bionic (2) signalfd.2.gz

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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 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 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

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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.