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SIGQUEUE(3) Linux Programmer's Manual SIGQUEUE(3)

NAME

     sigqueue - queue a signal and data to a process

SYNOPSIS

     #include <signal.h>

     int sigqueue(pid_t pid, int sig, const union sigval value);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     sigqueue(): _POSIX_C_SOURCE >= 199309L

DESCRIPTION

     sigqueue()  sends  the signal specified in sig to the process whose PID
     is given in pid.  The permissions required to send  a  signal  are  the
     same  as for kill(2).  As with kill(2), the null signal (0) can be used
     to check if a process with a given PID exists.

     The value argument is used to specify  an  accompanying  item  of  data
     (either  an integer or a pointer value) to be sent with the signal, and
     has the following type:

         union sigval {
             int   sival_int;
             void *sival_ptr; };

     If the receiving process has installed a handler for this signal  using
     the  SA_SIGINFO  flag to sigaction(2), then it can obtain this data via
     the si_value field of the siginfo_t  structure  passed  as  the  second
     argument to the handler.  Furthermore, the si_code field of that struc-
     ture will be set to SI_QUEUE.

RETURN VALUE

     On success, sigqueue() returns 0, indicating that the signal  was  suc-
     cessfully  queued  to the receiving process.  Otherwise, -1 is returned
     and errno is set to indicate the error.

ERRORS

     EAGAIN The limit of signals which may be queued has been reached.  (See
            signal(7) for further information.)

     EINVAL sig was invalid.

     EPERM  The  process  does not have permission to send the signal to the
            receiving process.  For the required permissions, see kill(2).

     ESRCH  No process has a PID matching pid.

VERSIONS

     sigqueue() and  the  underlying  rt_sigqueueinfo()  system  call  first
     appeared in Linux 2.2.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +-----------+---------------+---------+
     |Interface  | Attribute     | Value   |
     +-----------+---------------+---------+
     |sigqueue() | Thread safety | MT-Safe |
     +-----------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

NOTES

     If this function results in the sending of a signal to the process that
     invoked  it, and that signal was not blocked by the calling thread, and
     no other threads were willing to handle this signal (either  by  having
     it  unblocked,  or  by  waiting for it using sigwait(3)), then at least
     some signal must be delivered  to  this  thread  before  this  function
     returns.

 C library/kernel differences
     On Linux, sigqueue() is implemented using the rt_sigqueueinfo(2) system
     call.  The system call differs in its third argument, which is the sig-
     info_t  structure that will be supplied to the receiving process's sig-
     nal handler or returned  by  the  receiving  process's  sigtimedwait(2)
     call.   Inside  the  glibc sigqueue() wrapper, this argument, uinfo, is
     initialized as follows:

         uinfo.si_signo = sig;      /* Argument supplied  to  sigqueue()  */
         uinfo.si_code  = SI_QUEUE; uinfo.si_pid = getpid();   /* Process ID
         of sender */ uinfo.si_uid = getuid();   /* Real UID  of  sender  */
         uinfo.si_value = val;      /* Argument supplied to sigqueue() */