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 structure will be set to SI_QUEUE.

RETURN VALUE
       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 in‐
       dicate 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(2) 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  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() wrap‐
       per, 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() */

SEE ALSO
       kill(2), rt_sigqueueinfo(2), sigaction(2), signal(2), pthread_sigqueue(3), sigwait(3), signal(7)

Linux                                                                          2021-03-22                                                                    SIGQUEUE(3)