NAME¶

signalfd - create a file descriptor for accepting signals

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 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.

Starting with Linux 2.6.27, the following values may be bitwise ORed in flags to change the behaviour of signalfd():

SFD_NONBLOCK

Set 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_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 non-blocking.

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.

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) */
};

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 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.

VERSIONS¶

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.

CONFORMING TO¶

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

NOTES¶

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.

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.

BUGS¶

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).

EXAMPLE¶

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");


        }


    }
}

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)

COLOPHON¶

This page is part of release 3.22 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/.