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    Command:

    signalfd4

    
    
    

    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  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 descriptor.
    
           Starting with Linux 2.6.27, the following values may be bitwise ORed in
           flags to change the behaviour 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-
           tions:
    
           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.
                  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 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) */
                   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.
    
       execve(2) semantics
           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 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

           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.
    
           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 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 descriptors.
    
       Underlying Linux system calls
           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
               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");
    
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