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).
   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 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 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.)

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
          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

   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

   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.

   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

   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).

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
       $

   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 4.09 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/.

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