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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  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.)

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.

       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

       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 5.02 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/.