RTPRIO(2) MidnightBSD 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 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 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


The specified prio was out of range.


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.


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 Greenman.

MidnightBSD 0.3 July 23, 1994 MidnightBSD 0.3