signalfd

SIGNALFD(2)                Linux Programmer's Manual               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 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/.



Linux                             2019-03-06                       SIGNALFD(2)