šŸ’¾ Archived View for gmi.noulin.net ā€ŗ man ā€ŗ man2 ā€ŗ signalfd4.2.gmi captured on 2024-08-31 at 13:41:28. Gemini links have been rewritten to link to archived content

View Raw

More Information

ā¬…ļø Previous capture (2023-09-08)

-=-=-=-=-=-=-

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 sigprocā€
       mask(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  sigā€
              nalfd_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  hanā€
              dlers, 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  sigā€
       nals 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 semanā€
       tics, 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 sigā€
       nalfd  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
       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 availā€
       able 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 diā€
       rectory.  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  memā€
       ory 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 descripā€
       tor 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).

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 <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(fdsi));
               if (s != sizeof(fdsi))
                   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)

Linux                                                                          2021-03-22                                                                    SIGNALFD(2)