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SIGNALFD(2) Linux Programmer's Manual SIGNALFD(2)
signalfd - create a file descriptor for accepting signals
#include <sys/signalfd.h>
int signalfd(int fd, const sigset_t *mask, int flags);
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 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 descriptor.
The flags argument is currently unused, and must be specified as zero. In the
future, it may be used to request additional functionality.
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 non-blocking (via the use of the fcntl(2) F_SETFL operation
to set the O_NONBLOCK flag).
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 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(2) */
uint64_t ssi_ptr; /* Pointer sent by sigqueue(2) */
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) */
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.
After a fork(2), the child inherits a copy of the signalfd file descriptor.
The file descriptor refers to the same underlying file object as the
corresponding descriptor in the parent, and read(2)s in the child will return
information about signals generated for the parent (the process that created
the object using signalfd()).
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).)
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.)
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.
EBADF The fd file descriptor is not a valid file descriptor.
EINVAL fd is not a valid signalfd file descriptor; or, flags is non-zero.
EMFILE The per-process limit 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.
signalfd() is available on Linux since kernel 2.6.22. Working support is
provided in glibc since version 2.8.
signalfd() is Linux-specific.
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.
The flags argument is a glibc addition to the underlying system call.
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 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 descriptors.
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(2).
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
$
#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't 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");
}
}
}
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)
This page is part of release 3.11 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-07-08 SIGNALFD(2)