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 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 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.
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.
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.
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.)
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() 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).
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.
- 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.
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.
signalfd() and
signalfd4() are Linux-specific.
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.
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).
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.
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).
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)