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



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).   Nor-
       mally,  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  sig-
       nalfd():

       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  associ-
              ated 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 fol-
       lows:

           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) */
           };

       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.

   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 sig-
       nals.  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.)

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.

   Underlying Linux system calls
       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
       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  termi-
       nates 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(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  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-01-13                       SIGNALFD(2)