signalfd4 (2) - Linux Manuals
signalfd4: create a file descriptor for accepting signals
NAME
signalfd - create a file descriptor for accepting signals
SYNOPSIS
#include <sys/signalfd.h>int signalfd(int fd, const sigset_t *mask, int flags);
DESCRIPTION
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.
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 {
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.
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.
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).
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.
$ ./signalfd_demo
haC # Control-C generates SIGINT
Got SIGINT
haC
Got SIGINT
ha\ # Control-\ generates SIGQUIT
Got SIGQUIT
$
#define handle_error(msg) \
int
main(int argc, char *argv[])
{
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
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.
ERRORS
VERSIONS
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.
CONFORMING TO
signalfd()
and
signalfd4()
are Linux-specific.
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
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.
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.
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.
BUGS
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).
EXAMPLES
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:
Program source
#include <sys/signalfd.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>