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#  $Id: README,v 1.2 2001-09-27 12:01:46 chris Exp $

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Introduction

============

This directory contains a stack bounds checker.  It provides two

primary features:

   + check for stack overflow at each context switch

   + provides an educated guess at each task's stack usage

Enabling

========

Add the stack checker extension to the initial user extension set.

If using confdefs.h to build your configuration table, this is

as simple as adding -DSTACK_CHECK_ON to the gcc command line which

compiles the file defining the configuration table.  In the RTEMS

test suites and samples, this is always init.c

Background

==========

The stack overflow check at context switch works by looking for

a 16 byte pattern at the logical end of the stack to be corrupted.

The "guesser" assumes that the entire stack was prefilled with a known

pattern and assumes that the pattern is still in place if the memory

has not been used as a stack.

Both of these can be fooled by pushing large holes onto the stack

and not writing to them... or (much more unlikely) writing the

magic patterns into memory.

This code has not been extensively tested.  It is provided as a tool

for RTEMS users to catch the most common mistake in multitasking

systems ... too little stack space.  Suggestions and comments are appreciated.

NOTES:

1.  Stack usage information is questionable on CPUs which push

    large holes on stack.

2.  The stack checker has a tendency to generate a fault when

    trying to print the helpful diagnostic message.  If it comes

    out, congratulations. If not, then the variable Stack_check_Blown_task

    contains a pointer to the TCB of the offending task.  This

    is usually enough to go on.

FUTURE:

1.  Determine how/if gcc will generate stack probe calls and support that.

2.  Get accurate stack usage numbers on i960.. it pushes very large

    holes on the stack.