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<li class="toctree-l4"><a class="reference internal" href="#include-syscall-h"><code class="docutils literal notranslate"><span class="pre">include/syscall.h</span></code></a></li>
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<li class="toctree-l4"><a class="reference internal" href="#include-arch-syscall-h-or-arch-cpu-include-syscall-h"><code class="docutils literal notranslate"><span class="pre">include/arch/syscall.h</span></code> (or <code class="docutils literal notranslate"><span class="pre">arch/&lt;cpu&gt;/include/syscall.h</span></code>)</a></li>
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<section id="syscall-layer">
<h1>Syscall Layer<a class="headerlink" href="#syscall-layer" title="Permalink to this heading"></a></h1>
<p>This page discusses supports a syscall layer from communication between a
monolithic, kernel-mode NuttX kernel and a separately built, user-mode
application set.</p>
<p>With most MCUs, NuttX is built as a flat, single executable image
containing the NuttX RTOS along with all application code. The RTOS code
and the application run in the same address space and at the same kernel-
mode privileges. In order to exploit security features of certain
processors, an alternative build model is also supported: NuttX can
be built separately as a monolithic, kernel-mode module and the applications
can be added as a separately built, user-mode module.</p>
<p>The syscall layer provided in this directory serves as the communication
layer from the user-mode application into the kernel-mode RTOS. The
switch from user-mode to kernel-mode is accomplished using software
interrupts (SWIs). SWIs are implemented differently and named differently
by different manufacturers but all work essentially the same: A special
instruction is executed in user-mode that causes a software generated
interrupt. The software generated interrupt is caught within the kernel
and handle in kernel-mode.</p>
<section id="header-files">
<h2>Header Files<a class="headerlink" href="#header-files" title="Permalink to this heading"></a></h2>
<section id="include-syscall-h">
<h3><code class="docutils literal notranslate"><span class="pre">include/syscall.h</span></code><a class="headerlink" href="#include-syscall-h" title="Permalink to this heading"></a></h3>
<p>This header file supports general access to SWI facilities. It is simply
a wrapper file that includes <code class="docutils literal notranslate"><span class="pre">include/sys/syscall.h</span></code> and
<code class="docutils literal notranslate"><span class="pre">include/arch/syscall.h</span></code>.</p>
</section>
<section id="include-sys-syscall-h">
<h3><code class="docutils literal notranslate"><span class="pre">include/sys/syscall.h</span></code><a class="headerlink" href="#include-sys-syscall-h" title="Permalink to this heading"></a></h3>
<p>The SWIs received by the kernel are distinguish by a code that identifies
how to process the SWI. This header file defines all such codes understood
by the NuttX kernel.</p>
</section>
<section id="include-arch-syscall-h-or-arch-cpu-include-syscall-h">
<h3><code class="docutils literal notranslate"><span class="pre">include/arch/syscall.h</span></code> (or <code class="docutils literal notranslate"><span class="pre">arch/&lt;cpu&gt;/include/syscall.h</span></code>)<a class="headerlink" href="#include-arch-syscall-h-or-arch-cpu-include-syscall-h" title="Permalink to this heading"></a></h3>
<p>This header file is provided by the platform-specific logic and declares
(or defines) the mechanism for providing software interrupts on this
platform. The following functions must be declared (or defined) in this
header file:</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number only:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call0</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number and one parameter:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call1</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm1</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number and two parameters:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call2</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm1</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm2</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number and three parameters:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call3</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm1</span><span class="p">,</span>
<span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm2</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm3</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number and four parameters:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call4</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm1</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm2</span><span class="p">,</span>
<span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm3</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm4</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number and five parameters:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call5</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm1</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm2</span><span class="p">,</span>
<span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm3</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm4</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm5</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">SWI</span></code> with <code class="docutils literal notranslate"><span class="pre">SYS_</span></code> call number and six parameters:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">uintptr_t</span><span class="w"> </span><span class="nf">sys_call6</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">nbr</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm1</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm2</span><span class="p">,</span>
<span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm3</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm4</span><span class="p">,</span><span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm5</span><span class="p">,</span>
<span class="w"> </span><span class="kt">uintptr_t</span><span class="w"> </span><span class="n">parm6</span><span class="p">);</span>
</pre></div>
</div>
</li>
</ul>
</section>
<section id="syscall-database">
<h3>Syscall Database<a class="headerlink" href="#syscall-database" title="Permalink to this heading"></a></h3>
<p>Sycall information is maintained in a database. That “database” is
implemented as a simple comma-separated-value file, <code class="docutils literal notranslate"><span class="pre">syscall.csv</span></code>. Most
spreadsheets programs will accept this format and can be used to maintain
the syscall database.</p>
<p>The format of the CSV file for each line is:</p>
<ul class="simple">
<li><p>Field 1: Function name</p></li>
<li><p>Field 2: The header file that contains the function prototype</p></li>
<li><p>Field 3: Condition for compilation</p></li>
<li><p>Field 4: The type of function return value.</p></li>
<li><p>Field 5 - N+5: The type of each of the N formal parameters of the function</p></li>
<li><p>Fields N+5 - : If the last parameter is “…”, then the following fields
provide the type and number of of possible optional parameters.
See note below about variadic functions</p></li>
</ul>
<p>Each type field has a format as follows:</p>
<ul>
<li><p>type name:</p>
<p>For all simpler types</p>
</li>
<li><p>formal type | actual type:</p>
<p>For array types where the form of the formal (eg. <code class="docutils literal notranslate"><span class="pre">int</span> <span class="pre">parm[2]</span></code>)
differs from the type of actual passed parameter (eg. <code class="docutils literal notranslate"><span class="pre">int*</span></code>).
This is necessary because you cannot do simple casts to array types.</p>
</li>
<li><p>formal type | union member actual type | union member fieldname:</p>
<p>A similar situation exists for unions. For example, the formal
parameter type union sigval – You cannot cast a uintptr_t to
a union sigval, but you can cast to the type of one of the union
member types when passing the actual parameter. Similarly, we
cannot cast a union sigval to a uinptr_t either. Rather, we need
to cast a specific union member fieldname to <code class="docutils literal notranslate"><span class="pre">uintptr_t</span></code>.</p>
</li>
</ul>
</section>
</section>
<section id="variadic-functions">
<h2>Variadic Functions<a class="headerlink" href="#variadic-functions" title="Permalink to this heading"></a></h2>
<p>General variadic functions which may have an arbitrary number of argument
or arbitrary types cannot be represented as system calls.
<code class="docutils literal notranslate"><span class="pre">syslog()</span></code> is a good example. Normally you would work around this by
using the non- variadic form of the OS interface that accepts a <code class="docutils literal notranslate"><span class="pre">va_list</span></code>
as an argument, <code class="docutils literal notranslate"><span class="pre">vsyslog()</span></code> in this case.</p>
<p>There are many functions that have a variadic form but take only
one or two optional arguments. There can be handled as system
calls, but only by treating them as though they had a fixed number of
arguments.</p>
<p>These are handled in <code class="docutils literal notranslate"><span class="pre">syscall.csv</span></code> by appending the number and type of
optional arguments. For example, consider the <code class="docutils literal notranslate"><span class="pre">open()</span></code> OS interface. Its
prototype is:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="nf">open</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="n">path</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">oflag</span><span class="p">,</span><span class="w"> </span><span class="p">...);</span>
</pre></div>
</div>
<p>In reality, open may take only a single optional argument of type <code class="docutils literal notranslate"><span class="pre">mode_t</span></code>
and is represented in <code class="docutils literal notranslate"><span class="pre">syscall.csv</span></code> like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&quot;open&quot;,&quot;fcntl.h&quot;,&quot;&quot;,&quot;int&quot;,&quot;const char*&quot;,&quot;int&quot;,&quot;...&quot;,&quot;mode_t&quot;
</pre></div>
</div>
<p>The existence of the <code class="docutils literal notranslate"><span class="pre">mode_t</span></code> tells <code class="docutils literal notranslate"><span class="pre">tools/mksyscall</span></code> that there is at most
one optional parameter and, if present, it is of type <code class="docutils literal notranslate"><span class="pre">mode_t</span></code>.</p>
<p>NOTE: This CSV file is used not only to support the generate of trap information,
but also for the generation of symbol tables. See <code class="docutils literal notranslate"><span class="pre">Documentation/components/tools/</span></code>
and <code class="docutils literal notranslate"><span class="pre">Documentation/components/libs/</span></code> for further information.</p>
</section>
<section id="auto-generated-files">
<h2>Auto-Generated Files<a class="headerlink" href="#auto-generated-files" title="Permalink to this heading"></a></h2>
<p>Stubs and proxies for the sycalls are automatically generated from this CSV
database. Here the following definition is used:</p>
<ul class="simple">
<li><p>Proxy - A tiny bit of code that executes in the user space. A proxy
has exactly the same function prototype as does the “real” function
for which it proxies. However, it only serves to map the function
call into a syscall, marshaling all of the system call parameters
as necessary.</p></li>
<li><p>Stub - Another tiny bit of code that executes within the NuttX kernel
that is used to map a software interrupt received by the kernel to
a kernel function call. The stubs receive the marshaled system
call data, and perform the actually kernel function call (in
kernel-mode) on behalf of the proxy function.</p></li>
</ul>
</section>
<section id="sub-directories">
<h2>Sub-Directories<a class="headerlink" href="#sub-directories" title="Permalink to this heading"></a></h2>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">stubs</span></code> - Autogenerated stub files are placed in this directory.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">proxies</span></code> - Autogenerated proxy files are placed in this directory.</p></li>
</ul>
</section>
<section id="mksyscall">
<h2>mksyscall<a class="headerlink" href="#mksyscall" title="Permalink to this heading"></a></h2>
<p>mksyscall is C program that is used during the initial NuttX build
by the logic in the top-level <code class="docutils literal notranslate"><span class="pre">syscall/</span></code> directory. Information about the
stubs and proxies is maintained in a comma separated value (CSV) file
in the <code class="docutils literal notranslate"><span class="pre">syscall/</span></code> directory. The mksyscall program will accept this CSV
file as input and generate all of the required proxy or stub files as
output. See <code class="docutils literal notranslate"><span class="pre">Documentation/components/tools/</span></code> for additional information.</p>
</section>
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