# 13.2. Background¶

A barrier can be viewed as a gate at which tasks wait until the gate is opened. This has many analogies in the real world. Horses and other farm animals may approach a closed gate and gather in front of it, waiting for someone to open the gate so they may proceed. Similarly, ticket holders gather at the gates of arenas before concerts or sporting events waiting for the arena personnel to open the gates so they may enter.

Barriers are useful during application initialization. Each application task can perform its local initialization before waiting for the application as a whole to be initialized. Once all tasks have completed their independent initializations, the “application ready” barrier can be released.

## 13.2.1. Automatic Versus Manual Barriers¶

Just as with a real-world gate, barriers may be configured to be manually opened or automatically opened. All tasks calling the rtems_barrier_wait directive will block until a controlling task invokes the rtems_barrier_release directive.

Automatic barriers are created with a limit to the number of tasks which may simultaneously block at the barrier. Once this limit is reached, all of the tasks are released. For example, if the automatic limit is ten tasks, then the first nine tasks calling the rtems_barrier_wait directive will block. When the tenth task calls the rtems_barrier_wait directive, the nine blocked tasks will be released and the tenth task returns to the caller without blocking.

## 13.2.2. Building a Barrier Attribute Set¶

In general, an attribute set is built by a bitwise OR of the desired attribute components. The following table lists the set of valid barrier attributes:

RTEMS_BARRIER_AUTOMATIC_RELEASE

automatically release the barrier when the configured number of tasks are blocked

RTEMS_BARRIER_MANUAL_RELEASE

only release the barrier when the application invokes the rtems_barrier_release directive. (default)

Note

Barriers only support FIFO blocking order because all waiting tasks are released as a set. Thus the released tasks will all become ready to execute at the same time and compete for the processor based upon their priority.

Attribute values are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each attribute appears exactly once in the component list. An attribute listed as a default is not required to appear in the attribute list, although it is a good programming practice to specify default attributes. If all defaults are desired, the attribute RTEMS_DEFAULT_ATTRIBUTES should be specified on this call.

This example demonstrates the attribute_set parameter needed to create a barrier with the automatic release policy. The attribute_set parameter passed to the rtems_barrier_create directive will be RTEMS_BARRIER_AUTOMATIC_RELEASE. In this case, the user must also specify the maximum_waiters parameter.