signalfd(2) - Linux manual page (original) (raw)
signalfd(2) System Calls Manual signalfd(2)
NAME top
signalfd - create a file descriptor for accepting signalsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/signalfd.h>**
**int signalfd(int** _fd_**, const sigset_t ***_mask_**, int** _flags_**);**DESCRIPTION top
**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)](../man2/sigwaitinfo.2.html), and has the
advantage that the file descriptor may be monitored by [select(2)](../man2/select.2.html),
[poll(2)](../man2/poll.2.html), and [epoll(7)](../man7/epoll.7.html).
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)](../man3/sigsetops.3.html). Normally, the set of signals to be
received via the file descriptor should be blocked using
[sigprocmask(2)](../man2/sigprocmask.2.html), 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)](../man2/open.2.html)) referred to by the new file
descriptor. Using this flag saves extra calls to [fcntl(2)](../man2/fcntl.2.html)
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)](../man2/open.2.html) 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)](../man2/read.2.html)
If one or more of the signals specified in _mask_ is pending
for the process, then the buffer supplied to [read(2)](../man2/read.2.html) is
used to return one or more _signalfdsiginfo_ structures (see
below) that describe the signals. The [read(2)](../man2/read.2.html) returns
information for as many signals as are pending and will fit
in the supplied buffer. The buffer must be at least
_sizeof(struct signalfdsiginfo)_ bytes. The return value of
the [read(2)](../man2/read.2.html) is the total number of bytes read.
As a consequence of the [read(2)](../man2/read.2.html), 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)](../man2/sigwaitinfo.2.html)).
If none of the signals in _mask_ is pending for the process,
then the [read(2)](../man2/read.2.html) 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)](../man2/poll.2.html)
[select(2)](../man2/select.2.html)
(and similar)
The file descriptor is readable (the [select(2)](../man2/select.2.html) _readfds_
argument; the [poll(2)](../man2/poll.2.html) **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)](../man2/pselect.2.html), [ppoll(2)](../man2/ppoll.2.html), and
[epoll(7)](../man7/epoll.7.html).
[close(2)](../man2/close.2.html)
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 signalfdsiginfo 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 _siginfot_ structure. The _siginfot_ structure
is described in [sigaction(2)](../man2/sigaction.2.html). Not all fields in the returned
_signalfdsiginfo_ structure will be valid for a specific signal;
the set of valid fields can be determined from the value returned
in the _ssicode_ field. This field is the analog of the _siginfot_
_sicode_ field; see [sigaction(2)](../man2/sigaction.2.html) 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).
RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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.VERSIONS top
C library/kernel differences The underlying Linux system call requires an additional argument, sizet 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.STANDARDS top
Linux.HISTORY top
**signalfd**()
Linux 2.6.22, glibc 2.8.
**signalfd4**()
Linux 2.6.27.NOTES top
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)](../man2/select.2.html), [poll(2)](../man2/poll.2.html), or [epoll(7)](../man7/epoll.7.html): 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)](../man5/proc.5.html) 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)](../man2/fork.2.html), you will normally
want to unblock those signals before calling [execve(2)](../man2/execve.2.html), 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)](../man2/select.2.html), [poll(2)](../man2/poll.2.html), or [epoll(7)](../man7/epoll.7.html).BUGS top
Before Linux 2.6.25, the _ssiptr_ and _ssiint_ fields are not filled
in with the data accompanying a signal sent by [sigqueue(3)](../man3/sigqueue.3.html).EXAMPLES top
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");
}
}
}SEE ALSO top
[eventfd(2)](../man2/eventfd.2.html), [poll(2)](../man2/poll.2.html), [read(2)](../man2/read.2.html), [select(2)](../man2/select.2.html), [sigaction(2)](../man2/sigaction.2.html),
[sigprocmask(2)](../man2/sigprocmask.2.html), [sigwaitinfo(2)](../man2/sigwaitinfo.2.html), [timerfd_create(2)](../man2/timerfd%5Fcreate.2.html), [sigsetops(3)](../man3/sigsetops.3.html),
[sigwait(3)](../man3/sigwait.3.html), [epoll(7)](../man7/epoll.7.html), [signal(7)](../man7/signal.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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)