<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>3.2. Procedure Call Conventions</title><link rel="stylesheet" href="ecl.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"><link rel="home" href="index.html" title="The ECL manual"><link rel="up" href="ch26.html" title="Chapter 3. The interpreter"><link rel="prev" href="ch26.html" title="Chapter 3. The interpreter"><link rel="next" href="ch26s03.html" title="3.3. The lexical environment"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">3.2. Procedure Call Conventions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch26.html">Prev</a> </td><th width="60%" align="center">Chapter 3. The interpreter</th><td width="20%" align="right"> <a accesskey="n" href="ch26s03.html">Next</a></td></tr></table><hr></div><div class="section" title="3.2. Procedure Call Conventions"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="Internals-Procedure-Call-Conventions"></a>3.2. Procedure Call Conventions</h2></div></div></div><p><span class="application">ECL</span> employs standard C calling conventions to achieve efficiency and
interoperability with other languages.
Each Lisp function is implemented as a C function which takes as many
argument as the Lisp original plus one additional integer argument
which holds the number of actual arguments. The function sets <code class="literal">NValues</code>
to the number of Lisp values produced, it returns the first one and the
remaining ones are kept in a global (per thread) array (<code class="literal">VALUES</code>).</p><p>To show the argument/value passing mechanism, here we list the actual
code for the Common-Lisp function <code class="literal">cons</code>.</p><pre class="screen">
cl_cons(int narg, object car, object cdr)
{ object x;
check_arg(2);
x = alloc_object(t_cons);
CAR(x) = car;
CDR(x) = cdr;
NValues = 1;
return x;
}
</pre><p><span class="application">ECL</span> adopts the convention that the name of a function that implements a
Common-Lisp function begins with a short package name (<code class="literal">cl</code> for COMMON-LISP,
<code class="literal">si</code> for SYSTEM, etc), followed by <code class="literal">L</code>, and followed by the name of
the Common-Lisp function. (Strictly speaking, `<code class="literal">-</code>' and `<code class="literal">*</code>' in the
Common-Lisp function name are replaced by `<code class="literal">_</code>' and `<code class="literal">A</code>', respectively,
to obey the syntax of C.)</p><p><code class="literal">check_arg(2)</code> in the code of <code class="literal">cl_cons</code> checks that exactly two
arguments are supplied to <code class="literal">cons</code>. That is, it checks that <code class="literal">narg</code> is
2, and otherwise, it causes an error. <code class="literal">allocate_object(t_cons)</code> allocates
a cons cell in the heap and returns the pointer to the cell. After the
<code class="literal">CAR</code> and the <code class="literal">CDR</code> fields of the cell are set, the cell pointer is
returned directly. The number assigned to NValues set by the function (1 in
this case) represents the number of values of the function.</p><p>In general, if one is to play with the C kernel of <span class="application">ECL</span> there is no need to
know about all these conventions. There is a preprocessor that takes care of
the details, by using a lisp representation of the statements that output
values, and of the function definitions. For instance, the actual source code
for <code class="literal">cl_cons</code> in <code class="filename">src/c/lists.d</code></p><pre class="screen">
@(defun cons (car cdr)
@
@(return CONS(car, cdr))
@)
</pre></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch26.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch26.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch26s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 3. The interpreter </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 3.3. The lexical environment</td></tr></table></div></body></html>
