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<section id="nep-45-c-style-guide">
<span id="nep45"></span><h1>NEP 45 — C style guide<a class="headerlink" href="#nep-45-c-style-guide" title="Link to this heading">#</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author<span class="colon">:</span></dt>
<dd class="field-odd"><p>Charles Harris <<a class="reference external" href="mailto:charlesr.harris%40gmail.com">charlesr<span>.</span>harris<span>@</span>gmail<span>.</span>com</a>></p>
</dd>
<dt class="field-even">Status<span class="colon">:</span></dt>
<dd class="field-even"><p>Active</p>
</dd>
<dt class="field-odd">Type<span class="colon">:</span></dt>
<dd class="field-odd"><p>Process</p>
</dd>
<dt class="field-even">Created<span class="colon">:</span></dt>
<dd class="field-even"><p>2012-02-26</p>
</dd>
<dt class="field-odd">Resolution<span class="colon">:</span></dt>
<dd class="field-odd"><p><a class="github reference external" href="https://github.com/numpy/numpy/issues/11911">numpy/numpy#11911</a></p>
</dd>
</dl>
<section id="abstract">
<h2>Abstract<a class="headerlink" href="#abstract" title="Link to this heading">#</a></h2>
<p>This document gives coding conventions for the C code comprising
the C implementation of NumPy.</p>
</section>
<section id="motivation-and-scope">
<h2>Motivation and scope<a class="headerlink" href="#motivation-and-scope" title="Link to this heading">#</a></h2>
<p>The NumPy C coding conventions are based on Python
<a class="reference external" href="https://www.python.org/dev/peps/pep-0007">PEP 7 – Style Guide for C Code</a>
by Guido van Rossum with a few added strictures.</p>
<p>Because the NumPy conventions are very close to those in PEP 7, that PEP is
used as a template with the NumPy additions and variations in the appropriate
spots.</p>
</section>
<section id="usage-and-impact">
<h2>Usage and impact<a class="headerlink" href="#usage-and-impact" title="Link to this heading">#</a></h2>
<p>There are many C coding conventions and it must be emphasized that the primary
goal of the NumPy conventions isn’t to choose the “best,” about which there is
certain to be disagreement, but to achieve uniformity.</p>
<p>Two good reasons to break a particular rule:</p>
<ol class="arabic simple">
<li><p>When applying the rule would make the code less readable, even
for someone who is used to reading code that follows the rules.</p></li>
<li><p>To be consistent with surrounding code that also breaks it
(maybe for historic reasons) – although this is also an
opportunity to clean up someone else’s mess.</p></li>
</ol>
</section>
<section id="backward-compatibility">
<h2>Backward compatibility<a class="headerlink" href="#backward-compatibility" title="Link to this heading">#</a></h2>
<p>No impact.</p>
</section>
<section id="detailed-description">
<h2>Detailed description<a class="headerlink" href="#detailed-description" title="Link to this heading">#</a></h2>
<section id="c-dialect">
<h3>C dialect<a class="headerlink" href="#c-dialect" title="Link to this heading">#</a></h3>
<ul>
<li><p>Use C99 (that is, the standard defined by ISO/IEC 9899:1999).</p></li>
<li><p>Don’t use GCC extensions (for instance, don’t write multi-line strings
without trailing backslashes). Preferably break long strings
up onto separate lines like so:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="s">"blah blah"</span>
<span class="s">"blah blah"</span>
</pre></div>
</div>
<p>This will work with MSVC, which otherwise chokes on very long
strings.</p>
</li>
<li><p>All function declarations and definitions must use full prototypes (that is,
specify the types of all arguments).</p></li>
<li><p>No compiler warnings with major compilers (gcc, VC++, a few others).</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>NumPy still produces compiler warnings that need to be addressed.</p>
</div>
</section>
<section id="code-layout">
<h3>Code layout<a class="headerlink" href="#code-layout" title="Link to this heading">#</a></h3>
<ul>
<li><p>Use 4-space indents and no tabs at all.</p></li>
<li><p>No line should be longer than 80 characters. If this and the
previous rule together don’t give you enough room to code, your code is
too complicated – consider using subroutines.</p></li>
<li><p>No line should end in whitespace. If you think you need
significant trailing whitespace, think again; somebody’s editor might
delete it as a matter of routine.</p></li>
<li><p>Function definition style: function name in column 1, outermost
curly braces in column 1, blank line after local variable declarations:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span><span class="w"> </span><span class="kt">int</span>
<span class="nf">extra_ivars</span><span class="p">(</span><span class="n">PyTypeObject</span><span class="w"> </span><span class="o">*</span><span class="n">type</span><span class="p">,</span><span class="w"> </span><span class="n">PyTypeObject</span><span class="w"> </span><span class="o">*</span><span class="n">base</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">t_size</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PyType_BASICSIZE</span><span class="p">(</span><span class="n">type</span><span class="p">);</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">b_size</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PyType_BASICSIZE</span><span class="p">(</span><span class="n">base</span><span class="p">);</span>
<span class="w"> </span><span class="n">assert</span><span class="p">(</span><span class="n">t_size</span><span class="w"> </span><span class="o">>=</span><span class="w"> </span><span class="n">b_size</span><span class="p">);</span><span class="w"> </span><span class="cm">/* type smaller than base! */</span>
<span class="w"> </span><span class="p">...</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>If the transition to C++ goes through it is possible that this form will
be relaxed so that short class methods meant to be inlined can have the
return type on the same line as the function name. However, that is yet to
be determined.</p>
</li>
<li><p>Code structure: one space between keywords like <code class="docutils literal notranslate"><span class="pre">if</span></code>, <code class="docutils literal notranslate"><span class="pre">for</span></code> and
the following left parenthesis; no spaces inside the parenthesis; braces
around all <code class="docutils literal notranslate"><span class="pre">if</span></code> branches, and no statements on the same line as the
<code class="docutils literal notranslate"><span class="pre">if</span></code>. They should be formatted as shown:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">mro</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="nb">NULL</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">one_line_statement</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">...</span>
<span class="p">}</span>
<span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">n</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">one_line_statement</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">isstuff</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">dostuff</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">do</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">stuff</span><span class="p">;</span>
<span class="p">}</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">isstuff</span><span class="p">);</span>
<span class="k">switch</span><span class="w"> </span><span class="p">(</span><span class="n">kind</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="cm">/* Boolean kind */</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="sc">'b'</span><span class="p">:</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="cm">/* Unsigned int kind */</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="sc">'u'</span><span class="p">:</span>
<span class="w"> </span><span class="p">...</span>
<span class="w"> </span><span class="cm">/* Anything else */</span>
<span class="w"> </span><span class="k">default</span><span class="o">:</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">3</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p>The return statement should <em>not</em> get redundant parentheses:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">return</span><span class="w"> </span><span class="n">Py_None</span><span class="p">;</span><span class="w"> </span><span class="cm">/* correct */</span>
<span class="k">return</span><span class="p">(</span><span class="n">Py_None</span><span class="p">);</span><span class="w"> </span><span class="cm">/* incorrect */</span>
</pre></div>
</div>
</li>
<li><p>Function and macro call style: <code class="docutils literal notranslate"><span class="pre">foo(a,</span> <span class="pre">b,</span> <span class="pre">c)</span></code>, no space before
the open paren, no spaces inside the parens, no spaces before
commas, one space after each comma.</p></li>
<li><p>Always put spaces around the assignment, Boolean, and comparison
operators. In expressions using a lot of operators, add spaces
around the outermost (lowest priority) operators.</p></li>
<li><p>Breaking long lines: If you can, break after commas in the
outermost argument list. Always indent continuation lines
appropriately:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyErr_SetString</span><span class="p">(</span><span class="n">PyExc_TypeError</span><span class="p">,</span>
<span class="w"> </span><span class="s">"Oh dear, you messed up."</span><span class="p">);</span>
</pre></div>
</div>
<p>Here appropriately means at least a double indent (8 spaces). It isn’t
necessary to line everything up with the opening parenthesis of the function
call.</p>
</li>
<li><p>When you break a long expression at a binary operator, the
operator goes at the end of the previous line, for example:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">type</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="n">tp_dictoffset</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">&&</span>
<span class="w"> </span><span class="n">base</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="n">tp_dictoffset</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">&&</span>
<span class="w"> </span><span class="n">type</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="n">tp_dictoffset</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">b_size</span><span class="w"> </span><span class="o">&&</span>
<span class="w"> </span><span class="p">(</span><span class="kt">size_t</span><span class="p">)</span><span class="n">t_size</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">b_size</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">PyObject</span><span class="w"> </span><span class="o">*</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Note that the terms in the multi-line Boolean expression are indented so
as to make the beginning of the code block clearly visible.</p>
</li>
<li><p>Put blank lines around functions, structure definitions, and
major sections inside functions.</p></li>
<li><p>Comments go before the code they describe. Multi-line comments should
be like so:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cm">/*</span>
<span class="cm"> * This would be a long</span>
<span class="cm"> * explanatory comment.</span>
<span class="cm"> */</span>
</pre></div>
</div>
<p>Trailing comments should be used sparingly. Instead of</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">yes</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// Success!</span>
</pre></div>
</div>
<p>do</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">yes</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// Success!</span>
</pre></div>
</div>
</li>
<li><p>All functions and global variables should be declared static
when they aren’t needed outside the current compilation unit.</p></li>
<li><p>Declare external functions and variables in a header file.</p></li>
</ul>
</section>
<section id="naming-conventions">
<h3>Naming conventions<a class="headerlink" href="#naming-conventions" title="Link to this heading">#</a></h3>
<ul class="simple">
<li><p>There has been no consistent prefix for NumPy public functions, but
they all begin with a prefix of some sort, followed by an underscore, and
are in camel case: <code class="docutils literal notranslate"><span class="pre">PyArray_DescrAlignConverter</span></code>, <code class="docutils literal notranslate"><span class="pre">NpyIter_GetIterNext</span></code>.
In the future the names should be of the form <code class="docutils literal notranslate"><span class="pre">Npy*_PublicFunction</span></code>,
where the star is something appropriate.</p></li>
<li><p>Public Macros should have a <code class="docutils literal notranslate"><span class="pre">NPY_</span></code> prefix and then use upper case,
for example, <code class="docutils literal notranslate"><span class="pre">NPY_DOUBLE</span></code>.</p></li>
<li><p>Private functions should be lower case with underscores, for example:
<code class="docutils literal notranslate"><span class="pre">array_real_get</span></code>. Single leading underscores should not be used, but
some current function names violate that rule due to historical accident.</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Functions whose names begin with a single underscore should be renamed at
some point.</p>
</div>
</section>
<section id="function-documentation">
<h3>Function documentation<a class="headerlink" href="#function-documentation" title="Link to this heading">#</a></h3>
<p>NumPy doesn’t have a C function documentation standard at this time, but
needs one. Most NumPy functions are not documented in the code, and that
should change. One possibility is Doxygen with a plugin so that the same
NumPy style used for Python functions can also be used for documenting
C functions, see the files in <code class="docutils literal notranslate"><span class="pre">doc/cdoc/</span></code>.</p>
</section>
</section>
<section id="related-work">
<h2>Related work<a class="headerlink" href="#related-work" title="Link to this heading">#</a></h2>
<p>Based on Van Rossum and Warsaw, <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0007/"><strong>PEP 7</strong></a></p>
</section>
<section id="discussion">
<h2>Discussion<a class="headerlink" href="#discussion" title="Link to this heading">#</a></h2>
<p><a class="github reference external" href="https://github.com/numpy/numpy/issues/11911">numpy/numpy#11911</a>
recommended that this proposal, which originated as <code class="docutils literal notranslate"><span class="pre">doc/C_STYLE_GUIDE.rst</span></code>,
be turned into an NEP.</p>
</section>
<section id="copyright">
<h2>Copyright<a class="headerlink" href="#copyright" title="Link to this heading">#</a></h2>
<p>This document has been placed in the public domain.</p>
</section>
</section>
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