signalfd(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

SIGNALFD(2)             Linux Programmer's Manual            SIGNALFD(2)

NAME         top

       signalfd - create a file descriptor for accepting signals

SYNOPSIS         top

       #include <sys/signalfd.h>

       int signalfd(int fd, const sigset_t *mask, int flags);

DESCRIPTION         top

       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 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 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.

   fork(2) semantics
       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.

   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 receiving process, a read(2) from the received file
       descriptor will return 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 analogous 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 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.)

   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) 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).

RETURN VALUE         top

       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         top

       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.

VERSIONS         top

       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         top

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

NOTES         top

       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.

   Limitations
       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).

   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.

BUGS         top

       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).

EXAMPLES         top

       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
           $

   Program source

       #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(fdsi));
               if (s != sizeof(fdsi))
                   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         top

       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         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                    SIGNALFD(2)

Pages that refer to this page: eventfd(2)sigaction(2)signal(2)sigwaitinfo(2)syscalls(2)timerfd_create(2)timer_getoverrun(2)sd-event(3)sd_event_add_signal(3)sigwait(3)proc(5)signal(7)system_data_types(7)