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_NONBLOCKSet the O_NONBLOCK
file status flag on the new open file description.
Using this flag saves extra calls to fcntl(2) to achieve
the same result.
SFD_CLOEXECSet 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;int32_t ssi_errno;int32_t ssi_code;uint32_t ssi_pid;uint32_t ssi_uid;int32_t ssi_fd;uint32_t ssi_tid;
uint32_t ssi_band; /* Band event (SIGIO) */uint32_t ssi_overrun;uint32_t ssi_trapno;int32_t ssi_status;int32_t ssi_int;uint64_t ssi_ptr;uint64_t ssi_utime;uint64_t ssi_stime;uint64_t ssi_addr;
(for hardware-generated signals) */uint8_t pad[X];
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.)
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.
The fd file
descriptor is not a valid file descriptor.
fd is not a
valid signalfd file descriptor.
flags is
invalid; or, in Linux 2.6.26 or earlier, flags is nonzero.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
Could not mount (internal) anonymous inode device.
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.
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.
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 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^CGot 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)
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and the latest version of this page, can be found at
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|
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