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     rtprio, rtprio_thread — examine or modify realtime or idle priority


     Standard C Library (libc, -lc)


     #include <sys/types.h>
     #include <sys/rtprio.h>

     rtprio(int function, pid_t pid, struct rtprio *rtp);

     rtprio_thread(int function, lwpid_t lwpid, struct rtprio *rtp);


     The rtprio() system call is used to lookup or change the realtime or idle priority of a
     process, or the calling thread.  The rtprio_thread() system call is used to lookup or change
     the realtime or idle priority of a thread.

     The function argument specifies the operation to be performed.  RTP_LOOKUP to lookup the
     current priority, and RTP_SET to set the priority.

     For the rtprio() system call, the pid argument specifies the process to operate on, 0 for
     the calling thread.  When pid is non-zero, the system call reports the highest priority in
     the process, or sets all threads' priority in the process, depending on value of the
     function argument.

     For the rtprio_thread() system call, the lwpid specifies the thread to operate on, 0 for the
     calling thread.

     The *rtp argument is a pointer to a struct rtprio which is used to specify the priority and
     priority type.  This structure has the following form:

     struct rtprio {
             u_short type;
             u_short prio;

     The value of the type field may be RTP_PRIO_REALTIME for realtime priorities,
     RTP_PRIO_NORMAL for normal priorities, and RTP_PRIO_IDLE for idle priorities.  The priority
     specified by the prio field ranges between 0 and RTP_PRIO_MAX (usually 31).  0 is the
     highest possible priority.

     Realtime and idle priority is inherited through fork() and exec().

     A realtime thread can only be preempted by a thread of equal or higher priority, or by an
     interrupt; idle priority threads will run only when no other real/normal priority thread is
     runnable.  Higher real/idle priority threads preempt lower real/idle priority threads.
     Threads of equal real/idle priority are run round-robin.


     The rtprio() and rtprio_thread() functions return the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the error.


     The rtprio() and rtprio_thread() system calls will fail if:

     [EFAULT]           The rtp pointer passed to rtprio() or rtprio_thread() was invalid.

     [EINVAL]           The specified prio was out of range.

     [EPERM]            The calling thread is not allowed to set the realtime priority.  Only
                        root is allowed to change the realtime priority of any thread, and non-
                        root may only change the idle priority of threads the user owns, when the
                        sysctl(8) variable security.bsd.unprivileged_idprio is set to non-zero.

     [ESRCH]            The specified process or thread was not found or visible.


     nice(1), ps(1), rtprio(1), setpriority(2), nice(3), renice(8), p_cansee(9)


     The original author was Henrik Vestergaard Draboel <>.  This implementation
     in FreeBSD was substantially rewritten by David Greenman.  The rtprio_thread() system call
     was implemented by David Xu.