source: rtems-docs/c_user/stack_bounds_checker.rst @ d389819

Stack Bounds Checker
####################

Introduction
============

The stack bounds checker is an RTEMS support component that determines
if a task has overrun its run-time stack.  The routines provided
by the stack bounds checker manager are:

- ``rtems_stack_checker_is_blown`` - Has the Current Task Blown its Stack

- ``rtems_stack_checker_report_usage`` - Report Task Stack Usage

Background
==========

Task Stack
----------

Each task in a system has a fixed size stack associated with it.  This
stack is allocated when the task is created.  As the task executes, the
stack is used to contain parameters, return addresses, saved registers,
and local variables.  The amount of stack space required by a task
is dependent on the exact set of routines used.  The peak stack usage
reflects the worst case of subroutine pushing information on the stack.
For example, if a subroutine allocates a local buffer of 1024 bytes, then
this data must be accounted for in the stack of every task that invokes that
routine.

Recursive routines make calculating peak stack usage difficult, if not
impossible.  Each call to the recursive routine consumes *n* bytes
of stack space.  If the routine recursives 1000 times, then ``1000 * *n*`` bytes of stack space are required.

Execution
---------

The stack bounds checker operates as a set of task extensions.  At
task creation time, the task's stack is filled with a pattern to
indicate the stack is unused.  As the task executes, it will overwrite
this pattern in memory.  At each task switch, the stack bounds checker's
task switch extension is executed.  This extension checks that:

- the last ``n`` bytes of the task's stack have
  not been overwritten.  If this pattern has been damaged, it
  indicates that at some point since this task was context
  switch to the CPU, it has used too much stack space.

- the current stack pointer of the task is not within
  the address range allocated for use as the task's stack.

If either of these conditions is detected, then a blown stack
error is reported using the ``printk`` routine.

The number of bytes checked for an overwrite is processor family dependent.
The minimum stack frame per subroutine call varies widely between processor
families.  On CISC families like the Motorola MC68xxx and Intel ix86, all
that is needed is a return address.  On more complex RISC processors,
the minimum stack frame per subroutine call may include space to save
a significant number of registers.

Another processor dependent feature that must be taken into account by
the stack bounds checker is the direction that the stack grows.  On some
processor families, the stack grows up or to higher addresses as the
task executes.  On other families, it grows down to lower addresses.  The
stack bounds checker implementation uses the stack description definitions
provided by every RTEMS port to get for this information.

Operations
==========

Initializing the Stack Bounds Checker
-------------------------------------

The stack checker is initialized automatically when its task
create extension runs for the first time.

The application must include the stack bounds checker extension set
in its set of Initial Extensions.  This set of extensions is
defined as ``STACK_CHECKER_EXTENSION``.  If using ``<rtems/confdefs.h>``
for Configuration Table generation, then all that is necessary is
to define the macro ``CONFIGURE_STACK_CHECKER_ENABLED`` before including``<rtems/confdefs.h>`` as shown below:
.. code:: c

    #define CONFIGURE_STACK_CHECKER_ENABLED
    ...
    #include <rtems/confdefs.h>

Checking for Blown Task Stack
-----------------------------

The application may check whether the stack pointer of currently
executing task is within proper bounds at any time by calling
the ``rtems_stack_checker_is_blown`` method.  This
method return ``FALSE`` if the task is operating within its
stack bounds and has not damaged its pattern area.

Reporting Task Stack Usage
--------------------------

The application may dynamically report the stack usage for every task
in the system by calling the``rtems_stack_checker_report_usage`` routine.
This routine prints a table with the peak usage and stack size of
every task in the system.  The following is an example of the
report generated:
.. code:: c

    ID      NAME       LOW        HIGH     AVAILABLE      USED
    0x04010001  IDLE  0x003e8a60  0x003e9667       2952        200
    0x08010002  TA1   0x003e5750  0x003e7b57       9096       1168
    0x08010003  TA2   0x003e31c8  0x003e55cf       9096       1168
    0x08010004  TA3   0x003e0c40  0x003e3047       9096       1104
    0xffffffff  INTR  0x003ecfc0  0x003effbf      12160        128

Notice the last time.  The task id is 0xffffffff and its name is "INTR".
This is not actually a task, it is the interrupt stack.

When a Task Overflows the Stack
-------------------------------

When the stack bounds checker determines that a stack overflow has occurred,
it will attempt to print a message using ``printk`` identifying the
task and then shut the system down.  If the stack overflow has caused
corruption, then it is possible that the message cannot be printed.

The following is an example of the output generated:
.. code:: c

    BLOWN STACK!!! Offending task(0x3eb360): id=0x08010002; name=0x54413120
    stack covers range 0x003e5750 - 0x003e7b57 (9224 bytes)
    Damaged pattern begins at 0x003e5758 and is 128 bytes long

The above includes the task id and a pointer to the task control block as
well as enough information so one can look at the task's stack and
see what was happening.

Routines
========

This section details the stack bounds checker's routines.
A subsection is dedicated to each of routines
and describes the calling sequence, related constants, usage,
and status codes.

.. COMMENT: rtems_stack_checker_is_blown

STACK_CHECKER_IS_BLOWN - Has Current Task Blown Its Stack
---------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    bool rtems_stack_checker_is_blown( void );

**STATUS CODES:**

``TRUE`` - Stack is operating within its stack limits
``FALSE`` - Current stack pointer is outside allocated area

**DESCRIPTION:**

This method is used to determine if the current stack pointer
of the currently executing task is within bounds.

**NOTES:**

This method checks the current stack pointer against
the high and low addresses of the stack memory allocated when
the task was created and it looks for damage to the high water
mark pattern for the worst case usage of the task being called.

STACK_CHECKER_REPORT_USAGE - Report Task Stack Usage
----------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    void rtems_stack_checker_report_usage( void );

**STATUS CODES: NONE**

**DESCRIPTION:**

This routine prints a table with the peak stack usage and stack space
allocation of every task in the system.

**NOTES:**

NONE

.. COMMENT: COPYRIGHT (c) 1988-2007.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.