I’m learning FreeRTOS currently, I will try to write down what I learned from the source code and share it in this forum. hope it can help the beginner, and if I have made some mistake, it will be highly appreciated if some one can point out.
Hi John, thanks for the clear pictures. That all looks correct to me.
As you can see, the List_t type has been implemented with a minimum RAM footprint. All items (elements) know to which list they belong, if any. Each node in the list is part its object.
For instance, an example taken from FreeRTOS+TCP: a network buffer is either linked into a free list or into a list of occupied buffers. In order to make it linkable, it gets a ListItem_t field:
typedef struct xNETWORK_BUFFER
{
ListItem_t xBufferListItem; /* Used to reference the buffer form the free buffer list or a socket. */
uint32_t ulIPAddress; /* Source or destination IP address, depending on usage scenario. */
uint8_t *pucEthernetBuffer; /* Pointer to the start of the Ethernet frame. */
...
} NetworkBufferDescriptor_t;
Now xBufferListItem will always point to one of the two lists. You can add more ListItem_t's to your objects to make them linkable to several lists.
The List_t type has been exposed in a header file, but normally developers will not make use of them. It is a type definition “internal to the kernel”. But you’re free to use it, of course. Note that with a few additions (wrappers), you can create both FIFO’s and stacks, based on List_t.
Other objects are more important for most developers: Queue’s, Semaphores, Event-Groups and the mechanisms to notify (and wake-up) a task.