SIGNALFD
Section: Linux Programmer's Manual (2)
Updated: 2009-01-13
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 3.27 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- The signalfd_siginfo structure
-
- fork(2) semantics
-
- execve(2) semantics
-
- Thread semantics
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- Underlying Linux system calls
-
- BUGS
-
- EXAMPLE
-
- Program source
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:34:57 GMT, March 26, 2013