signalfd(2) System Calls Manual signalfd(2)
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 descrip-
tor. 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 de-
scription (see open(2)) referred to by the new file de-
scriptor. 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 opera-
tions:
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 re-
turn 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 sig-
waitinfo(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-de-
scriptor 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 sig-
nalfd 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 de-
scribed in sigaction(2). Not all fields in the returned signalfd_sig-
info 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 sigac-
tion(2) for details.
fork(2) semantics
After a fork(2), the child inherits a copy of the signalfd file de-
scriptor. 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 re-
ceiving process, a read(2) from the received file descriptor will re-
turn 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 anal-
ogous 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 di-
rected 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) sig-
nals 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 ei-
ther a new file descriptor (if fd was -1), or fd if fd was a valid sig-
nalfd 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 de-
scriptor.
VERSIONS
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.
STANDARDS
Linux.
HISTORY
signalfd()
Linux 2.6.22, glibc 2.8.
signalfd4()
Linux 2.6.27.
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 dif-
ferent 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 syn-
chronously generated, such as the SIGSEGV signal that results from ac-
cessing 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 sig-
nals 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).
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 sig-
nal. 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)
Linux man-pages 6.7 2023-10-31 signalfd(2)
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