NAME
signalfd - create a file descriptor for accepting signals
SYNOPSIS
#include <sys/signalfd.h>
int signalfd(int fd, const sigset_t *mask, int flags);
DESCRIPTION
signalfd() creates a file descriptor that can be used to accept signals
targeted at the caller. This provides an alternative to the use of a
signal handler or sigwaitinfo(2), and has the advantage that the file
descriptor may be monitored by select(2), poll(2), and epoll(7).
The mask argument specifies the set of signals that the caller wishes
to accept via the file descriptor. This argument is a signal set whose
contents can be initialized using the macros described in sigsetops(3).
Normally, the set of signals to be received via the file descriptor
should be blocked using sigprocmask(2), to prevent the signals being
handled according to their default dispositions. It is not possible to
receive SIGKILL or SIGSTOP signals via a signalfd file descriptor;
these signals are silently ignored if specified in mask.
If the fd argument is -1, then the call creates a new file descriptor
and associates the signal set specified in mask with that descriptor.
If fd is not -1, then it must specify a valid existing signalfd file
descriptor, and mask is used to replace the signal set associated with
that descriptor.
Starting with Linux 2.6.27, the following values may be bitwise ORed in
flags to change the behaviour of signalfd():
SFD_NONBLOCK Set the O_NONBLOCK file status flag on the new open file
description. Using this flag saves extra calls to
fcntl(2) to achieve the same result.
SFD_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag in
open(2) for reasons why this may be useful.
In Linux up to version 2.6.26, the flags argument is unused, and must
be specified as zero.
signalfd() returns a file descriptor that supports the following
operations:
read(2)
If one or more of the signals specified in mask is pending for
the process, then the buffer supplied to read(2) is used to
return one or more signalfd_siginfo structures (see below) that
describe the signals. The read(2) returns information for as
many signals as are pending and will fit in the supplied buffer.
The buffer must be at least sizeof(struct signalfd_siginfo)
bytes. The return value of the read(2) is the total number of
bytes read.
As a consequence of the read(2), the signals are consumed, so
that they are no longer pending for the process (i.e., will not
be caught by signal handlers, and cannot be accepted using
sigwaitinfo(2)).
If none of the signals in mask is pending for the process, then
the read(2) either blocks until one of the signals in mask is
generated for the process, or fails with the error EAGAIN if the
file descriptor has been made nonblocking.
poll(2), select(2) (and similar)
The file descriptor is readable (the select(2) readfds argument;
the poll(2) POLLIN flag) if one or more of the signals in mask
is pending for the process.
The signalfd file descriptor also supports the other file-
descriptor multiplexing APIs: pselect(2), ppoll(2), and
epoll(7).
close(2)
When the file descriptor is no longer required it should be
closed. When all file descriptors associated with the same
signalfd object have been closed, the resources for object are
freed by the kernel.
The signalfd_siginfo structure
The format of the signalfd_siginfo structure(s) returned by read(2)s
from a signalfd file descriptor is as follows:
struct signalfd_siginfo {
uint32_t ssi_signo; /* Signal number */
int32_t ssi_errno; /* Error number (unused) */
int32_t ssi_code; /* Signal code */
uint32_t ssi_pid; /* PID of sender */
uint32_t ssi_uid; /* Real UID of sender */
int32_t ssi_fd; /* File descriptor (SIGIO) */
uint32_t ssi_tid; /* Kernel timer ID (POSIX timers)
uint32_t ssi_band; /* Band event (SIGIO) */
uint32_t ssi_overrun; /* POSIX timer overrun count */
uint32_t ssi_trapno; /* Trap number that caused signal */
int32_t ssi_status; /* Exit status or signal (SIGCHLD) */
int32_t ssi_int; /* Integer sent by sigqueue(2) */
uint64_t ssi_ptr; /* Pointer sent by sigqueue(2) */
uint64_t ssi_utime; /* User CPU time consumed (SIGCHLD) */
uint64_t ssi_stime; /* System CPU time consumed (SIGCHLD) */
uint64_t ssi_addr; /* Address that generated signal
(for hardware-generated signals) */
uint8_t pad[X]; /* Pad size to 128 bytes (allow for
additional fields in the future) */
};
Each of the fields in this structure is analogous to the similarly
named field in the siginfo_t structure. The siginfo_t structure is
described in sigaction(2). Not all fields in the returned
signalfd_siginfo structure will be valid for a specific signal; the set
of valid fields can be determined from the value returned in the
ssi_code field. This field is the analog of the siginfo_t si_code
field; see sigaction(2) for details.
fork(2) semantics
After a fork(2), the child inherits a copy of the signalfd file
descriptor. A read(2) from the file descriptor in the child will
return information about signals queued to the child.
execve(2) semantics
Just like any other file descriptor, a signalfd file descriptor remains
open across an execve(2), unless it has been marked for close-on-exec
(see fcntl(2)). Any signals that were available for reading before the
execve(2) remain available to the newly loaded program. (This is
analogous to traditional signal semantics, where a blocked signal that
is pending remains pending across an execve(2).)
Thread semantics
The semantics of signalfd file descriptors in a multithreaded program
mirror the standard semantics for signals. In other words, when a
thread reads from a signalfd file descriptor, it will read the signals
that are directed to the thread itself and the signals that are
directed to the process (i.e., the entire thread group). (A thread
will not be able to read signals that are directed to other threads in
the process.)
RETURN VALUE
On success, signalfd() returns a signalfd file descriptor; this is
either a new file descriptor (if fd was -1), or fd if fd was a valid
signalfd file descriptor. On error, -1 is returned and errno is set to
indicate the error.
ERRORS
EBADF The fd file descriptor is not a valid file descriptor.
EINVAL fd is not a valid signalfd file descriptor.
EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is
nonzero.
EMFILE The per-process limit of open file descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has been
reached.
ENODEV Could not mount (internal) anonymous inode device.
ENOMEM There was insufficient memory to create a new signalfd file
descriptor.
VERSIONS
signalfd() is available on Linux since kernel 2.6.22. Working support
is provided in glibc since version 2.8. The signalfd4() system call
(see NOTES) is available on Linux since kernel 2.6.27.
CONFORMING TO
signalfd() and signalfd4() are Linux-specific.
NOTES
The underlying Linux system call requires an additional argument,
size_t sizemask, which specifies the size of the mask argument. The
glibc signalfd() wrapper function does not include this argument, since
it provides the required value for the underlying system call.
A process can create multiple signalfd file descriptors. This makes it
possible to accept different signals on different file descriptors.
(This may be useful if monitoring the file descriptors using select(2),
poll(2), or epoll(7): the arrival of different signals will make
different descriptors ready.) If a signal appears in the mask of more
than one of the file descriptors, then occurrences of that signal can
be read (once) from any one of the descriptors.
Underlying Linux system calls
There are two underlying Linux system calls: signalfd() and the more
recent signalfd4(). The former system call does not implement a flags
argument. The latter system call implements the flags values described
above. Starting with glibc 2.9, the signalfd() wrapper function will
use signalfd4() where it is available.
BUGS
In kernels before 2.6.25, the ssi_ptr and ssi_int fields are not filled
in with the data accompanying a signal sent by sigqueue(2).
EXAMPLE
The program below accepts the signals SIGINT and SIGQUIT via a signalfd
file descriptor. The program terminates after accepting a SIGQUIT
signal. The following shell session demonstrates the use of the
program:
$ ./signalfd_demo
^C # Control-C generates SIGINT
Got SIGINT
^C
Got SIGINT
^\ # Control-\ generates SIGQUIT
Got SIGQUIT
$
Program source
#include <sys/signalfd.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
sigset_t mask;
int sfd;
struct signalfd_siginfo fdsi;
ssize_t s;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
sigaddset(&mask, SIGQUIT);
/* Block signals so that they aren't handled
according to their default dispositions */
if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
handle_error("sigprocmask");
sfd = signalfd(-1, &mask, 0);
if (sfd == -1)
handle_error("signalfd");
for (;;) {
s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
if (s != sizeof(struct signalfd_siginfo))
handle_error("read");
if (fdsi.ssi_signo == SIGINT) {
printf("Got SIGINT\n");
} else if (fdsi.ssi_signo == SIGQUIT) {
printf("Got SIGQUIT\n");
exit(EXIT_SUCCESS);
} else {
printf("Read unexpected signal\n");
}
}
}
SEE ALSO
eventfd(2), poll(2), read(2), select(2), sigaction(2), sigprocmask(2),
sigwaitinfo(2), timerfd_create(2), sigsetops(3), sigwait(3), epoll(7),
signal(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.