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 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

              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

              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).

              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

   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.)

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

       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

       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

       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
       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.

       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

       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.

       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
       signal.  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(fdsi));
               if (s != sizeof(fdsi))

               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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       latest version of this page, can be found at

Linux                              2021-03-22                        SIGNALFD(2)