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       sigqueue, rt_sigqueueinfo - queue a signal and data to a process


       #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


       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
       structure will be set to SI_QUEUE.


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


       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.


       This system call first appeared in Linux 2.2.




       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

       On    Linux,   the   underlying   system   call   is   actually   named
       rt_sigqueueinfo(), and differs in its  third  argument,  which  is  the
       siginfo_t  structure  that  will be supplied to the receiving process’s
       signal handler or returned by the receiving  process’s  sigtimedwait(2)
       call.   Inside  the  glibc  sigqueue() wrapper, this argument, info, is
       initialized as follows:

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


       kill(2), sigaction(2), signal(2), sigwait(3), signal(7)


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