Ubuntu Manpage: signalfd - create a file descriptor for accepting signals

NAME

       signalfd - create a file descriptor for accepting signals

LIBRARY

       Standard C library (libc, -lc)

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.

       Up to Linux 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.)

   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).

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

   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.

STANDARDS

       Linux.

HISTORY

       signalfd()
              Linux 2.6.22, glibc 2.8.

       signalfd4()
              Linux 2.6.27.

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).

BUGS

       Before Linux 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

       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 <err.h>
       #include <signal.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/signalfd.h>
       #include <unistd.h>

       int
       main(void)
       {
           int                      sfd;
           ssize_t                  s;
           sigset_t                 mask;
           struct signalfd_siginfo  fdsi;

           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)
               err(EXIT_FAILURE, "sigprocmask");

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

           for (;;) {
               s = read(sfd, &fdsi, sizeof(fdsi));
               if (s != sizeof(fdsi))
                   err(EXIT_FAILURE, "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");
               }
           }
       }