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



NAME
       signalfd - create a file descriptor for accepting signals

SYNOPSIS
       #include <&lt;sys/signalfd.h>&gt;

       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.

       The  flags argument is currently unused, and must be specified as zero.
       In the future, it may be used to request additional functionality.

       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
              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  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 non-blocking (via the use  of  the
              fcntl(2) F_SETFL operation to set the O_NONBLOCK flag).

       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 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(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 sig-
       nalfd_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.  The file descriptor refers to  the  same  underlying  file
       object  as  the corresponding descriptor in the parent, and read(2)s in
       the child will return information about signals generated for the  par-
       ent (the process that created the object using signalfd()).

   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
       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; or, 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.

CONFORMING TO
       signalfd() is 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.

       The flags argument is a glibc addition to 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 dif-
       ferent 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 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
           $

       #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 3.05 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/.



Linux                             2008-07-08                       SIGNALFD(2)