Coma FW-C2800 User's Guide download pdf (Page 160)

Semaphores

Thread Scheduling 4-55

4.6 Semaphores

DSP/BIOS provides a fundamental set of functions for intertask

synchronization and communication based upon semaphores. Semaphores

are often used to coordinate access to a shared resource among a set of

competing tasks. The SEM module provides functions that manipulate

semaphore objects accessed through handles of type SEM_Handle.

SEM objects are counting semaphores that can be used for both task

synchronization and mutual exclusion. Counting semaphores keep an

internal count of the number of corresponding resources available. When

count is greater than 0, tasks do not block when acquiring a semaphore.

The functions SEM_create and SEM_delete are used to create and delete

semaphore objects, respectively, as shown in Example 4-9. You can also use

the Configuration Tool to create semaphore objects. See section 2.2.8,

Creating, Referencing, and Deleting Dynamically-Created DSP/BIOS

Objects, page 2-10, for a discussion of the benefits of creating objects with

the Configuration Tool.

Example 4-9. Creating and Deleting a Semaphore

The semaphore count is initialized to count when it is created. In general,

count is set to the number of resources that the semaphore is synchronizing.

SEM_pend waits for a semaphore. If the semaphore count is greater than 0,

SEM_pend simply decrements the count and returns. Otherwise, SEM_pend

waits for the semaphore to be posted by SEM_post.

Note:

When called within an HWI, the code sequence calling SEM_post or

SEM_ipost must be either wrapped within an HWI_enter/HWI_exit pair or

invoked by the HWI dispatcher.

The timeout parameter to SEM_pend, as shown in Example 4-10, allows the

task to wait until a timeout, to wait indefinitely (SYS_FOREVER), or to not

wait at all (0). SEM_pend’s return value is used to indicate if the semaphore

was acquired successfully.

SEM_Handle SEM_create(count, attrs);

Uns count;

SEM_Attrs *attrs;

Void SEM_delete(sem);

SEM_Handle sem;

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Coma FW-C2800 User's Guide Page 160