How to define a stack for each task on an ARM - c

Question

Today I have a little proble and I think that the origin is about the stack.
This is my problem :
I declare three user task like this :
void task1Function(void) {
print_uart0("-usertask : First task is started...\r\n");
while(1){
//syscall(1);
}
}
void task2Function(void) {
print_uart0("-usertask : Second task is running...\r\n");
//syscall(); /* To return in the kernel's mode */
while(1){
}
}
void task3Function(void) {
print_uart0("-usertask : Third task is running...\r\n");
//syscall(); /* To return in the kernel's mode */
while(1){
}
}
I have an array of three task whose the strucure is below :
typedef struct
{
unsigned int *sp;
unsigned int registers[12];
unsigned int lr;
unsigned int pc;
unsigned int cpsr;
unsigned int mode;
unsigned int num;
void *stack;
int stacksize;
int priority;
int state; /* Running, Ready, Waiting, Suspended */
/* Next and previous task in the queue */
struct taskstruct *qnext, *qprev;
}taskstruct;
Here the inialisation of my tasks :
void init_task(taskstruct * task, void (*function)(void) ){
task->sp = (unsigned int*)&function;
task->registers[0] = 0; // r0
task->registers[1] = 0; // r1
task->registers[2] = 0; // r2
task->registers[3] = 0; // r3
task->registers[4] = 0; // r4
task->registers[5] = 0; // r5
task->registers[6] = 0; // r6
task->registers[7] = 0; // r7
task->registers[8] = 0; // r8
task->registers[9] = 0; // r9
task->registers[10] = 0; // r10
task->registers[11] = 0; // r11
task->registers[12] = 0; // r12
task->lr = 0;
task->pc = 0;
task->cpsr = 0;
task->mode = 0x10;
}
init_task(&task[0],&task1Function);
init_task(&task[1],&task2Function);
init_task(&task[2],&task3Function);
But when I passed the task[0].sp to my activate function, it's always the last declared task that is launched (i.e the third):
.global activate
activate:
LDR r12, [r0]
/*STMFD sp!,{r1-r11,lr}*/
NOP
msr CPSR_c, #0x10 /* User mode with IRQ enabled and FIQ disabled*/
mov pc, r12
So I guess that I have a problem with my user stack and that I have to setup a different stack for each of them, am I right ? In this case, can somebody tell me how I have to proceed ?
Regards, Vincent

Answer 1

Yes, you are right.
You must have separate stacks for each task, since the state of the stack is part of the task's total state.
You could add something like
uint32_t stack[512];
to your taskstruct, and then set/restore the task pointer accordingly when switching tasks, of course. It's hard to provide enough detail in this context.

Answer 2

'Ok but how I can allocate the stack task ? ' - malloc it.
You can economize on mallocs by adding the stack space on the end of the taskstruct - all your current members are fixed-size, so calculating the total size to malloc is easy.
The hard bit with these taskers is getting the interrupt entry correct, ie. when an interrupt handler needs to change a task state and so has to exit via the scheduler. This is especially good fun with nested interrupts. Good luck with that!