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README.html
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README.html
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<body>
<h1>Worst Case Stack</h1>
<h2>Overview</h2>
<p>This program is used to do static stack analysis on C source code to determine the maximum stack space used by each function. Source must be compiled using gcc with a number of special flags (see below for details)</p>
<h2>Useage</h2>
<ol>
<li>compile *.c sources using gcc and the flags <code>-c</code>, <code>-fdump-rtl-dfinish</code> and <code>-fstack-usage</code>
</li>
<li>Run the script python wcs.py</li>
</ol>
<pre><code>gcc -c -fdump-rtl-dfinish -fstack-usage main.c my_library.c other.c
wcs.py
</code></pre>
<p><strong>Note:</strong> When running <code>wcs.py</code> the current working directory must contain all the relevant input files.</p>
<h2>Dependencies</h2>
<p>This script requires python 3. It was written and tested using version 3.4.3</p>
<h2>Inputs - Files from gcc.</h2>
<p>The script will search the current directory for sets of files with the names <code><name>.o</code>, <code><name>.su</code> and <code><name>.c.270r.dfinish</code>. If all three are found the script will calculate the worst case stack for every function in the <code><name>.c</code>.</p>
<p>See the usage section for information about how to generate these files.</p>
<h2>Inputs - Manual Stack Usage Files</h2>
<p>The scripts also look in files ending with *.msu. These files should contain a whitespace delimited table with function names in the first column, and a decimal integer with the worst case stack usage in the second line </p>
<pre><code>my_function 20
do_something 120
__exit 144
</code></pre>
<p>Every line must contain a function / stack pair (empty lines are not permitted). These files can be useful for specifying the worst case stack for functions for which the c-source is not available but the stack usage is known by other means such as inspecting the assembly or run-time testing.</p>
<h2>Output</h2>
<p>The script will output a list of functions in a table with the following columns:</p>
<ol>
<li>
<strong>Translation Unit</strong> (e.g. the name of the file where the function is implemented)</li>
<li><strong>Function Name</strong></li>
<li>
<strong>Stack</strong> Either the maximum number of bytes during a call to this function (including nested calls at all depths)
or the string <code>unbounded</code> if the maximum cannot be determined because some function in the call tree is recursively defined or makes calls via function pointer.</li>
<li>
<strong>Unresolved Dependancies</strong> A list functions that are called somewhere in the call tree for which there is no
definition in any of the given input files.</li>
</ol>
<h2>Known Limitations:</h2>
<ol>
<li>wcs.py can only determine stack usage from <code>*.c</code> source. Calls to compiled libraries (e.g. libc.a) or to assembly functions will result in <code>unbounded</code> (e.g. unknown) stack usage.</li>
<li>Functions compiled with the <code>weak</code> attribute will crash this script</li>
<li>The actual worst case stack may be greater than reported by this function if outside actors modify the stack. Common offenders are:
<ol>
<li>Interrupt handlers</li>
<li>Operating system context changes</li>
</ol>
</li>
<li>The use of inline assembly will result in potentially incorrect results. Specifically, if a function uses inline assembly to load or store from the stack, modify the stack pointer, or branch to code that does likewise, expect incorrect results.<br>
</li>
</ol>
<p><strong>The script has no way to detect situations 3 and 4. In the presence of these conditions the script will still complete successfully. Use caution.</strong></p>
<h2>Notes</h2>
<ol>
<li>This script assumes little endian byte ordering for object files. If you are compiling for a big-endian system set the <code>byte_order</code> variable to <code>">"</code> in the file <code>elf.py</code>
</li>
</ol>
<pre><code>byte_order = ">"
</code></pre>
</body>
</html>
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