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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 new open  file
                   description.   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
            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 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-
            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 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
     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
     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 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
     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.)


     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  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
     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 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  ^C Got SIGINT ^\                    # Control-\ gener-
         ates 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;
         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),


     This  page  is  part of release 4.16 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 2017-05-03 SIGNALFD(2)

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