rtprio man page on FreeBSD

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RTPRIO(2)		    BSD System Calls Manual		     RTPRIO(2)

     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 process.

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

     The *rtp argument is a pointer to a struct rtprio which is used to spec‐
     ify the priority and priority type.  This structure has the following

     struct rtprio {
	     u_short type;
	     u_short prio;

     The value of the type field may be RTP_PRIO_REALTIME for realtime priori‐
     ties, 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 pri‐
     ority 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

     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 ⟨hvd@terry.ping.dk⟩.
     This implementation in FreeBSD was substantially rewritten by David

BSD				 July 23, 1994				   BSD

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