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13 .Dd "May 05, 2016"
14 .Dt SIGNALFD 3C
15 .Os
16 .Sh NAME
17 .Nm signalfd
18 .Nd create or modify a file descriptor for signal handling
19 .Sh SYNOPSIS
20 .In sys/signalfd.h
21 .
22 .Ft int
23 .Fo signalfd
24 .Fa "int fd"
25 .Fa "const sigset_t *mask"
26 .Fa "int flags"
27 .Fc
28 .
29 .Sh DESCRIPTION
30 The
31 .Fn signalfd
32 function returns a file descriptor that can be used
33 for synchronous consumption of signals. The file descriptor can be operated
34 upon via
35 .Xr read 2
36 and the facilities that notify of file descriptor activity (e.g.
37 .Xr poll 2 ,
38 .Xr port_get 3C ,
39 .Xr epoll_wait 3C
40 ). To dispose of the instance
41 .Xr close 2
42 should be called on the file descriptor.
43 .Pp
44 If the
45 .Va fd
46 argument is -1, a new signalfd file descriptor will be
47 returned, otherwise the
48 .Va fd
49 argument should be an existing signalfd file descriptor whose signal mask will
50 be updated.
51 .Pp
52 The
53 .Va mask
54 argument specifies the set of signals that are relevant to the file descriptor.
55 It may be manipulated with the standard signal set manipulation functions
56 documented in
57 .Xr sigsetops 3C .
58 Signals in the mask which cannot be caught (e.g.
59 .Fa SIGKILL )
60 are ignored.
61 .Pp
62 The
63 .Va flags
64 argument specifies additional parameters for the instance, and can have any of
65 the following values:
66 .Bl -tag -width Dv
67 .It Sy SFD_CLOEXEC
68 Instance will be closed upon an
69 .Xr exec 2 ;
70 see description for
71 .Fa O_CLOEXEC
72 in
73 .Xr open 2 .
74 .It Sy SFD_NONBLOCK
75 Instance will be set to be non-blocking. A
76 .Xr read 2
77 on a signalfd instance that has been initialized with
78 .Fa SFD_NONBLOCK ,
79 or made non-blocking in other ways, will return
80 .Er EAGAIN
81 in lieu of blocking if there are no signals from the
82 .Va mask
83 that are pending.
84 .El
85 .Pp
86 As with
87 .Xr sigwait 2 ,
88 reading a signal from the file descriptor will consume the signal. The signals
89 used with signalfd file descriptors are normally first blocked so that their
90 handler does not run when a signal arrives. If the signal is not blocked the
91 behavior matches that of
92 .Xr sigwait 2 ;
93 if a
94 .Xr read 2
95 is pending then the signal is consumed by the read, otherwise the signal is
96 consumed by the handler.
97 .Pp
98 The following operations can be performed upon a signalfd file descriptor:
99 .Bl -tag -width Dv
100 .It Sy read(2)
101 Reads and consumes one or more of the pending signals that match the file
102 descriptor's
103 .Va mask .
104 The read buffer must be large enough to hold one or more
105 .Vt signalfd_siginfo
106 structures, which is described below.
107 .Xr read 2
108 will block if there are no matching signals pending, or return
109 .Er EAGAIN
110 if the instance was created with
111 .Fa SFD_NONBLOCK .
112 After a
113 .Xr fork 2 ,
114 if the child reads from the descriptor it will only consume signals from itself.
115 .It Sy poll(2)
116 Provide notification when one of the signals from the
117 .Va mask
118 arrives.
119 .Fa POLLIN
120 and
121 .Fa POLLRDNORM
122 will be set.
123 .It Sy close(2)
124 Closes the desriptor.
125 .El
126 .Pp
127 The
128 .Vt signalfd_siginfo
129 structure returned from
130 .Xr read 2
131 is a fixed size 128 byte structure defined as follows:
132 .Bd -literal
133 typedef struct signalfd_siginfo {
134 uint32_t ssi_signo; /* signal from signal.h */
135 int32_t ssi_errno; /* error from errno.h */
136 int32_t ssi_code; /* signal code */
137 uint32_t ssi_pid; /* PID of sender */
138 uint32_t ssi_uid; /* real UID of sender */
139 int32_t ssi_fd; /* file descriptor (SIGIO) */
140 uint32_t ssi_tid; /* unused */
141 uint32_t ssi_band; /* band event (SIGIO) */
142 uint32_t ssi_overrun; /* unused */
143 uint32_t ssi_trapno; /* trap number that caused signal */
144 int32_t ssi_status; /* exit status or signal (SIGCHLD) */
145 int32_t ssi_int; /* unused */
146 uint64_t ssi_ptr; /* unused */
147 uint64_t ssi_utime; /* user CPU time consumed (SIGCHLD) */
148 uint64_t ssi_stime; /* system CPU time consumed (SIGCHLD) */
149 uint64_t ssi_addr; /* address that generated signal */
150 uint8_t ssi_pad[48]; /* pad size to 128 bytes */
151 } signalfd_siginfo_t;
152 .Ed
153 .Sh NOTES
154 File descriptor duplication during fork presents a challenge to the
155 .Sy signalfd
156 facility since signal data and events are dependent on the process from which
157 they are queried. Its use with caching event systems such as
158 .Xr epoll 5 ,
159 .Sy /dev/poll ,
160 or
161 .Xr port_create 3C
162 can result in undefined behavior if signalfd and polling descriptors are used
163 together after being shared across a fork. Such restrictions do not apply if
164 the child only calls
165 .Xr close 2
166 on the descriptors.
167 .Sh RETURN VALUES
168 Upon successful completion, a file descriptor associated with the instance
169 is returned. Otherwise, -1 is returned and errno is set to indicate the error.
170 When
171 .Va fd
172 is not -1 and there is no error, the value of
173 .Va fd
174 is returned.
175 .Sh ERRORS
176 The
177 .Fn signalfd function
178 will fail if:
179 .Bl -tag -width Er
180 .It Er EBADF
181 The
182 .Va fd
183 descriptor is invalid.
184 .It Er EFAULT
185 The
186 .Va mask
187 address is invalid.
188 .It Er EINVAL
189 The
190 .Va fd
191 descriptor is not a signalfd descriptor or the
192 .Va flags
193 are invalid.
194 .It Er EMFILE
195 There are currently
196 .Va OPEN_MAX
197 file descriptors open in the calling process.
198 .It Er ENODEV
199 Unable to allocate state for the file descriptor.
200 .El
201 .Sh SEE ALSO
202 .Xr poll 2 ,
203 .Xr sigwait 2 ,
204 .Xr port_create 3C ,
205 .Xr sigsetops 3C ,
206 .Xr sigwaitinfo 3C ,
207 .Xr signal.h 3HEAD ,
208 .Xr epoll 5