.\" Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>

.\" starting from a version by Davide Libenzi <davidel@xmailserver.org>

.\"

.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)

.\" This program is free software; you can redistribute it and/or modify

.\" it under the terms of the GNU General Public License as published by

.\" the Free Software Foundation; either version 2 of the License, or

.\" (at your option) any later version.

.\"

.\" This program is distributed in the hope that it will be useful,

.\" but WITHOUT ANY WARRANTY; without even the implied warranty of

.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

.\" GNU General Public License for more details.

.\"

.\" You should have received a copy of the GNU General Public

.\" License along with this manual; if not, see

.\" <http://www.gnu.org/licenses/>.

.\" %%%LICENSE_END

.\"

.TH SIGNALFD 2 2017-05-03 Linux "Linux Programmer's Manual"

.SH NAME

signalfd \- create a file descriptor for accepting signals

.SH SYNOPSIS

.B #include <sys/signalfd.h>

.PP

.BI "int signalfd(int " fd ", const sigset_t *" mask ", int " flags );

.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).

.PP

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 .

.PP

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.

.PP

Starting with Linux 2.6.27, the following values may be bitwise ORed in

.IR flags

to change the behavior of

.BR signalfd ():

.TP 14

.B SFD_NONBLOCK

Set the

.BR O_NONBLOCK

file status flag on the new open file description.

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.

.PP

In Linux up to version 2.6.26, the

.I flags

argument is unused, and must be specified as zero.

.PP

.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), " select "(2) (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:

.PP

.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

.PP

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.)

.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

.BR signalfd ()

is available on Linux since kernel 2.6.22.

Working support is provided in glibc since version 2.8.

.\" signalfd() is in glibc 2.7, but reportedly does not build

The

.BR signalfd4 ()

system call (see NOTES) is available on Linux since kernel 2.6.27.

.SH CONFORMING TO

.BR signalfd ()

and

.BR signalfd4 ()

are Linux-specific.

.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.

.PP

Attempts to include

.B SIGKILL

and

.B SIGSTOP

in

.I mask

are silently ignored.

.PP

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

.BR SIGSEGV

signal that results from accessing an invalid memory address

or the

.BR SIGFPE

signal that results from an arithmetic error.

Such signals can be caught only via signal handler.

.PP

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),

.\"

.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.

.PP

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 BUGS

In kernels before 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 2.6.24.5

.SH EXAMPLE

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:

.PP

.in +4n

.EX

.RB "$" " ./signalfd_demo"

.BR "^C" "                   # Control\-C generates SIGINT"

Got SIGINT

.B ^C

Got SIGINT

\fB^\\\fP                    # Control\-\\ generates SIGQUIT

Got SIGQUIT

$

.EE

.in

.SS Program source

\&

.EX

#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\(aqt 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");

        }

    }

}

.EE

.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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.