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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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signalfd(2) System Calls Manual signalfd(2)
signalfd - create a file descriptor for accepting signals
Standard C library (libc, -lc)
#include <sys/signalfd.h>
int signalfd(int fd, const sigset_t *mask, int flags);
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 file 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 file descriptor.
Starting with Linux 2.6.27, the following values may be bitwise
ORed in flags to change the behavior of signalfd():
SFD_NONBLOCK
Set the O_NONBLOCK file status flag on the open file
description (see open(2)) referred to by the new file
descriptor. 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.
Up to Linux 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(3) */
uint64_t ssi_ptr; /* Pointer sent by sigqueue(3) */
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) */
uint16_t ssi_addr_lsb; /* Least significant bit of address
(SIGBUS; since Linux 2.6.37) */
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.
Semantics of file descriptor passing
As with other file descriptors, signalfd file descriptors can be
passed to another process via a UNIX domain socket (see unix(7)).
In the receiving process, a read(2) from the received file
descriptor will return information about signals queued to that
process.
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.)
epoll(7) semantics
If a process adds (via epoll_ctl(2)) a signalfd file descriptor
to an epoll(7) instance, then epoll_wait(2) returns events only
for signals sent to that process. In particular, if the process
then uses fork(2) to create a child process, then the child will
be able to read(2) signals that are sent to it using the signalfd
file descriptor, but epoll_wait(2) will not indicate that the
signalfd file descriptor is ready. In this scenario, a possible
workaround is that after the fork(2), the child process can close
the signalfd file descriptor that it inherited from the parent
process and then create another signalfd file descriptor and add
it to the epoll instance. Alternatively, the parent and the
child could delay creating their (separate) signalfd file
descriptors and adding them to the epoll instance until after the
call to fork(2).
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.
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 on the number 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.
C library/kernel differences
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.
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.
Linux.
signalfd()
Linux 2.6.22, glibc 2.8.
signalfd4()
Linux 2.6.27.
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 file 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 file descriptors.
Attempts to include SIGKILL and SIGSTOP in mask are silently
ignored.
The signal mask employed by a signalfd file descriptor can be
viewed via the entry for the corresponding file descriptor in the
process's /proc/pid/fdinfo directory. See proc(5) for further
details.
Limitations
The signalfd mechanism can't be used to receive signals that are
synchronously generated, such as the SIGSEGV signal that results
from accessing an invalid memory address or the SIGFPE signal
that results from an arithmetic error. Such signals can be
caught only via signal handler.
As described above, in normal usage one blocks the signals that
will be accepted via signalfd(). If spawning a child process to
execute a helper program (that does not need the signalfd file
descriptor), then, after the call to fork(2), you will normally
want to unblock those signals before calling execve(2), so that
the helper program can see any signals that it expects to see.
Be aware, however, that this won't be possible in the case of a
helper program spawned behind the scenes by any library function
that the program may call. In such cases, one must fall back to
using a traditional signal handler that writes to a file
descriptor monitored by select(2), poll(2), or epoll(7).
Before Linux 2.6.25, the ssi_ptr and ssi_int fields are not
filled in with the data accompanying a signal sent by
sigqueue(3).
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 <err.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/signalfd.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void)
{
int sfd;
ssize_t s;
sigset_t mask;
struct signalfd_siginfo fdsi;
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)
err(EXIT_FAILURE, "sigprocmask");
sfd = signalfd(-1, &mask, 0);
if (sfd == -1)
err(EXIT_FAILURE, "signalfd");
for (;;) {
s = read(sfd, &fdsi, sizeof(fdsi));
if (s != sizeof(fdsi))
err(EXIT_FAILURE, "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");
}
}
}
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
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man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 signalfd(2)
Pages that refer to this page: eventfd(2), sigaction(2), signal(2), sigwaitinfo(2), syscalls(2), timerfd_create(2), timer_getoverrun(2), sd-event(3), sd_event_add_signal(3), sigwait(3), proc_pid_fd(5), proc_pid_fdinfo(5), signal(7), system_data_types(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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