file descriptor; this is
144 either a new file descriptor (if fd was -1), or fd if fd was a valid
145 signalfd file descriptor. On error, -1 is returned and errno is set to
146 indicate the error.
147
149 EBADF The fd file descriptor is not a valid file descriptor.
150 EINVAL fd is not a valid signalfd file descriptor.
152 EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is
154 nonzero.
155 EMFILE The per-process limit of open file descriptors has been reached.
157 ENFILE The system-wide limit on the total number of open files has been
159 reached.
160 ENODEV Could not mount (internal) anonymous inode device.
162 ENOMEM There was insufficient memory to create a new signalfd file
164 descriptor.
165
167 signalfd() is available on Linux since kernel 2.6.22. Working support
168 is provided in glibc since version 2.8. The signalfd4() system call
169 (see NOTES) is available on Linux since kernel 2.6.27.
170
172 signalfd() and signalfd4() are Linux-specific.
173
175 The underlying Linux system call requires an additional argument,
176 size_t sizemask, which specifies the size of the mask argument. The
177 glibc signalfd() wrapper function does not include this argument, since
178 it provides the required value for the underlying system call.
179 A process can create multiple signalfd file descriptors. This makes it
181 possible to accept different signals on different file descriptors.
182 (This may be useful if monitoring the file descriptors using select(2),
183 poll(2), or epoll(7): the arrival of different signals will make dif‐
184 ferent descriptors ready.) If a signal appears in the mask of more
185 than one of the file descriptors, then occurrences of that signal can
186 be read (once) from any one of the descriptors.
187 Underlying Linux system calls
189 There are two underlying Linux system calls: signalfd() and the more
190 recent signalfd4(). The former system call does not implement a flags
191 argument. The latter system call implements the flags values described
192 above. Starting with glibc 2.9, the signalfd() wrapper function will
193 use signalfd4() where it is available.
194
196 In kernels before 2.6.25, the ssi_ptr and ssi_int fields are not filled
197 in with the data accompanying a signal sent by sigqueue(2).
198
200 The program below accepts the signals SIGINT and SIGQUIT via a signalfd
201 file descriptor. The program terminates after accepting a SIGQUIT sig‐
202 nal. The following shell session demonstrates the use of the program:
203 $ ./signalfd_demo
205 ^C # Control-C generates SIGINT
206 Got SIGINT
207 ^C
208 Got SIGINT
209 ^\ # Control-\ generates SIGQUIT
210 Got SIGQUIT
211 $
212 Program source
214 #include <sys/signalfd.h>
216 #include <signal.h>
217 #include <unistd.h>
218 #include <stdlib.h>
219 #include <stdio.h>
220 #define handle_error(msg) \
222 do { perror(msg); exit(EXIT_FAILURE); } while (0)
223 int
225 main(int argc, char *argv[])
226 {
227 sigset_t mask;
228 int sfd;
229 struct signalfd_siginfo fdsi;
230 ssize_t s;
231 sigemptyset(&mask);
233 sigaddset(&mask, SIGINT);
234 sigaddset(&mask, SIGQUIT);
235 /* Block signals so that they aren't handled
237 according to their default dispositions */
238 if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
240 handle_error("sigprocmask");
241 sfd = signalfd(-1, &mask, 0);
243 if (sfd == -1)
244 handle_error("signalfd");
245 for (;;) {
247 s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
248 if (s != sizeof(struct signalfd_siginfo))
249 handle_error("read");
250 if (fdsi.ssi_signo == SIGINT) {
252 printf("Got SIGINT\n");
253 } else if (fdsi.ssi_signo == SIGQUIT) {
254 printf("Got SIGQUIT\n");
255 exit(EXIT_SUCCESS);
256 } else {
257 printf("Read unexpected signal\n");
258 }
259 }
260 }
261
263 eventfd(2), poll(2), read(2), select(2), sigaction(2), sigprocmask(2),
264 sigwaitinfo(2), timerfd_create(2), sigsetops(3), sigwait(3), epoll(7),
265 signal(7)
266
268 This page is part of release 3.25 of the Linux man-pages project. A
269 description of the project, and information about reporting bugs, can
270 be found at http://www.kernel.org/doc/man-pages/.
271Linux 2009-01-13 SIGNALFD(2)