Manual Pages  — RTPRIO

rtprio, rtprio_thread – examine or modify realtime or idle priority

LIBRARY SYNOPSIS DESCRIPTION RETURN VALUES ERRORS SEE ALSO AUTHORS

Standard C Library (libc, -lc)

#include <sys/types.h>

#include <sys/rtprio.h>

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

int
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 rtprioand 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.

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