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NAME
signalfd - create a file descriptor for accepting signals
LIBRARY
Standard C library (libc, -lc)
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.
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)
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 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.
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
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
signalfd() is available since Linux 2.6.22. Working support is provided since glibc 2.8.
The signalfd4() system call (see NOTES) is available since Linux 2.6.27.
STANDARDS
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.
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.
BUGS
Before Linux 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:
$ ./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 <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
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)