µC/OS-II and Event Flags
You should note that the wait list for event flags is different than the other wait lists in µC/OS-II. With
event flags, the wait list is accomplished through a doubly linked list as shown in figure 4. Three data
structures are involved. OS_FLAG_GRP (mentioned above), OS_TCB which is the task control block and
OS_FLAG_NODE which is used to keep track of which bits the task is waiting for as well as what type of
wait (AND or OR). As you can see, there are a lot of pointers involved.
An OS_FLAG_NODE is created when a task desires to wait on bits of an event flag group and the node is
‘destroyed’ when the event(s) occur. In other words, a node is created by OSFlagPend() as we will see
shortly. Before we discuss this, let’s look at the OS_FLAG_NODE data structure.
typedef struct {
void *OSFlagNodeNext;
void *OSFlagNodePrev;
void *OSFlagNodeTCB;
void *OSFlagNodeFlagGrp;
OS_FLAGS OSFlagNodeFlags;
INT8U OSFlagNodeWaitType;
} OS_FLAG_NODE;
The OSFlagNodeNext and OSFlagNodePrev are used to maintain a doubly linked list of
OS_FLAG_NODEs. The doubly linked list allows us to easily insert and especially remove nodes from the
wait list.
OSFlagNodeTCB is used to point to the TCB of the task waiting on event flags belonging to the event flag
group. This pointer thus allows us to know which tasks is waiting for the specified flags.
OSFlagNodeFlagGrp allows a link back to the event flag group. This pointer is used when removing the
node from the doubly linked list and is there because a node might need to be removed because a task is
deleted (see OSTaskDel()).
The OSFlagNodeFlags contains the bit-pattern of the flags that the task is waiting for. For example,
your task might have performed an OSFlagPend() and specified that the task wants to wait for bits 0, 4,
6 and 7 (bit 0 is the rightmost bit). In this case, OSFlagFlags would contain 0xD1. Depending on the
size of the data type OS_FLAGS, OSFlagFlags is either 8, 16 or 32 bits. OS_FLAGS is specified in your
application configuration file, i.e OS_CFG.H. Because µC/OS-II and the ports are provided in source
form, you can easily change the number of bits in an event flag group to satisfy your requirements for a
specific application or product. The reason you would limit the number of bits to 8 is to reduce both RAM
and ROM for your application.
The last member of the OS_FLAG_NODE data structure is OSFlagNodeWaitType which determines
whether the task is waiting for ALL (AND wait) the bits in the event flag group that matches
OSFlagNodeFlags or, ANY (OR wait) of the bits in the event flag group that matches
OSFlagNodeFlags. OSFlagNodeWaitType can be set to:
OS_FLAG_WAIT_CLR_ALL
OS_FLAG_WAIT_CLR_AND
OS_FLAG_WAIT_CLR_ANY
OS_FLAG_WAIT_CLR_OR
OS_FLAG_WAIT_SET_ALL
OS_FLAG_WAIT_SET_AND
OS_FLAG_WAIT_SET_ANY
OS_FLAG_WAIT_SET_OR
AN1007 - 5
