Have a question about Micrium's embedded software? Ask our Expert, Matt Gordon

Hi Matt,

You have been a great help, just one little thing, how to use the flags in uC/OS-III. I define the bits, create the flag, pend the flag and after the task is executed I post the flag, but I guess its not as simple as using a semaphore, beacuse its not working. Please help Matt.

Regards,

jumbowat

Hi Matt,

You have been a great help, just one little thing, how to use the flags in uC/OS-III. I define the bits, create the flag, pend the flag and after the task is executed I post the flag, but I guess its not as simple as using a semaphore, beacuse its not working. Please help Matt.

Regards,

jumbowat

To some extent, event flags are like collections of binary semaphores.  Each flag represents an event and can be in one of two possible states: the event has taken place, or it hasn’t.  Your application can wait on these events using a pend function similar to the one provided as part of uC/OS-III’s semaphore API. 

The pend function, which is named OSFlagPend(), accepts a number of arguments, the second of which is a bit pattern indicating the particular flags that are of importance to the calling task.  The function also accepts an options value that allows the calling task to specify whether it will wait for all (an “and” pend) or any (an “or” pend) of the designated events to take place.  A return from the pend function typically occurs once the conditions established by the calling task through these two arguments have come into existence.  However, the calling task can use another of the function’s arguments to specify a timeout, a period of time after which the function will return regardless of which events have taken place. 

The pend function’s counterpart is the event flag post function, OSFlagPost().  Like pend, this function accepts an argument that specifies a set of flags.  In the case of post, though, the calling task will not wait on these flags; instead, the kernel will update the flags’ values to indicate that the events to which they correspond have taken place. 

Example pend and post calls are shown below.  Additional example code can be found in the uC/OS-III books available from the knode.   

                                                                                 /* Flag creation code                          */

OSFlagCreate(&AppFlagEx,                                        /* Pointer to OS_FLAG_GRP structure */

                     "Example Flag",                                    /* String name for flags                        */

                      0,                                                        /* Initial flag values                              */

                      &err);                                                   /* Error pointer                                    */

void AppTaskA (void *p_arg)

{

    while (1) {

        OSFlagPend(&AppFlagEx,                                   /* Pointer to OS_FLAG_GRP structure */

                            0x0003,                                          /* Specify bits 0 and 1                         */

                            100,                                               /* Wait for a maximum of 100 ticks       */

                            OS_OPT_PEND_FLAG_SET_ALL,   /* Wait for both flags to be set              */

                            0,                                                   /* No timestamp for this example          */

                            &err);                                              /* Error pointer                                    */

        Take actions that must follow occurrence of events;

    }

}

void AppTaskB (void *p_arg)

{

    while (1) {

        OSFlagPost(&AppFlagEx,                                   /* Pointer to OS_FLAG_GRP structure   */

                           0x0003,                                          /* Specify bits 0 and 1                           */

                           OS_OPT_POST_FLAG_SET,           /* Indicate that bits will be set                */

                           &err);                                              /* Error pointer                                      */

        Perform additional work;

    }

}

Hey Matt, thank you so much

Also, if I want to make a task start from the beginning if I interrupt it in the middle of a task, then do I have an option of using something other than OSTaskDel(). What i mean is suppose my task is printing 1 to 100 and then I interrupt it at 55, now when I want to restart the task but it should start the counting again from 1, insteading of continuing it from 55.

Regards.