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FreeBSD 11.0 - man page for rtprio_thread (freebsd section 2)

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

rtprio, rtprio_thread -- examine or modify realtime or idle priority
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 sys- tem call reports the highest priority in the process, or sets all threads' priority in the process, depending on value of the function argu- ment. 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 prior- ity. 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: [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.
nice(1), ps(1), rtprio(1), setpriority(2), nice(3), renice(8), p_cansee(9)
The original author was Henrik Vestergaard Draboel <>. This implementation in FreeBSD was substantially rewritten by David Greenman. The rtprio_thread() system call was implemented by David Xu.
December 27, 2011 BSD

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