.\" Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\" starting from a version by Davide Libenzi <davidel@xmailserver.org>
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH signalfd 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
signalfd \- create a file descriptor for accepting signals
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/signalfd.h>
.P
.BI "int signalfd(int " fd ", const sigset_t *" mask ", int " flags );
.fi
.SH DESCRIPTION
.BR 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
.BR sigwaitinfo (2),
and has the advantage that the file descriptor may be monitored by
.BR select (2),
.BR poll (2),
and
.BR epoll (7).
.P
The
.I 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
.BR sigsetops (3).
Normally, the set of signals to be received via the
file descriptor should be blocked using
.BR sigprocmask (2),
to prevent the signals being handled according to their default
dispositions.
It is not possible to receive
.B SIGKILL
or
.B SIGSTOP
signals via a signalfd file descriptor;
these signals are silently ignored if specified in
.IR mask .
.P
If the
.I fd
argument is \-1,
then the call creates a new file descriptor and associates the
signal set specified in
.I mask
with that file descriptor.
If
.I fd
is not \-1,
then it must specify a valid existing signalfd file descriptor, and
.I mask
is used to replace the signal set associated with that file descriptor.
.P
Starting with Linux 2.6.27, the following values may be bitwise ORed in
.I flags
to change the behavior of
.BR signalfd ():
.TP 14
.B SFD_NONBLOCK
Set the
.B O_NONBLOCK
file status flag on the open file description (see
.BR open (2))
referred to by the new file descriptor.
Using this flag saves extra calls to
.BR fcntl (2)
to achieve the same result.
.TP
.B SFD_CLOEXEC
Set the close-on-exec
.RB ( FD_CLOEXEC )
flag on the new file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.P
Up to Linux 2.6.26, the
.I flags
argument is unused, and must be specified as zero.
.P
.BR signalfd ()
returns a file descriptor that supports the following operations:
.TP
.BR read (2)
If one or more of the signals specified in
.I mask
is pending for the process, then the buffer supplied to
.BR read (2)
is used to return one or more
.I signalfd_siginfo
structures (see below) that describe the signals.
The
.BR read (2)
returns information for as many signals as are pending and will
fit in the supplied buffer.
The buffer must be at least
.I "sizeof(struct signalfd_siginfo)"
bytes.
The return value of the
.BR read (2)
is the total number of bytes read.
.IP
As a consequence of the
.BR 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
.BR sigwaitinfo (2)).
.IP
If none of the signals in
.I mask
is pending for the process, then the
.BR read (2)
either blocks until one of the signals in
.I mask
is generated for the process,
or fails with the error
.B EAGAIN
if the file descriptor has been made nonblocking.
.TP
.BR poll (2)
.TQ
.BR select (2)
.TQ
(and similar)
The file descriptor is readable
(the
.BR select (2)
.I readfds
argument; the
.BR poll (2)
.B POLLIN
flag)
if one or more of the signals in
.I mask
is pending for the process.
.IP
The signalfd file descriptor also supports the other file-descriptor
multiplexing APIs:
.BR pselect (2),
.BR ppoll (2),
and
.BR epoll (7).
.TP
.BR 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.
.SS The signalfd_siginfo structure
The format of the
.I signalfd_siginfo
structure(s) returned by
.BR read (2)s
from a signalfd file descriptor is as follows:
.P
.in +4n
.EX
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 */
.\" ssi_trapno is unused on most arches
    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) */
.\" ssi_addr_lsb: commit b8aeec34175fc8fe8b0d40efea4846dfc1ba663e
    uint8_t  pad[\fIX\fP];       /* Pad size to 128 bytes (allow for
                              additional fields in the future) */
};
.EE
.in
.P
Each of the fields in this structure
is analogous to the similarly named field in the
.I siginfo_t
structure.
The
.I siginfo_t
structure is described in
.BR sigaction (2).
Not all fields in the returned
.I signalfd_siginfo
structure will be valid for a specific signal;
the set of valid fields can be determined from the value returned in the
.I ssi_code
field.
This field is the analog of the
.I siginfo_t
.I si_code
field; see
.BR sigaction (2)
for details.
.SS fork(2) semantics
After a
.BR fork (2),
the child inherits a copy of the signalfd file descriptor.
A
.BR read (2)
from the file descriptor in the child will return information
about signals queued to the child.
.SS 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
.BR unix (7)).
In the receiving process, a
.BR read (2)
from the received file descriptor will return information
about signals queued to that process.
.SS execve(2) semantics
Just like any other file descriptor,
a signalfd file descriptor remains open across an
.BR execve (2),
unless it has been marked for close-on-exec (see
.BR fcntl (2)).
Any signals that were available for reading before the
.BR 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
.BR execve (2).)
.SS 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.)
.\"
.SS epoll(7) semantics
If a process adds (via
.BR epoll_ctl (2))
a signalfd file descriptor to an
.BR epoll (7)
instance, then
.BR epoll_wait (2)
returns events only for signals sent to that process.
In particular, if the process then uses
.BR fork (2)
to create a child process, then the child will be able to
.BR read (2)
signals that are sent to it using the signalfd file descriptor, but
.BR epoll_wait (2)
will
.B not
indicate that the signalfd file descriptor is ready.
In this scenario, a possible workaround is that after the
.BR 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
.BR fork (2).
.SH RETURN VALUE
On success,
.BR signalfd ()
returns a signalfd file descriptor;
this is either a new file descriptor (if
.I fd
was \-1), or
.I fd
if
.I fd
was a valid signalfd file descriptor.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
The
.I fd
file descriptor is not a valid file descriptor.
.TP
.B EINVAL
.I fd
is not a valid signalfd file descriptor.
.\" or, the
.\" .I sizemask
.\" argument is not equal to
.\" .IR sizeof(sigset_t) ;
.TP
.B EINVAL
.I flags
is invalid;
or, in Linux 2.6.26 or earlier,
.I flags
is nonzero.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been
reached.
.TP
.B ENODEV
Could not mount (internal) anonymous inode device.
.TP
.B ENOMEM
There was insufficient memory to create a new signalfd file descriptor.
.SH VERSIONS
.SS C library/kernel differences
The underlying Linux system call requires an additional argument,
.IR "size_t sizemask" ,
which specifies the size of the
.I mask
argument.
The glibc
.BR signalfd ()
wrapper function does not include this argument,
since it provides the required value for the underlying system call.
.P
There are two underlying Linux system calls:
.BR signalfd ()
and the more recent
.BR signalfd4 ().
The former system call does not implement a
.I flags
argument.
The latter system call implements the
.I flags
values described above.
Starting with glibc 2.9, the
.BR signalfd ()
wrapper function will use
.BR signalfd4 ()
where it is available.
.SH STANDARDS
Linux.
.SH HISTORY
.TP
.BR signalfd ()
Linux 2.6.22,
glibc 2.8.
.\" signalfd() is in glibc 2.7, but reportedly does not build
.TP
.BR signalfd4 ()
Linux 2.6.27.
.SH NOTES
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
.BR select (2),
.BR poll (2),
or
.BR epoll (7):
the arrival of different signals will make different file descriptors ready.)
If a signal appears in the
.I 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.
.P
Attempts to include
.B SIGKILL
and
.B SIGSTOP
in
.I mask
are silently ignored.
.P
The signal mask employed by a signalfd file descriptor can be viewed
via the entry for the corresponding file descriptor in the process's
.IR /proc/ pid /fdinfo
directory.
See
.BR proc (5)
for further details.
.\"
.SS Limitations
The signalfd mechanism can't be used to receive signals that
are synchronously generated, such as the
.B SIGSEGV
signal that results from accessing an invalid memory address
or the
.B SIGFPE
signal that results from an arithmetic error.
Such signals can be caught only via signal handler.
.P
As described above,
in normal usage one blocks the signals that will be accepted via
.BR signalfd ().
If spawning a child process to execute a helper program
(that does not need the signalfd file descriptor),
then, after the call to
.BR fork (2),
you will normally want to unblock those signals before calling
.BR 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
.BR select (2),
.BR poll (2),
or
.BR epoll (7).
.SH BUGS
Before Linux 2.6.25, the
.I ssi_ptr
and
.I ssi_int
fields are not filled in with the data accompanying a signal sent by
.BR sigqueue (3).
.\" The fix also was put into Linux 2.6.24.5
.SH EXAMPLES
The program below accepts the signals
.B SIGINT
and
.B SIGQUIT
via a signalfd file descriptor.
The program terminates after accepting a
.B SIGQUIT
signal.
The following shell session demonstrates the use of the program:
.P
.in +4n
.EX
.RB "$" " ./signalfd_demo"
.BR "\[ha]C" "                   # Control\-C generates SIGINT"
Got SIGINT
.B \[ha]C
Got SIGINT
\fB\[ha]\e\fP                    # Control\-\e generates SIGQUIT
Got SIGQUIT
$
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (signalfd.c)
.EX
#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\[aq]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\en");
        } else if (fdsi.ssi_signo == SIGQUIT) {
            printf("Got SIGQUIT\en");
            exit(EXIT_SUCCESS);
        } else {
            printf("Read unexpected signal\en");
        }
    }
}
.EE
.\" SRC END
.SH SEE ALSO
.BR eventfd (2),
.BR poll (2),
.BR read (2),
.BR select (2),
.BR sigaction (2),
.BR sigprocmask (2),
.BR sigwaitinfo (2),
.BR timerfd_create (2),
.BR sigsetops (3),
.BR sigwait (3),
.BR epoll (7),
.BR signal (7)