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<section class="tex2jax_ignore mathjax_ignore" id="petsc-style-and-usage-guide">
<span id="style"></span><h1>PETSc Style and Usage Guide<a class="headerlink" href="#petsc-style-and-usage-guide" title="Link to this heading">#</a></h1>
<p>The PETSc team uses certain conventions to make the source code
consistent and easier to maintain. We will interchangeably use the
terminology <em>subclass</em>, <em>implementation</em>, or <em>type</em> <a class="footnote-reference brackets" href="#footnote-1" id="id1" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a> to refer to a
concrete realization of an abstract base class. For example,
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSPGMRES.html">KSPGMRES</a></span></code> is a type for the base class <code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSP.html">KSP</a></span></code>.</p>
<section id="names">
<h2>Names<a class="headerlink" href="#names" title="Link to this heading">#</a></h2>
<p>Consistency of names for variables, functions, and so on is extremely
important. We use several conventions</p>
<ol class="arabic">
<li><p>All function names and enum types consist of acronyms or words, each
of which is capitalized, for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSPSolve.html">KSPSolve</a>()</span></code> and
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/MatGraphOperations/MatGetOrdering.html">MatGetOrdering</a>()</span></code>.</p></li>
<li><p>All enum elements and macro variables are named with all capital
letters. When they consist of several complete words, there is an
underscore between each word. For example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/MatAssemblyType.html">MAT_FINAL_ASSEMBLY</a></span></code>.</p></li>
<li><p>Functions that are private to PETSc (not callable by the application
code) either</p>
<ul class="simple">
<li><p>have an appended <code class="docutils notranslate"><span class="pre">_Private</span></code> (for example, <code class="docutils notranslate"><span class="pre">StashValues_Private</span></code>)
or</p></li>
<li><p>have an appended <code class="docutils notranslate"><span class="pre">_Subtype</span></code> (for example, <code class="docutils notranslate"><span class="pre">MatMultSeq_AIJ</span></code>).</p></li>
</ul>
<p>In addition, functions that are not intended for use outside of a
particular file are declared <code class="docutils notranslate"><span class="pre">static</span></code>. Also, see the item
on symbol visibility in <a class="reference internal" href="#usage-of-petsc-functions-and-macros"><span class="std std-ref">Usage of PETSc Functions and Macros</span></a>.</p>
</li>
<li><p>Function names in structures (for example, <code class="docutils notranslate"><span class="pre">_matops</span></code>) are the same
as the base application function name without the object prefix and
in lowercase. For example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/MatMultTranspose.html">MatMultTranspose</a>()</span></code> has a
structure name of <code class="docutils notranslate"><span class="pre">multtranspose</span></code>.</p></li>
<li><p>Names of implementations of class functions should begin with the
function name, an underscore, and the name of the implementation, for
example, <code class="docutils notranslate"><span class="pre">KSPSolve_GMRES()</span></code>.</p></li>
<li><p>Each application-usable function begins with the name of the class
object, followed by any subclass name, for example,
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/IS/ISInvertPermutation.html">ISInvertPermutation</a>()</span></code>, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/MatMult.html">MatMult</a>()</span></code>, or
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSPGMRESSetRestart.html">KSPGMRESSetRestart</a>()</span></code>.</p></li>
<li><p>Functions that PETSc provides as defaults for user-providable
functions end with <code class="docutils notranslate"><span class="pre">Default</span></code> (for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscSignalHandlerDefault.html">PetscSignalHandlerDefault</a>()</span></code>).</p></li>
<li><p>Options database keys are lower case, have an underscore between
words, and match the function name associated with the option without
the word “set” or “get”, for example, <code class="docutils notranslate"><span class="pre">-ksp_gmres_restart</span></code>.</p></li>
<li><p>Specific <code class="docutils notranslate"><span class="pre">XXXType</span></code> values (for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/MATSEQAIJ.html">MATSEQAIJ</a></span></code>) do not have
an underscore in them unless they refer to another package that uses
an underscore, for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/MATSOLVERSUPERLU_DIST.html">MATSOLVERSUPERLU_DIST</a></span></code>.</p></li>
<li><p>Typedefs for functions should end in <code class="docutils notranslate"><span class="pre">Fn</span></code> as in, for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/SNES/SNESFunctionFn.html">SNESFunctionFn</a></span></code>.</p></li>
</ol>
</section>
<section id="petsc-and-standard-datatypes">
<span id="stylepetsccount"></span><h2>PETSc and standard datatypes<a class="headerlink" href="#petsc-and-standard-datatypes" title="Link to this heading">#</a></h2>
<ol class="arabic">
<li><p><code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code> is generally used for array indices, and array lengths. It
is a signed 32-bit or 64-bit <code class="docutils notranslate"><span class="pre">int</span></code> depending on the <code class="docutils notranslate"><span class="pre">./configure</span></code> option
<code class="docutils notranslate"><span class="pre">--with-64-bit-indices</span></code>. There is the possibility of integer overflow with the
32-bit version.</p></li>
<li><p><code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> is <code class="docutils notranslate"><span class="pre">ptrdiff_t</span></code>, (on 64-bit systems it is 64-bits) and should be used for array sizes (number of entries)
and indices that may become large. <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscIntCast.html">PetscIntCast</a>()</span></code> should always be used when converting
to <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code> from <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code>. Since, in most configurations an array of <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> requires twice the memory
of an array of <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code> most index arrays (such as <code class="docutils notranslate"><span class="pre"><a href="../manualpages/IS/ISGetIndices.html">ISGetIndices</a>()</span></code> use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code>,
when these arrays get too large then <code class="docutils notranslate"><span class="pre">--with-64-bit-indices</span></code> must be used to
<code class="docutils notranslate"><span class="pre">./configure</span></code> PETSc. In most cases it is appropriate to use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> in lieu of <code class="docutils notranslate"><span class="pre">PetscInt64</span></code>.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">size_t</span></code> is used for variables that contain the amount of memory, generally in bytes.
It should <strong>not</strong> be used for the number of
entries in an array, or to index into an array, that should be <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code>, or <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code>.
Though <code class="docutils notranslate"><span class="pre">size_t</span></code> is unsigned and hence can have values larger then those that can be stored
in a <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> those sizes will never be reached in practice so it is ok to cast with <code class="docutils notranslate"><span class="pre">(<a href="../manualpages/Sys/PetscCount.html">PetscCount</a>)</span></code>
from a <code class="docutils notranslate"><span class="pre">size_t</span></code> variable to a <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> variable, but <strong>not</strong> a <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code>.
One should not blindly cast from a <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> or a <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code>
to <code class="docutils notranslate"><span class="pre">size_t</span></code> since, when the value is negative, it will produce garbage.</p></li>
<li><p><strong>Never</strong> blindly put in a cast from a potentially longer (in number of bits) to a shorter integer such as</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PetscInt64</span><span class="w"> </span><span class="n">a</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">b</span>
<span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="p">)</span><span class="n">a</span>
</pre></div>
</div>
<p>simply to prevent a compiler warning. Use the appropriate PETSc cast function unless you
absolutely know the value will fit in the lesser number of bits.</p>
</li>
<li><p>MPI 4.0 supports the use of <code class="docutils notranslate"><span class="pre">MPI_Count</span></code> (large count) for many MPI functions that previously used <code class="docutils notranslate"><span class="pre">int</span></code> in a new API where the MPI function
names end in <code class="docutils notranslate"><span class="pre">_c</span></code>. Since not all installed MPI implementations have such support, use <code class="docutils notranslate"><span class="pre">MPIU_XXX()</span></code> routines
that use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span></code> for count arguments and use the large count MPI versions when possible.
When not possible they first check the size of the input count arguments and error if they
will not fit in the MPI required <code class="docutils notranslate"><span class="pre">int</span></code>, if they fit then the standard MPI functions are automatically called.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">sizeof</span><span class="p">(</span><span class="n">MPI_Count</span><span class="p">)</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"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span><span class="p">)</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"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="p">)</span>
<span class="k">sizeof</span><span class="p">(</span><span class="n">PetscInt64</span><span class="p">)</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"><a href="../manualpages/Sys/PetscCount.html">PetscCount</a></span><span class="p">)</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"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="p">)</span>
<span class="k">sizeof</span><span class="p">(</span><span class="n">MPI_Count</span><span class="p">)</span><span class="w"> </span><span class="n">may</span><span class="w"> </span><span class="n">be</span><span class="w"> </span><span class="n">strictly</span><span class="w"> </span><span class="n">greater</span><span class="w"> </span><span class="n">than</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">PetscInt64</span><span class="p">)</span>
</pre></div>
</div>
</li>
</ol>
</section>
<section id="coding-conventions-and-style">
<h2>Coding Conventions and Style<a class="headerlink" href="#coding-conventions-and-style" title="Link to this heading">#</a></h2>
<p>Within the PETSc source code, we adhere to the following guidelines so
that the code is uniform and easily maintained.</p>
<section id="c-formatting">
<h3>C Formatting<a class="headerlink" href="#c-formatting" title="Link to this heading">#</a></h3>
<p>The <code class="docutils notranslate"><span class="pre">.clang-format</span></code> file in the PETSc root directory controls the white space and basic layout. You can run the formatter in the entire repository with <code class="docutils notranslate"><span class="pre">make</span> <span class="pre">clangformat</span></code>. All merge requests must be properly formatted; this is automatically checked for merge requests with <code class="docutils notranslate"><span class="pre">make</span> <span class="pre">checkclangformat</span></code>.</p>
<p>Even with the use of <code class="docutils notranslate"><span class="pre">clang-format</span></code> there are still many decisions about code formatting that must be constantly made. A subset of these is automatically checked for merge requests with <code class="docutils notranslate"><span class="pre">make</span> <span class="pre">checkbadSource</span></code>.</p>
<ol class="arabic">
<li><p>The prototypes for functions should not include the names of the
variables</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PETSC_EXTERN</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscErrorCode.html">PetscErrorCode</a></span><span class="w"> </span><span class="n">MyFunction</span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="p">);</span><span class="w"> </span><span class="c1">// Correct</span>
<span class="n">PETSC_EXTERN</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscErrorCode.html">PetscErrorCode</a></span><span class="w"> </span><span class="n">MyFunction</span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">myvalue</span><span class="p">);</span><span class="w"> </span><span class="c1">// Incorrect</span>
</pre></div>
</div>
</li>
<li><p>All local variables of a particular type (for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code>) should be listed
on the same line if possible; otherwise, they should be listed on adjacent lines. Note
that pointers of different arity (levels of indirection) are considered to be different types. <code class="docutils notranslate"><span class="pre">clang-format</span></code> automatically
handles the indenting shown below.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Correct</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">c</span><span class="p">;</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="o">*</span><span class="n">d</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">e</span><span class="p">;</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="o">**</span><span class="n">f</span><span class="p">;</span>
<span class="c1">// Incorrect</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">c</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">d</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">e</span><span class="p">,</span><span class="w"> </span><span class="o">**</span><span class="n">f</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><p>Local variables should be initialized in their declaration when possible</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Correct</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">11</span><span class="p">;</span>
<span class="n"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span><span class="p">;</span>
<span class="c1">// use a</span>
<span class="c1">// Incorrect</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">a</span><span class="p">;</span>
<span class="n"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span><span class="p">;</span>
<span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">11</span><span class="p">;</span>
<span class="c1">// use a</span>
</pre></div>
</div>
</li>
<li><p>All PETSc subroutine code blocks <em>must</em> start with a single blank line between the local variable
declarations followed by <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span></code>.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Correct</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="n"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span><span class="p">;</span>
<span class="c1">// Incorrect</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="n"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span><span class="p">;</span>
<span class="c1">// Incorrect</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="n">y</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">11</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><p>Functions in PETSc examples, including <code class="docutils notranslate"><span class="pre">main()</span></code> should have <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionBeginUser.html">PetscFunctionBeginUser</a></span></code> as the first line after the local variable declarations.</p></li>
<li><p>PETSc functions that begin <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span></code> must always return via <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionReturn.html">PetscFunctionReturn</a>()</span></code>, or <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionReturnVoid.html">PetscFunctionReturnVoid</a>()</span></code>, not <code class="docutils notranslate"><span class="pre">return</span></code>. If the function returns a <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscErrorCode.html">PetscErrorCode</a></span></code>, then it must always return with <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionReturn.html">PetscFunctionReturn</a>(<a href="../manualpages/Sys/PetscErrorCode.html">PETSC_SUCCESS</a>)</span></code>.</p></li>
<li><p>Functions that do use return should use <code class="docutils notranslate"><span class="pre">return</span> <span class="pre">xx;</span></code> rather than <code class="docutils notranslate"><span class="pre">return(xx);</span></code></p></li>
<li><p>All PETSc function calls must have their return value checked for errors using the
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCall.html">PetscCall</a>()</span></code> macro. This should be wrapped around the function in question.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n"><a href="../manualpages/Sys/PetscCall.html">PetscCall</a></span><span class="p">(</span><span class="n">MyFunction</span><span class="p">(...));</span><span class="w"> </span><span class="c1">// Correct</span>
<span class="n"><a href="../manualpages/Sys/PetscErrorCode.html">PetscErrorCode</a></span><span class="w"> </span><span class="n">ierr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MyFunction</span><span class="p">(...);</span><span class="n"><a href="../manualpages/Sys/PetscCall.html">PetscCall</a></span><span class="p">(</span><span class="n">ierr</span><span class="p">);</span><span class="w"> </span><span class="c1">// Incorrect</span>
</pre></div>
</div>
<p>The only exceptions to this rule are begin-end style macros which embed local variables
or loops as part of their expansion
(e.g. <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscOptionsBegin.html">PetscOptionsBegin</a>()</span></code>/<code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscOptionsEnd.html">PetscOptionsEnd</a>()</span></code>). These handle errors internally
and do not need error checking.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Correct</span>
<span class="n"><a href="../manualpages/Sys/PetscOptionsBegin.html">PetscOptionsBegin</a></span><span class="p">(...);</span>
<span class="n"><a href="../manualpages/Sys/PetscOptionsEnd.html">PetscOptionsEnd</a></span><span class="p">();</span>
</pre></div>
</div>
<p>As a rule, always try to wrap the function first; if this fails to compile, you do
not need to add the error checking.</p>
<p>Calls to external package functions are generally made with <code class="docutils notranslate"><span class="pre">PetscCallExternal()</span></code> or its variants that are specialized for particular packages, for example <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCallBLAS.html">PetscCallBLAS</a>()</span></code></p>
</li>
<li><p>Single operation <code class="docutils notranslate"><span class="pre">if</span></code> and <code class="docutils notranslate"><span class="pre">else</span></code> commands should not be wrapped in braces. They should be done as follows,</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="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">XXXX</span><span class="p">;</span>
<span class="k">else</span><span class="w"> </span><span class="n">YYY</span><span class="p">;</span>
</pre></div>
</div>
<p>not</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="w"> </span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="n">XXXX</span><span class="p">;}</span>
<span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="n">YYY</span><span class="p">;}</span>
</pre></div>
</div>
</li>
<li><p>Do not leave sections of commented-out code or dead source code protected with <code class="docutils notranslate"><span class="pre">ifdef</span> <span class="pre">foo</span></code> in the source files.</p></li>
<li><p>Use classic block comments (<code class="docutils notranslate"><span class="pre">/*</span> <span class="pre">There</span> <span class="pre">must</span> <span class="pre">be</span> <span class="pre">a</span> <span class="pre">space</span> <span class="pre">before</span> <span class="pre">the</span> <span class="pre">first</span> <span class="pre">word</span> <span class="pre">in</span> <span class="pre">the</span> <span class="pre">comment</span> <span class="pre">and</span> <span class="pre">a</span> <span class="pre">space</span> <span class="pre">at</span> <span class="pre">the</span> <span class="pre">end</span> <span class="pre">*/</span></code>,
(<code class="docutils notranslate"><span class="pre">/*Do</span> <span class="pre">not</span> <span class="pre">do</span> <span class="pre">this*/</span></code>) for multi-line comments, and <code class="docutils notranslate"><span class="pre">//</span> <span class="pre">Comment</span></code> for single-line comments in source files.</p></li>
<li><p>Do not put a <code class="docutils notranslate"><span class="pre">*</span></code> at the beginning or end of each line of a multi-line comment.</p></li>
<li><p>Do not use <code class="docutils notranslate"><span class="pre">/*</span> <span class="pre">----</span> <span class="pre">...</span> <span class="pre">-----</span> <span class="pre">*/</span></code> or similar constructs to separate parts of source code files.</p></li>
<li><p>Use appropriate grammar and spelling in the comments.</p></li>
<li><p>All variables must be declared at the beginning of the code block (C89
style), never mixed in with code. However, when variables are only used in a limited
scope, it is encouraged to declare them in that scope. 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">cond</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscScalar.html">PetscScalar</a></span><span class="w"> </span><span class="o">*</span><span class="n">tmp</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscCall.html">PetscCall</a></span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscMalloc1.html">PetscMalloc1</a></span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">tmp</span><span class="p">));</span>
<span class="w"> </span><span class="c1">// use tmp</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscCall.html">PetscCall</a></span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscFree.html">PetscFree</a></span><span class="p">(</span><span class="n">tmp</span><span class="p">));</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The only exception to this is variables used exclusively within a <code class="docutils notranslate"><span class="pre">for</span></code> loop, which must
be declared inside the loop initializer:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Correct</span>
<span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </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="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// loop body</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Correct, variable used outside of loop</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">i</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></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="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// loop body</span>
<span class="p">}</span>
<span class="n">j</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">i</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// Incorrect</span>
<span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">i</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="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// loop body</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p>Developers can use // to split very long lines when it improves code readability. For example</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">f</span><span class="p">[</span><span class="n">j</span><span class="p">][</span><span class="n">i</span><span class="p">].</span><span class="n">omega</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">xdot</span><span class="p">[</span><span class="n">j</span><span class="p">][</span><span class="n">i</span><span class="p">].</span><span class="n">omega</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">uxx</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">uyy</span><span class="w"> </span><span class="c1">//</span>
<span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="p">(</span><span class="n">vxp</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="p">(</span><span class="n">u</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">x</span><span class="p">[</span><span class="n">j</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">1</span><span class="p">].</span><span class="n">omega</span><span class="p">)</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">vxm</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="n">j</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">1</span><span class="p">].</span><span class="n">omega</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">u</span><span class="p">))</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">hy</span><span class="w"> </span><span class="c1">//</span>
<span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="p">(</span><span class="n">vyp</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="p">(</span><span class="n">u</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">x</span><span class="p">[</span><span class="n">j</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">][</span><span class="n">i</span><span class="p">].</span><span class="n">omega</span><span class="p">)</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">vym</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="n">j</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="p">][</span><span class="n">i</span><span class="p">].</span><span class="n">omega</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">u</span><span class="p">))</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">hx</span><span class="w"> </span><span class="c1">//</span>
<span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mf">.5</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">grashof</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="n">j</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">1</span><span class="p">].</span><span class="n">temp</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">x</span><span class="p">[</span><span class="n">j</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">1</span><span class="p">].</span><span class="n">temp</span><span class="p">)</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">hy</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><p>The use of <code class="docutils notranslate"><span class="pre">//</span> <span class="pre">clang-format</span> <span class="pre">off</span></code> is allowed in the source code but should only be used when necessary. It should not
be used when trailing // to split lines works.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// clang-format off</span>
<span class="n">f</span><span class="w"> </span><span class="p">...</span>
<span class="c1">// clang-format on</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils notranslate"><span class="pre">size</span></code> and <code class="docutils notranslate"><span class="pre">rank</span></code> should be used exclusively for the results of <code class="docutils notranslate"><span class="pre"><a href="http://www.mpich.org/static/docs/latest/www3/MPI_Comm_size.html#MPI_Comm_size">MPI_Comm_size</a>()</span></code> and <code class="docutils notranslate"><span class="pre"><a href="http://www.mpich.org/static/docs/latest/www3/MPI_Comm_rank.html#MPI_Comm_rank">MPI_Comm_rank</a>()</span></code> and other variable names for these values should be avoided unless necessary.</p></li>
</ol>
</section>
<section id="c-usage">
<h3>C Usage<a class="headerlink" href="#c-usage" title="Link to this heading">#</a></h3>
<ol class="arabic">
<li><p>Do not use language features that are not in the intersection of C99, C++11, and MSVC
v1900+ (Visual Studio 2015). Examples of such banned features include variable-length arrays.
Note that variable-length arrays (including VLA-pointers) are not supported in C++ and
were made optional in C11. You may use designated initializers via the
<code class="docutils notranslate"><span class="pre">PetscDesignatedInitializer()</span></code> macro.</p></li>
<li><p>Array and pointer arguments where the array values are not changed
should be labeled as <code class="docutils notranslate"><span class="pre">const</span></code> arguments.</p></li>
<li><p>Scalar values passed to functions should <em>never</em> be labeled as
<code class="docutils notranslate"><span class="pre">const</span></code>.</p></li>
<li><p>Subroutines that would normally have a <code class="docutils notranslate"><span class="pre">void</span> <span class="pre">**</span></code> argument to return
a pointer to some data should be prototyped as <code class="docutils notranslate"><span class="pre">void</span> <span class="pre">*</span></code>.
This prevents the caller from having to put a <code class="docutils notranslate"><span class="pre">(void</span> <span class="pre">**)</span></code> cast in
each function call. See, for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/DMDA/DMDAVecGetArray.html">DMDAVecGetArray</a>()</span></code>.</p></li>
<li><p>Do not use the <code class="docutils notranslate"><span class="pre">register</span></code> directive.</p></li>
<li><p>Use <code class="docutils notranslate"><span class="pre">if</span> <span class="pre">(v</span> <span class="pre">==</span> <span class="pre">NULL)</span></code> or <code class="docutils notranslate"><span class="pre">if</span> <span class="pre">(flg</span> <span class="pre">==</span> <span class="pre"><a href="../manualpages/Sys/PETSC_TRUE.html">PETSC_TRUE</a>)</span></code>, instead of using <code class="docutils notranslate"><span class="pre">if</span> <span class="pre">(!v)</span></code> or <code class="docutils notranslate"><span class="pre">if</span> <span class="pre">(flg)</span></code> or <code class="docutils notranslate"><span class="pre">if</span> <span class="pre">(!flg)</span></code>.</p></li>
<li><p>Avoid <code class="docutils notranslate"><span class="pre">#ifdef</span></code> or <code class="docutils notranslate"><span class="pre">#ifndef</span></code> when possible. Rather, use <code class="docutils notranslate"><span class="pre">#if</span> <span class="pre">defined</span></code> or <code class="docutils notranslate"><span class="pre">#if</span> <span class="pre">!defined</span></code>. Better, use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscDefined.html">PetscDefined</a>()</span></code> (see below). The only exception to this
rule is for header guards, where the <code class="docutils notranslate"><span class="pre">#ifndef</span></code> form is preferred (see below).</p></li>
<li><p>Header guard macros should be done using <code class="docutils notranslate"><span class="pre">#pragma</span> <span class="pre">once</span></code>. This must be the very first
non-comment line of the file. There must be no leading or trailing empty (non-comment)
lines in the header. For example, do</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cm">/*</span>
<span class="cm"> It's OK to have</span>
<span class="cm"> comments</span>
<span class="cm">*/</span>
<span class="c1">// before the guard</span>
<span class="cp">#pragma once</span>
<span class="c1">// OK, other headers included after the guard</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><petscdm.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><petscdevice.h></span>
<span class="c1">// OK, other preprocessor symbols defined after the guard</span>
<span class="cp">#define FOO_BAR_BAZ</span>
<span class="c1">// OK, regular symbols defined after the guard</span>
<span class="k">typedef</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">_p_PetscFoo</span><span class="w"> </span><span class="o">*</span><span class="n">PetscFoo</span><span class="p">;</span>
<span class="p">...</span>
</pre></div>
</div>
<p>Do not do</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// ERROR, empty lines at the beginning of the header</span>
<span class="c1">// ERROR, included other headers before the guard</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><petscdm.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><petscdevice.h></span>
<span class="c1">// ERROR, defined other preprocessor symbols before the guard</span>
<span class="cp">#define FOO_BAR_BAZ</span>
<span class="c1">// ERROR, defined regular symbols before the guard</span>
<span class="k">typedef</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">_p_PetscFoo</span><span class="w"> </span><span class="o">*</span><span class="n">PetscFoo</span><span class="p">;</span>
<span class="cp">#pragma once</span>
</pre></div>
</div>
</li>
<li><p>Never use system random number generators such as <code class="docutils notranslate"><span class="pre">rand()</span></code> in PETSc
code or examples because these can produce different results on
different systems, thus making portability testing difficult. Instead,
use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscRandom.html">PetscRandom</a></span></code> which produces the same results regardless
of the system used.</p></li>
<li><p>Variadic macros may be used in PETSc, but must work with MSVC v1900+ (Visual Studio
2015). Most compilers have conforming implementations of the C99/C++11 rules for
<code class="docutils notranslate"><span class="pre">__VA_ARGS__</span></code>, but MSVC’s implementation is not conforming and may need workarounds.
See <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscDefined.html">PetscDefined</a>()</span></code> for an example of how to work around MSVC’s limitations to write
a macro that is usable in both.</p></li>
</ol>
</section>
<section id="usage-of-petsc-functions-and-macros">
<span id="id2"></span><h3>Usage of PETSc Functions and Macros<a class="headerlink" href="#usage-of-petsc-functions-and-macros" title="Link to this heading">#</a></h3>
<ol class="arabic">
<li><p>Lengthy conditional preprocessor blocks should mark any <code class="docutils notranslate"><span class="pre">#else</span></code> or <code class="docutils notranslate"><span class="pre">#endif</span></code>
directives with a comment containing (or explaining) either the boolean condition or
the macro’s name if the first directive tests whether one is defined. One
should be able to read any part of the macroblock and find or deduce the
initial <code class="docutils notranslate"><span class="pre">#if</span></code>. That is:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#if defined(MY_MACRO)</span>
<span class="c1">// many lines of code</span>
<span class="cp">#else </span><span class="c1">// MY_MACRO (use name of macro)</span>
<span class="c1">// many more lines of code</span>
<span class="cp">#endif </span><span class="c1">// MY_MACRO</span>
<span class="cp">#if MY_MACRO > 10</span>
<span class="c1">// code</span>
<span class="cp">#else </span><span class="c1">// MY_MACRO < 10</span>
<span class="c1">// more code</span>
<span class="cp">#endif </span><span class="c1">// MY_MACRO > 10</span>
</pre></div>
</div>
</li>
<li><p>Public PETSc include files, <code class="docutils notranslate"><span class="pre">petsc*.h</span></code>, should not reference
private PETSc <code class="docutils notranslate"><span class="pre">petsc/private/*impl.h</span></code> include files.</p></li>
<li><p>Public and private PETSc include files cannot reference include files
located in the PETSc source tree.</p></li>
<li><p>All public functions must sanity-check their arguments using the appropriate
<code class="docutils notranslate"><span class="pre">PetscValidXXX()</span></code> macros. These must appear between <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span></code> and
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionReturn.html">PetscFunctionReturn</a>()</span></code> For example</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n"><a href="../manualpages/Sys/PetscErrorCode.html">PetscErrorCode</a></span><span class="w"> </span><span class="nf">PetscPublicFunction</span><span class="p">(</span><span class="n"><a href="../manualpages/Vec/Vec.html">Vec</a></span><span class="w"> </span><span class="n">v</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscScalar.html">PetscScalar</a></span><span class="w"> </span><span class="o">*</span><span class="n">array</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span><span class="w"> </span><span class="n">collectiveInt</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a></span><span class="p">;</span>
<span class="w"> </span><span class="n">PetscValidHeaderSpecific</span><span class="p">(</span><span class="n">v</span><span class="p">,</span><span class="w"> </span><span class="n">VEC_CLASSID</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">PetscAssertPointer</span><span class="p">(</span><span class="n">array</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">);</span>
<span class="w"> </span><span class="n">PetscValidLogicalCollectiveInt</span><span class="p">(</span><span class="n">v</span><span class="p">,</span><span class="w"> </span><span class="n">collectiveInt</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="p">...</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscFunctionReturn.html">PetscFunctionReturn</a></span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PetscErrorCode.html">PETSC_SUCCESS</a></span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>See <code class="docutils notranslate"><span class="pre">include/petsc/private/petscimpl.h</span></code> and search for “PetscValid” to see all
available checker macros.</p>
</li>
<li><p>When possible, use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscDefined.html">PetscDefined</a>()</span></code> instead of preprocessor conditionals.
For example, use:</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"><a href="../manualpages/Sys/PetscDefined.html">PetscDefined</a></span><span class="p">(</span><span class="n">USE_DEBUG</span><span class="p">))</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="p">}</span>
</pre></div>
</div>
<p>instead of:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#if defined(PETSC_USE_DEBUG)</span>
<span class="w"> </span><span class="p">...</span>
<span class="cp">#endif</span>
</pre></div>
</div>
<p>The former usage allows syntax and type-checking in all configurations of
PETSc, whereas the latter needs to be compiled with and without debugging
to confirm that it compiles.</p>
</li>
<li><p><em>Never</em> put a function call in a <code class="docutils notranslate"><span class="pre">return</span></code> statement; do not write</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n"><a href="../manualpages/Sys/PetscFunctionReturn.html">PetscFunctionReturn</a></span><span class="p">(</span><span class="w"> </span><span class="n">somefunction</span><span class="p">(...)</span><span class="w"> </span><span class="p">);</span><span class="w"> </span><span class="cm">/* Incorrect */</span>
</pre></div>
</div>
</li>
<li><p>Do <em>not</em> put a blank line immediately after <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionBegin.html">PetscFunctionBegin</a>;</span></code>
or a blank line immediately before <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFunctionReturn.html">PetscFunctionReturn</a>(<a href="../manualpages/Sys/PetscErrorCode.html">PETSC_SUCCESS</a>);</span></code>.</p></li>
<li><p>Do not include <code class="docutils notranslate"><span class="pre">assert.h</span></code> in PETSc source code. Do not use
<code class="docutils notranslate"><span class="pre">assert()</span></code>, it doesn’t play well in the parallel MPI world.
You may use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscAssert.html">PetscAssert</a>()</span></code> where appropriate. See <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCall.html">PetscCall</a>()</span></code> documentation
for guidance of when to use <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscCheck.html">PetscCheck</a>()</span></code> vs. <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscAssert.html">PetscAssert</a>()</span></code>.</p></li>
<li><p>Make error messages short but informative. The user should be able to reasonably
diagnose the greater problem from your error message.</p></li>
<li><p>Except in code that may be called before PETSc is fully initialized,
always use <code class="docutils notranslate"><span class="pre">PetscMallocN()</span></code> (for example, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscMalloc1.html">PetscMalloc1</a>()</span></code>),
<code class="docutils notranslate"><span class="pre">PetscCallocN()</span></code>, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscNew.html">PetscNew</a>()</span></code>, and <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscFree.html">PetscFree</a>()</span></code>, not
<code class="docutils notranslate"><span class="pre">malloc()</span></code> and <code class="docutils notranslate"><span class="pre">free()</span></code>.</p></li>
<li><p>MPI routines and macros that are not part of the 2.1 standard
should not be used in PETSc without appropriate <code class="docutils notranslate"><span class="pre">configure</span></code>
checks and <code class="docutils notranslate"><span class="pre">#if</span> <span class="pre"><a href="../manualpages/Sys/PetscDefined.html">PetscDefined</a>()</span></code> checks. Code should also be provided
that works if the MPI feature is not available; for example,</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#if <a href="../manualpages/Sys/PetscDefined.html">PetscDefined</a>(HAVE_MPI_REDUCE_LOCAL)</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscCallMPI.html">PetscCallMPI</a></span><span class="p">(</span><span class="n"><a href="http://www.mpich.org/static/docs/latest/www3/MPI_Reduce_local.html#MPI_Reduce_local">MPI_Reduce_local</a></span><span class="p">(</span><span class="n">inbuf</span><span class="p">,</span><span class="w"> </span><span class="n">inoutbuf</span><span class="p">,</span><span class="w"> </span><span class="n">count</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/MPIU_INT.html">MPIU_INT</a></span><span class="p">,</span><span class="w"> </span><span class="n">MPI_SUM</span><span class="p">));</span>
<span class="cp">#else</span>
<span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscCallMPI.html">PetscCallMPI</a></span><span class="p">(</span><span class="n"><a href="http://www.mpich.org/static/docs/latest/www3/MPI_Reduce.html#MPI_Reduce">MPI_Reduce</a></span><span class="p">(</span><span class="n">inbuf</span><span class="p">,</span><span class="w"> </span><span class="n">inoutbuf</span><span class="p">,</span><span class="w"> </span><span class="n">count</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/MPIU_INT.html">MPIU_INT</a></span><span class="p">,</span><span class="w"> </span><span class="n">MPI_SUM</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/PETSC_COMM_SELF.html">PETSC_COMM_SELF</a></span><span class="p">);</span>
<span class="cp">#endif</span>
</pre></div>
</div>
</li>
<li><p>Do not introduce PETSc routines that provide essentially the same
functionality as an available MPI routine. For example, do not write
a routine <code class="docutils notranslate"><span class="pre">PetscGlobalSum()</span></code> that takes a scalar value and performs
an <code class="docutils notranslate"><span class="pre"><a href="http://www.mpich.org/static/docs/latest/www3/MPI_Allreduce.html#MPI_Allreduce">MPI_Allreduce</a>()</span></code> on it. Instead, use the MPI routine
<code class="docutils notranslate"><span class="pre"><a href="http://www.mpich.org/static/docs/latest/www3/MPI_Allreduce.html#MPI_Allreduce">MPI_Allreduce</a>()</span></code> directly in the code.</p></li>
<li><p>Never use a local variable counter such as <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span> <span class="pre">flops</span> <span class="pre">=</span> <span class="pre">0;</span></code> to
accumulate flops and then call <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Log/PetscLogFlops.html">PetscLogFlops</a>();</span></code> <em>always</em> just
call <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Log/PetscLogFlops.html">PetscLogFlops</a>()</span></code> directly when needed.</p></li>
<li><p>Library symbols meant to be directly usable by the user should be declared
<code class="docutils notranslate"><span class="pre">PETSC_EXTERN</span></code> in their respective public header file. Symbols intended for internal use should instead be declared <code class="docutils notranslate"><span class="pre">PETSC_INTERN</span></code>. Note that doing so is
unnecessary in the case of symbols local to a single translation unit; these should
be declared <code class="docutils notranslate"><span class="pre">static</span></code>. PETSc can be configured to build a separate shared
library for each top-level class (<code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/Mat.html">Mat</a></span></code>, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Vec/Vec.html">Vec</a></span></code>, <code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSP.html">KSP</a></span></code>, and so on), and that plugin
implementations of these classes can be included as separate shared libraries; thus,
otherwise private symbols may need to be marked <code class="docutils notranslate"><span class="pre">PETSC_SINGLE_LIBRARY_INTERN</span></code>. For
example</p>
<ul class="simple">
<li><p><code class="docutils notranslate"><span class="pre">MatStashCreate_Private()</span></code> is marked <code class="docutils notranslate"><span class="pre">PETSC_INTERN</span></code> as it is used
across compilation units, but only within the <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Mat/Mat.html">Mat</a></span></code> package;</p></li>
<li><p>all functions, such as <code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSPCreate.html">KSPCreate</a>()</span></code>, included in the public
headers (<code class="docutils notranslate"><span class="pre">include/petsc*.h</span></code>) should be marked <code class="docutils notranslate"><span class="pre">PETSC_EXTERN</span></code>;</p></li>
<li><p><code class="docutils notranslate"><span class="pre">VecLoad_Default()</span></code> is marked
<code class="docutils notranslate"><span class="pre">PETSC_SINGLE_LIBRARY_INTERN</span></code> as it may be used across library boundaries, but is
not intended to be visible to users;</p></li>
</ul>
</li>
<li><p>Before removing or renaming an API function, type, or enumerator,
<code class="docutils notranslate"><span class="pre">PETSC_DEPRECATED_XXX()</span></code> should be used in the relevant header file
to indicate the new usage and the PETSc version number where the
deprecation will first appear. The old function or type, with the
deprecation warning, should remain for at least one major release. We do not remove support for the
deprecated functionality unless there is a specific reason to remove it; it is not removed simply because
it has been deprecated for “a long time.”</p>
<p>The function or type’s manual page should be updated (see <a class="reference internal" href="#manual-page-format"><span class="std std-ref">Manual Page Format</span></a>).
For example,</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span><span class="w"> </span><span class="n">NewType</span><span class="w"> </span><span class="n">OldType</span><span class="w"> </span><span class="n">PETSC_DEPRECATED_TYPEDEF</span><span class="p">(</span><span class="s">"Use NewType (since version 3.9)"</span><span class="p">);</span>
<span class="n">PETSC_DEPRECATED_FUNCTION</span><span class="p">(</span><span class="s">"Use NewFunction() (since version 3.9)"</span><span class="p">)</span><span class="w"> </span><span class="n"><a href="../manualpages/Sys/PetscErrorCode.html">PetscErrorCode</a></span><span class="w"> </span><span class="n">OldFunction</span><span class="p">();</span>
<span class="cp">#define OLD_ENUMERATOR_DEPRECATED OLD_ENUMERATOR PETSC_DEPRECATED_ENUM("Use NEW_ENUMERATOR (since version 3.9)")</span>
<span class="k">typedef</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">OLD_ENUMERATOR_DEPRECATED</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span>
<span class="w"> </span><span class="n">NEW_ENUMERATOR</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span>
<span class="p">}</span><span class="w"> </span><span class="n">MyEnum</span><span class="p">;</span>
</pre></div>
</div>
<p>Note that after compiler preprocessing, the enum above would be transformed into something like</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">OLD_ENUMERATOR</span><span class="w"> </span><span class="nf">__attribute__</span><span class="p">((</span><span class="n">deprecated</span><span class="p">))</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span>
<span class="w"> </span><span class="n">NEW_ENUMERATOR</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span>
<span class="p">}</span><span class="w"> </span><span class="n">MyEnum</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><p>Before removing or renaming an options database key,
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscOptionsDeprecated.html">PetscOptionsDeprecated</a>()</span></code> should be used for at least one major
release. We do not remove support for the
deprecated functionality unless there is a specific reason to remove it; it is not removed simply because
it has been deprecated for “a long time.”</p></li>
<li><p>The format strings in PETSc ASCII output routines, such as
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscPrintf.html">PetscPrintf</a>()</span></code>, take a <code class="docutils notranslate"><span class="pre">%"</span> <span class="pre">PetscInt_FMT</span> <span class="pre">"</span></code> for all PETSc variables of type <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscInt.html">PetscInt</a></span></code>,
not a <code class="docutils notranslate"><span class="pre">%d</span></code>.</p></li>
<li><p>All arguments of type <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscReal.html">PetscReal</a></span></code> to PETSc ASCII output routines,
such as <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscPrintf.html">PetscPrintf</a></span></code>, must be cast to <code class="docutils notranslate"><span class="pre">double</span></code>, for example,</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n"><a href="../manualpages/Sys/PetscPrintf.html">PetscPrintf</a></span><span class="p">(</span><span class="n"><a href="../manualpages/Sys/PETSC_COMM_WORLD.html">PETSC_COMM_WORLD</a></span><span class="p">,</span><span class="w"> </span><span class="s">"Norm %g</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="kt">double</span><span class="p">)</span><span class="n">norm</span><span class="p">);</span>
</pre></div>
</div>
</li>
</ol>
</section>
</section>
<section id="formatted-comments">
<h2>Formatted Comments<a class="headerlink" href="#formatted-comments" title="Link to this heading">#</a></h2>
<p>PETSc uses formatted comments and the Sowing packages <span id="id3">[<a class="reference internal" href="#id1933" title="W Gropp. Users manual for bfort: producing Fortran interfaces to C source code. Technical Report ANL/MCS-TM-208, Argonne National Laboratory, 1995.">Gro95b</a>]</span> <span id="id4">[<a class="reference internal" href="#id1934" title="W Gropp. Users manual for doctext: producing documentation from source code. Technical Report ANL/MCS-TM-206, Argonne National Laboratory, 1995.">Gro95a</a>]</span>
to generate documentation (manual pages) and the Fortran interfaces. Documentation
for Sowing and the formatting may be found at
<a class="reference external" href="http://wgropp.cs.illinois.edu/projects/software/sowing/">http://wgropp.cs.illinois.edu/projects/software/sowing/</a>; in particular,
see the documentation for <code class="docutils notranslate"><span class="pre">doctext</span></code>. Currently, doctext produces Markdown files ending in <code class="docutils notranslate"><span class="pre">.md</span></code>, which
Sphinx later processes.</p>
<ul class="simple">
<li><p><code class="docutils notranslate"><span class="pre">/*@</span></code>
a formatted comment of a function that will be used for documentation and a Fortran interface.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">/*@C</span></code>
a formatted comment of a function that will be used only for documentation, not to generate a Fortran interface. Certain constructs and usages do not yet support automatically generating a Fortran interface. In general, such labeled C functions should have a custom Fortran interface provided.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">/*E</span></code>
a formatted comment of an enum used for documentation only.
.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">/*S</span></code>
a formatted comment for a data type such as
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSP.html">KSP</a></span></code>
.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">/*J</span></code>
a formatted comment for a string type such as
<code class="docutils notranslate"><span class="pre"><a href="../manualpages/KSP/KSPType.html">KSPType</a></span></code>
.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">/*MC</span></code>
a formatted comment of a CPP macro or enum value for documentation.</p></li>
</ul>
<p>The Fortran interface files supplied manually by the developer go into the
directory <code class="docutils notranslate"><span class="pre">ftn-custom</span></code>, while those automatically generated
go into directories in the $PETSC_ARCH/ftn`` directory tree.</p>
<p>Each include file that contains formatted comments needs to have a line of the form</p>
<blockquote>
<div><div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cm">/* SUBMANSEC = submansec (for example Sys) */</span>
</pre></div>
</div>
</div></blockquote>
<p>preceded by and followed by a blank line. For source code, this information is found in the makefile in that source code’s directory in the format</p>
<blockquote>
<div><div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">MANSEC</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../manualpages/DM/DM.html">DM</a></span>
<span class="n">SUBMANSEC</span><span class="o">=</span><span class="w"> </span><span class="n">DMPlex</span>
</pre></div>
</div>
</div></blockquote>
<section id="manual-page-format">
<span id="id5"></span><h3>Manual Page Format<a class="headerlink" href="#manual-page-format" title="Link to this heading">#</a></h3>
<p>Each function, typedef, class, macro, enum, and so on in the public API
should include the following data, correctly formatted (see codes
section) to generate complete manual pages and (possibly) Fortran interfaces with
Sowing. All entries below should be separated by blank lines. Except
where noted, add a newline after the section headings.</p>
<ol class="arabic">
<li><p>The item’s name, followed by a dash and brief (one-sentence)
description.</p></li>
<li><p>If documenting a function implemented with a preprocessor macro
(e.g., <code class="docutils notranslate"><span class="pre"><a href="../manualpages/Sys/PetscOptionsBegin.html">PetscOptionsBegin</a>()</span></code>), an explicit <code class="docutils notranslate"><span class="pre">Synopsis:</span></code> section
noting the required header and the function signature.</p></li>
<li><p>If documenting a function, a description of the function’s
“collectivity”.</p>
<ul class="simple">
<li><p><code class="docutils notranslate"><span class="pre">Not</span> <span class="pre">Collective</span></code> if the function need not be called on multiple (or possibly all) MPI
processes</p></li>
<li><p><code class="docutils notranslate"><span class="pre">Collective</span></code> if the function is a collective operation.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">Logically</span> <span class="pre">Collective;</span> <span class="pre">yyy</span> <span class="pre">must</span> <span class="pre">contain</span> <span class="pre">common</span> <span class="pre">value]</span></code>
if the function is collective but does not require any actual
synchronization (e.g., setting class parameters uniformly). Any
argument yyy, which must have the same value on all ranks of the
MPI communicator should be noted here.</p></li>
</ul>
</li>
<li><p>If the function is not supported in Fortran, then after the collective information, on the same line,
one should provide <code class="docutils notranslate"><span class="pre">;</span> <span class="pre">No</span> <span class="pre">Fortran</span> <span class="pre">support</span></code>.</p></li>
<li><p>If documenting a function with input parameters, a list of input
parameter descriptions in an <code class="docutils notranslate"><span class="pre">Input</span> <span class="pre">Parameter(s):</span></code> section.</p></li>
<li><p>If documenting a function with output parameters, a list of output
parameter descriptions in an <code class="docutils notranslate"><span class="pre">Output</span> <span class="pre">Parameter(s):</span></code> section.</p></li>
<li><p>If any input or output parameters are function pointers, they should be documented in the style</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="go">Calling sequence of `func()`:</span>
<span class="go">. arg - the integer argument description</span>
</pre></div>
</div>
</li>
<li><p>If documenting a function that interacts with the options database, a
list of options database keys in an <code class="docutils notranslate"><span class="pre">Options</span> <span class="pre">Database</span> <span class="pre">Key(s):</span></code>
section.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">Level:</span></code> (no newline) followed by <code class="docutils notranslate"><span class="pre">beginner</span></code>,
<code class="docutils notranslate"><span class="pre">intermediate</span></code>, <code class="docutils notranslate"><span class="pre">advanced</span></code>, <code class="docutils notranslate"><span class="pre">developer</span></code>, or <code class="docutils notranslate"><span class="pre">deprecated</span></code>. This
should be listed before the various <code class="docutils notranslate"><span class="pre">Note(s):</span></code> sections.</p></li>
<li><p>(Optional) a <code class="docutils notranslate"><span class="pre">Note(s):</span></code> section containing in-depth discussion,
technical caveats, special cases, and so on. If it is ambiguous
whether returned pointers/objects need to be freed/destroyed by the
user or not, this information should be mentioned here.</p></li>
<li><p>(If applicable) a <code class="docutils notranslate"><span class="pre">Fortran</span> <span class="pre">Note(s):</span></code> section detailing any relevant
differences in calling or using the item from Fortran.</p></li>
<li><p>(If applicable) a <code class="docutils notranslate"><span class="pre">Developer</span> <span class="pre">Note(s):</span></code> section detailing any relevant
information about the code for developers, for example, why a
particular algorithm was implemented.</p></li>
<li><p>(If applicable) references should be indicated inline with {cite}`Bibtex-key` where
Bibtex-key is in the file <code class="docutils notranslate"><span class="pre">doc/petsc.bib</span></code>, as in the manual page for <code class="docutils notranslate"><span class="pre"><a href="../manualpages/PC/PCFIELDSPLIT.html">PCFIELDSPLIT</a></span></code>.</p></li>
<li><p><code class="docutils notranslate"><span class="pre">.seealso:</span></code> (no newline, no spaces to the left of this text), followed by a list of related manual
pages. These manual pages should usually also point back to this
manual page in their <code class="docutils notranslate"><span class="pre">.seealso:</span></code> sections. This is the final entry in the
comment. There should be no blank line after the <code class="docutils notranslate"><span class="pre">.seealso:</span></code> items.</p></li>
<li><p>All PETSc functions that appear in a manual page (except the one in the header at the top) should end with a <code class="docutils notranslate"><span class="pre">()</span></code> and be enclosed
in single back tick marks. All PETSc enum types and macros etc, should also be enclosed in single back tick marks.
This includes each item listed in the <code class="docutils notranslate"><span class="pre">.seealso:</span></code> lines.</p></li>
</ol>
</section>
<section id="spelling-and-capitalization">
<h3>Spelling and Capitalization<a class="headerlink" href="#spelling-and-capitalization" title="Link to this heading">#</a></h3>
<ol class="arabic simple">
<li><p>Proper nouns, including Unix, Linux, X Windows, and Microsoft Windows, should be fully written and capitalized. This includes all operating systems.
The Apple computer operating system is written as macOS.</p></li>
<li><p>Company names and product names should be capitalized.</p></li>
<li><p>Company names and terms that are traditionally all capitalized, for example, MATLAB, NVIDIA, and CUDA, should be all capitalized.</p></li>
<li><p>ARM is a family of processor designs, while Arm is the company that licenses them.</p></li>
<li><p>Unix should not be all capitalized.</p></li>
<li><p>Microsoft Windows should always be written out with two words. That is, it should not be shortened to Windows or MS Win etc.</p></li>
<li><p>CMake should be capitalized as shown.</p></li>
<li><p>BLAS and LAPACK are written in full capitalization.</p></li>
<li><p>Open MPI is written as two words.</p></li>
</ol>
</section>
</section>
<section id="references">
<h2>References<a class="headerlink" href="#references" title="Link to this heading">#</a></h2>
<div class="docutils container" id="id6">
<div role="list" class="citation-list">
<div class="citation" id="id1934" role="doc-biblioentry">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id4">Gro95a</a><span class="fn-bracket">]</span></span>
<p>W Gropp. Users manual for doctext: producing documentation from source code. Technical Report ANL/MCS-TM-206, Argonne National Laboratory, 1995.</p>
</div>
<div class="citation" id="id1933" role="doc-biblioentry">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id3">Gro95b</a><span class="fn-bracket">]</span></span>
<p>W Gropp. Users manual for bfort: producing Fortran interfaces to C source code. Technical Report ANL/MCS-TM-208, Argonne National Laboratory, 1995.</p>
</div>
</div>
</div>
</section>
</section>
<hr class="footnotes docutils" />
<aside class="footnote-list brackets">
<aside class="footnote brackets" id="footnote-1" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id1">1</a><span class="fn-bracket">]</span></span>
<p>Type also refers to the string name of the subclass.</p>
</aside>
</aside>
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