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     rtprio — examine or modify a process 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);


     The rtprio() system call is used to lookup or change the realtime or idle priority of a

     The function argument specifies the operation to be performed.  RTP_LOOKUP to lookup the
     current priority, and RTP_SET to set the priority.  The pid argument specifies the process
     to be used, 0 for the current process.

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


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


     The rtprio() system call will fail if

     [EINVAL]           The specified prio was out of range.

     [EPERM]            The calling process is not allowed to set the realtime priority.  Only
                        root is allowed to change the realtime priority of any process, and non-
                        root may only change the idle priority of the current process.

     [ESRCH]            The specified process was not found.


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


     The original author was Henrik Vestergaard Draboel <>.  This implementation
     in FreeBSD was substantially rewritten by David Greenman.