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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
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SIGNALFD(2) Linux Programmer's Manual SIGNALFD(2)
signalfd - create a file descriptor for accepting signals
#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 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(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 sig‐
nalfd_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 sig‐
nals 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.)
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.
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.
signalfd() and signalfd4() are Linux-specific.
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),
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.
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(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 <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");
}
}
}
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 release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 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(5), signal(7)
Copyright and license for this manual page
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