# sigqueue

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           sigqueue — queue a signal to a process
    


## SYNOPSIS



           #include <signal.h>
    
           int sigqueue(pid_t _pid_ , int _signo_ , union sigval _value_);
    


## DESCRIPTION



           The _sigqueue_() function shall cause the signal specified by _signo_
           to be sent with the value specified by _value_ to the process
           specified by _pid_.  If _signo_ is zero (the null signal), error
           checking is performed but no signal is actually sent. The null
           signal can be used to check the validity of _pid_.
    
           The conditions required for a process to have permission to queue
           a signal to another process are the same as for the _kill_()
           function.
    
           The _sigqueue_() function shall return immediately. If SA_SIGINFO is
           set for _signo_ and if the resources were available to queue the
           signal, the signal shall be queued and sent to the receiving
           process. If SA_SIGINFO is not set for _signo_ , then _signo_ shall be
           sent at least once to the receiving process; it is unspecified
           whether _value_ shall be sent to the receiving process as a result
           of this call.
    
           If the value of _pid_ causes _signo_ to be generated for the sending
           process, and if _signo_ is not blocked for the calling thread and if
           no other thread has _signo_ unblocked or is waiting in a _sigwait_()
           function for _signo_ , either _signo_ or at least the pending,
           unblocked signal shall be delivered to the calling thread before
           the _sigqueue_() function returns. Should any multiple pending
           signals in the range SIGRTMIN to SIGRTMAX be selected for
           delivery, it shall be the lowest numbered one.  The selection
           order between realtime and non-realtime signals, or between
           multiple pending non-realtime signals, is unspecified.
    


## RETURN VALUE



           Upon successful completion, the specified signal shall have been
           queued, and the _sigqueue_() function shall return a value of zero.
           Otherwise, the function shall return a value of -1 and set _errno_
           to indicate the error.
    


## ERRORS



           The _sigqueue_() function shall fail if:
    
           **EAGAIN** No resources are available to queue the signal. The process
                  has already queued {SIGQUEUE_MAX} signals that are still
                  pending at the receiver(s), or a system-wide resource limit
                  has been exceeded.
    
           **EINVAL** The value of the _signo_ argument is an invalid or
                  unsupported signal number.
    
           **EPERM** The process does not have appropriate privileges to send
                  the signal to the receiving process.
    
           **ESRCH** The process _pid_ does not exist.
    
           _The following sections are informative._
    


## EXAMPLES



           None.
    


## APPLICATION USAGE



           None.
    


## RATIONALE



           The _sigqueue_() function allows an application to queue a realtime
           signal to itself or to another process, specifying the
           application-defined value. This is common practice in realtime
           applications on existing realtime systems.  It was felt that
           specifying another function in the _sig_...  name space already
           carved out for signals was preferable to extending the interface
           to _kill_().
    
           Such a function became necessary when the put/get event function
           of the message queues was removed. It should be noted that the
           _sigqueue_() function implies reduced performance in a security-
           conscious implementation as the access permissions between the
           sender and receiver have to be checked on each send when the _pid_
           is resolved into a target process. Such access checks were
           necessary only at message queue open in the previous interface.
    
           The standard developers required that _sigqueue_() have the same
           semantics with respect to the null signal as _kill_(), and that the
           same permission checking be used. But because of the difficulty of
           implementing the ``broadcast'' semantic of _kill_() (for example, to
           process groups) and the interaction with resource allocation, this
           semantic was not adopted. The _sigqueue_() function queues a signal
           to a single process specified by the _pid_ argument.
    
           The _sigqueue_() function can fail if the system has insufficient
           resources to queue the signal. An explicit limit on the number of
           queued signals that a process could send was introduced. While the
           limit is ``per-sender'', this volume of POSIX.1‐2017 does not
           specify that the resources be part of the state of the sender.
           This would require either that the sender be maintained after exit
           until all signals that it had sent to other processes were handled
           or that all such signals that had not yet been acted upon be
           removed from the queue(s) of the receivers. This volume of
           POSIX.1‐2017 does not preclude this behavior, but an
           implementation that allocated queuing resources from a system-wide
           pool (with per-sender limits) and that leaves queued signals
           pending after the sender exits is also permitted.
    


## FUTURE DIRECTIONS



           None.
    


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