RTPRIO(2) FreeBSD System Calls Manual RTPRIO(2)


rtprio, rtprio_threadexamine 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:
The rtp pointer passed to rtprio() or rtprio_thread() was invalid.
The specified prio was out of range.
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.
The specified process or thread was not found or visible.


The original author was Henrik Vestergaard Draboel <hvd@terry.ping.dk>. This implementation in FreeBSD was substantially rewritten by David Greenman. The rtprio_thread() system call was implemented by David Xu.
December 27, 2011 FreeBSD