printing the stack of a task in FreeRTOS - freertos

Question

I am working on a STM32F4-discovery board, i installed FreeRTOS on the board and was able to run two tasks created by main function.
Now i want task 1 to access the local variables of task 2 with any passing of variable by reference or by value.
I thought it would be good to print the stack content of task 2 and then locate the local variables and use that in task1
Can somebody guide me with this?
I tried to print the address of each variable and tried to use in task1, but my program did not compile and returned -1.

Answer 1

It is not clear to me what you are trying to achieve, or what you mean by the program returning -1 because it didn't compile, but it would not be normal for one task to access a stack variable of another task.
Each task has its own stack, and the stack is private to the task. There are lots of ways tasks can communicate with each other though, without the need to access each other's stack. The simplest way is just to make the variable global, although global variables are rarely a good thing. Other than that you could send the value from one task to another on a queue (http://www.freertos.org/Inter-Task-Communication.html), or even using a task notification as a mailbox (http://www.freertos.org/RTOS_Task_Notification_As_Mailbox.html).

Answer 2

First, a cautionary note: what you're trying to do -- access local variables of one task from another -- can be error-prone. Particularly if the local is declared in Task A and goes out of scope before Task B accesses it. Since stack memory can be reused for different variables in different functions, Task B might be accessing some other variable.
But I've actually used this pattern in practice -- specifically, to allow one task to stack-allocate a buffer for communication serviced by another task.
So I'll just assume that you know what you're doing. :-)
It's difficult to compute the address of a local variable ahead of time. If you derive it today, it's likely to change if you change the code or compiler version. What you probably want to do is to capture its address at runtime and somehow make it available to the other task. This can be be tricky, however: it's possible for the other task to try to use the local before your task starts up, and nothing prevents other tasks from getting to it.
A slightly cleaner approach would be to provide the address to the other task through a queue shard by the two tasks.
QueueHandle_t shared_queue;
void common_startup() {
// This code might go in main, or wherever you initialize things.
shared_queue = xQueueCreate(1, sizeof(unsigned *));
}
void task_a() {
// This task wants to share the address of 'local' with task_b.
unsigned local;
// Stuff the address of our local in the queue. We use a
// non-blocking send here because the queue will be empty (nobody
// else puts things in it). In real code you'd probably do some
// error checking!
unsigned * ptr = &local;
xQueueSend(shared_queue, &ptr, 0);
while (1) {
do_stuff();
}
}
void task_b() {
// This task wants to use task_a's 'local'.
// Real code would do error checking!
unsigned * ptr;
xQueueReceive(shared_queue, &ptr, portMAX_DELAY);
while (1) {
// Just increment the shared variable
(*ptr)++;
}
}