signalfd(2) — Linux manual page


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

NAME         top

       signalfd - create a file descriptor for accepting signals

SYNOPSIS         top

       #include <sys/signalfd.h>

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

DESCRIPTION         top

       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

       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

              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 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 de‐
       scriptor.  A read(2) from the file descriptor in the child will re‐
       turn 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 re‐
       mains 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 sig‐
       nals 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 descrip‐
       tor, but epoll_wait(2) will not indicate that the signalfd file de‐
       scriptor is ready.  In this scenario, a possible workaround is that
       after the fork(2), the child process can close the signalfd file de‐
       scriptor 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 in‐
       stance until after the call to fork(2).

RETURN VALUE         top

       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         top

       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 reached.

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There was insufficient memory to create a new signalfd file

VERSIONS         top

       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         top

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

NOTES         top

       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

   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         top

       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).

EXAMPLES         top

       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");

SEE ALSO         top

       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         top

       This page is part of release 5.09 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

Linux                            2020-11-01                      SIGNALFD(2)

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