Kinetis Microcontrollers中的主题 How to access the stack pointer from C?

How to access the stack pointer from C?

laszlomonda — Tue, 18 Apr 2017 20:16:11 GMT

Hi there,

It was trivial to access the stack pointer from C for AVRs. The following code worked:

void __attribute__ ((__section__ (".init3"), __naked__)) move_sp(void) {
SP -= sizeof(wormhole_t);
message_ptr = (wormhole_t*)(SP + 1);
}

How can I access the stack pointer for the MK22FN512VLH12?

Thanks in advance!

- Laci

Re: How to access the stack pointer from C?

BlackNight — Wed, 19 Apr 2017 05:31:47 GMT

Hi Laci,

I'm using this:

void* KIN1_GetPC(void)
{
#ifdef __GNUC__
  void *pc;

  __asm__ __volatile__ ("mov %0, pc" : "=r"(pc));
  return pc;
#else
  #warning "only for GCC"
  return NULL;
#endif
}
‍‍‍‍‍‍‍‍‍‍‍‍

I hope this helps,

Erich

Re: How to access the stack pointer from C?

mjbcswitzerland — Wed, 19 Apr 2017 11:53:44 GMT

Compiler independent C-only version for Cortex:

void *fnGetSP(void)
{
volatile unsigned long var = 0;
return (void *)((unsigned long)&var + 4);
}

If the variable is not set to 0 the code will generally be in-lined and result in the compiler directly generating a single instruction similar to
ADD R0, SP, #0x04

which is the SP value but I don't trust that because it may be slightly off when it optimises out the temporary variable (and the +4 may not be needed - making it compiler/optimisation dependent - although still accurate to one stack pointer location).

This code does generate a warning though because it uses the address of a temporary variable...

Regards

Mark

Re: How to access the stack pointer from C?

laszlomonda — Thu, 20 Apr 2017 15:15:09 GMT

Thanks Erich,

Am I right that your function returns the PC, not the SP? Should I simply replace pc with sp in the assembly statement to adapt your function?

- Laci

Re: How to access the stack pointer from C?

laszlomonda — Thu, 20 Apr 2017 15:16:33 GMT

Thanks Mark,

I love the simplicity of your solution, but I'm not in peace with the warning. Is there any way to disable this warning for this function?

Re: How to access the stack pointer from C?

mjbcswitzerland — Thu, 20 Apr 2017 15:27:48 GMT

Warning need to be disabled according to the compiler use - maybe a pragma can be added to quieten it.

Regards

Mark

Re: How to access the stack pointer from C?

bobpaddock — Mon, 24 Apr 2017 12:30:06 GMT

Here is GCC example of how to selectively disable a warning per function.

The word 'diagnostic' may need to be replaced with 'error'.

Note that there are some warnings and errors that simply refuse to be turned off by pragma.

Using the GNU Compiler Collection (GCC): Diagnostic Pragmas

void power_off( void )
{

USART_Shutdown();

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Winline"

POWER_OFF();

#pragma GCC diagnostic pop
}

Re: How to access the stack pointer from C?

laszlomonda — Mon, 01 May 2017 19:14:35 GMT

BlackNight‌,

Your original suggestion contained pc, not sp. Can you confirm that the following is correct?

void* getSP(void)
{
void *sp;
__asm__ __volatile__ ("mov %0, sp" : "=r"(sp));
return sp;
}

Alternatively, the following has also been suggested to me. I'm wondering whether it's also correct:

void* getSP(void)
{
void *sp;
__asm__ __volatile__ ("mrs %0, msp" : "=r"(sp));
return sp;
}

Thank you!

- Laci

Re: How to access the stack pointer from C?

BlackNight — Tue, 02 May 2017 12:43:25 GMT

Hi Laci,

yes, this is what I'm using for getting the PC too.

Erich

Re: How to access the stack pointer from C?

laszlomonda — Thu, 04 May 2017 02:44:41 GMT

BlackNight‌,

But which one? :smileyhappy: The 1st or the 2nd code code snippet that I pasted?

Thanks!

- Laci

Re: How to access the stack pointer from C?

BlackNight — Thu, 04 May 2017 05:26:17 GMT

Hi Laci,

simply try them out? :-).

I'm using the following:

void* KIN1_GetPC(void)
{
  void *pc;

  __asm__ __volatile__ ("mov %0, pc" : "=r"(pc));
  return pc;
}
‍‍‍‍‍‍‍

Erich

Re: How to access the stack pointer from C?

laszlomonda — Fri, 05 May 2017 15:10:57 GMT

Thank you, Erich!

Usually, I can try suggested solutions, but in this case I wasn't even sure what should be the value of SP. I will make sure to use your version.

Thanks again!

- Laci