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

       signalfd - create a file descriptor for accepting signals

       #include <sys/signalfd.h>

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

       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 descrip-
       tor.   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  de-
                     scription  (see  open(2)) referred to by the new file de-
                     scriptor.  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 opera-

              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  re-
              turn  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-

              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-de-
              scriptor multiplexing APIs: pselect(2), ppoll(2), and epoll(7).

              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(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 de-
       scribed  in sigaction(2).  Not all fields in the returned signalfd_sig-
       info 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  sigac-
       tion(2) for details.

   fork(2) semantics
       After  a  fork(2),  the  child inherits a copy of the signalfd file de-
       scriptor.  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  re-
       ceiving  process,  a read(2) from the received file descriptor will re-
       turn 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 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  di-
       rected  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

   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()
       to create a child process, then the child will be able to read(2)  sig-
       nals  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).

       On  success, signalfd() returns a signalfd file descriptor; this is ei-
       ther a new file descriptor (if fd was -1), or fd if fd was a valid sig-
       nalfd  file  descriptor.   On error, -1 is returned and errno is set to
       indicate the error.

       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-

       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

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There  was insufficient memory to create a new signalfd file de-

       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.

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

       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 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.

       The signalfd mechanism can't be used to receive signals that  are  syn-
       chronously  generated, such as the SIGSEGV signal that results from ac-
       cessing an invalid memory address or the  SIGFPE  signal  that  results
       from  an  arithmetic error.  Such signals can be caught only via signal

       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 sig-
       nals 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.

       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).

       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
           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)

       main(int argc, char *argv[])
           sigset_t mask;
           int sfd;
           struct signalfd_siginfo fdsi;
           ssize_t s;

           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)

           sfd = signalfd(-1, &mask, 0);
           if (sfd == -1)

           for (;;) {
               s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
               if (s != sizeof(struct signalfd_siginfo))

               if (fdsi.ssi_signo == SIGINT) {
                   printf("Got SIGINT\n");
               } else if (fdsi.ssi_signo == SIGQUIT) {
                   printf("Got SIGQUIT\n");
               } else {
                   printf("Read unexpected signal\n");

       eventfd(2),  poll(2), read(2), select(2), sigaction(2), sigprocmask(2),
       sigwaitinfo(2), timerfd_create(2), sigsetops(3), sigwait(3),  epoll(7),

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Linux                             2019-10-10                       SIGNALFD(2)
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