SYNOPSIS

#include <ggi/gg.h>
struct gg_task {
gg_task_callback_fn *cb; /* Function to call to run task */
void *hook; /* Task data can be hung here */
int pticks; /* Run once every pticks ticks. */
int ncalls; /* Run ncalls times (0 = infinite) */
int lasttick; /* last tick run (read-only) */
/* Other members present but are for internal use only. */
};
typedef int (gg_task_callback_fn)(struct gg_task *task);
GG_SCHED_TICKS2USECS(uint32_t ticks);
GG_SCHED_USECS2TICKS(uint32_t usecs);
uint32_t ggTimeBase(void);
int ggAddTask(struct gg_task *task);
int ggDelTask(struct gg_task *task);

DESCRIPTION

LibGG implements a task scheduler in both threaded and non-threaded environments. Tasks can be registered with the scheduler to run short, asynchronous routines called "handlers" which may interrupt or run in parallel with the normal flow-of-control. It is recommended to use LibGG tasks in lieue of threads when writing for maximum portability, if they can meet the demands of the application, since not all environments support threads.

The LibGG task scheduler uses a unit of time called a "tick", which may vary between architectures. The tick is guaranteed to be no more than one second, however, most environments will support at least 60 ticks per second. By default LibGG will select 60 ticks per second if it is supported, see below for instructions on modifying this behavior. The function ggTimeBase is used to find out the size of a tick.

GG_SCHED_TICKS2USECS and GG_SCHED_USECS2TICKS are convenient macros that simplifies conversion between ticks and microseconds and vice versa.

The maximum rate at which a periodic task may run is once per tick. The maximum period (minimum rate) of a LibGG task is the value of the macro GG_SCHED_TICK_WRAP minus one, and is also measured in ticks.

ggAddTask will examine the values in the offered task control structure task. Before calling ggAddTask the task control structure must be initialized by filling it with zeros, including the internal-use-only area. The task control structure should be further initialized by providing at least a pointer to a callback handler function in the member cb, and initializing the pticks member to contain the number of ticks between each call to the handler function. The ncalls member may be left at zero, in which case the task remains scheduled to run once every pticks until explicitly deleted, or it may be set to a positive integer to indicate that the task should be automatically deleted after the handler has been called ncalls times. The int return type on the callback hook is only there for possible future expansion. For now callbacks should always return 0. Other values are undefined.

The task control structure must only be used for one task, however a task handler may be called by multiple tasks. The member hook is provided for the application's use in the task control structure as a means to easily transport task-local data to the handler. If a tick arrives during a call to ggAddTask, the handler may be invoked before ggAddTask returns; A memory barrier is included in ggAddTask which ensures that all values in the task control structure are up to date on multiprocessor systems even in this case. The task control structure should not be altered, except by a task handler as noted below, while the task is scheduled.

ggDelTask will remove a task from the scheduler. The task may be called after ggDelTask is called, but is guaranteed not to be called after ggDelTask has returned, until such a point as it is added again with ggAddTask.

A task can be put to sleep for a certain amount of time in microseconds by altering the period of the task to the correct number of ticks, and then that task itself can reset it's period back based on a value in it's private hook when it next runs.

A task can wait for an other task to finish either by writing code to poll the other task's flags, or by writing a callback into the latter task when it is done to reschedule a list of waiting tasks. How a task terminates is entirely up to the author.

Each scheduled task is guaranteed never to be reentered by the scheduler. That is, only one call to a task handler for a given task control structure will be run at a time, though a single handler function that handles more than one task control structure may be entered simultaneously once per structure.

When a task executes, the handler is invoked and the parameter task given to the handler contains the same pointer value as was given to ggAddTask. The ncalls member will be updated to contain the number of calls, including the current call, which remain before the task is automatically deleted (or zero if the task will never be automatically deleted.) Thus it is safe to call ggAddTask again to reuse the task control structure once the handler has returned with ncalls equal to 1. The lasttick member will contain the number of the LibGG scheduler tick being executed, which should increase monotonically unless a problem occurs as noted below, wrapping around modulus the value GG_SCHED_TICK_WRAP.

ggAddTask and ggDelTask may not be called from within a task handler, however, the task handler is free to alter the pticks and ncalls members in the task control structure task in order to change its period, or increase or decrease the number of calls before auto-deletion. For example, to cancel itself, a task need only set ncalls to 1 before returning. The task handler may also change it's callback function or data hook members. A write memory barrier is included in the scheduler to prevent old values from being seen by other processors on SMP systems.

LibGG ticks are measured in real (wall clock) time and LibGG makes every effort to ensure that drift due to runtime factors is kept at a minimum. When a process is suspended, however, LibGG ticks stop and resume where they left off. Likewise, when system utilization is very high or tasks are misused the LibGG scheduler may fail to count ticks. However the ggCurTime(3) function will still be accurate in these cases and can be used to detect such situations.

All scheduled LibGG tasks may in the worst case have to be run serialized, and may be postponed slightly while a call to ggAddTask or ggDelTask is in progress, so there may be some delay between the start of a LibGG tick and the actual execution of the task. This can be minimized by limiting the duties of task handlers to very short, quick operations.

When utilization is high or tasks misbehave, the scheduler may elect simply not to call a task handler even though it is scheduled to be called on a given tick. This may happen either to all tasks or to select individual tasks. The "lasttick" member of the task control structure can be safely read from within a task handler in order to detect such a circumstance (it will always contain the current tick, but can be compared to a previously stored value.)

Since LibGG tasks may be called in a signal handler or other non-interruptible context, they should not call ggLock(3) on any locks that may already be locked. In addition, there may be limits imposed on the functions which are safe to use inside task handlers (that is, only reentrant functions may be safe.) More detailed information on using locks inside LibGG task handlers is contained in the manpage for ggLock(3).

Scheduled tasks will be canceled, in a somewhat precarious fashion, by a normal call to ggExit(3). As such, it is considered best practice to use ggDelTask to cancel tasks when gracefully deinitializing LibGG or a library that uses LibGG.

RETURN VALUE

ggAddTask returns GGI_OK on success or:

• GGI_EARGREQ if called with NULL argument;
• GGI_EARGINVAL if the task is incorrectly set;
• GGI_EBUSY if the task is already added;
• GGI_ENOMEM if the task lock could not be created.

ggDelTask returns GGI_OK on success or:

• GGI_EARGREQ if called with NULL argument;
• GGI_EARGINVAL if the task is not currently scheduled.

ggTimeBase returns an integer between 1 and 1000000, inclusive, which represents the number on microseconds between each tick of the LibGG scheduler.