T-Kernel Specification (1.B0.02)

4.9. SYSTEM MANAGEMENT FUNCTIONS 191
tk rot rdq
Rotate Ready Queue
[C Language Interface]
ER ercd = tk_rot_rdq ( PRI tskpri ) ;
[Parameters]
PRI tskpri Task priority
[Return Parameters]
ER ercd Error code
[Error Codes]
E OK
E PAR
Normal completion
Parameter error (tskpri is invalid)
[Description]
Rotates the precedence among tasks having the priority designated in tskpri.
This system call changes the precedence of tasks in RUN or READY state having the designated priority,
so that the task with the highest precedence among those tasks is given the lowest precedence.
By setting tskpri = TPRI RUN = 0, this system call rotates the precedence of tasks having the priority
level of the task currently in RUN state. When tk rot rdq is called from an ordinary task, it rotates the
precedence of tasks having the same priority as the invoking task. When calling from a cyclic handler
or other task-independent portion, it is also possible to call tk rot rdq (tskpri = TPRI RUN).
[Additional Notes]
If there are no tasks in a run state having the designated priority, or only one such task, the system call
completes normally with no operation (no error code is returned).
When this system call is issued in dispatch enabled state, designating as the priority either TPRI RUN or
the current priority of the invoking task, the precedence of the invoking task will be the lowest among
tasks of the same priority. In this way the system call can be used to relinquish execution privilege.
In dispatch disabled state, the task with highest precedence among tasks of the same priority is not
always the currently executing task. The precedence of the invoking task will therefore not always
become the lowest among tasks having the same priority when the above method is used in dispatch
disabled state.
Examples of tk rot rdq execution are given in Figure 4.9 and Figure 4.10. When this system call is
issued in the state shown in Figure 4.9 designating tskpri = 2, the new precedence order becomes that
in Figure 4.10, and Task C becomes the executing task.
Copyright c○ 2002, 2003 by T-Engine Forum
T-Kernel 1.B0.02