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<reference id="strings">
  <title>Strings</title>
  <partintro>
    <title>Overview</title>
    <para>
      &uffi; has functions to two types of <varname>C</varname>-compatible
      strings: <emphasis>cstring</emphasis> and <emphasis>foreign</emphasis>
      strings.  cstrings are used <emphasis>only</emphasis> as parameters to
      and from functions. In some implementations a cstring is not a foreign
      type but rather the Lisp string itself. On other platforms a cstring
      is a newly allocated foreign vector for storing characters. The
      following is an example of using cstrings to both send and return a
      value.
    </para>
    
    <screen>
(uffi:def-function ("getenv" c-getenv) 
   ((name :cstring))
   :returning :cstring)

(defun my-getenv (key)
  "Returns an environment variable, or NIL if it does not exist"
  (check-type key string)
  (uffi:with-cstring (key-native key)
    (uffi:convert-from-cstring (c-getenv key-native))))
    </screen>

    <para>
      In contrast, foreign strings are always a foreign vector of
      characters which have memory allocated. Thus, if you need to
      allocate memory to hold the return value of a string, you must
      use a foreign string and not a cstring.  The following is an
      example of using a foreign string for a return value.
    </para>

    <screen>
(uffi:def-function ("gethostname" c-gethostname)
    ((name (* :unsigned-char))
     (len :int))
  :returning :int)

(defun gethostname ()
  "Returns the hostname"
  (let* ((name (uffi:allocate-foreign-string 256))
         (result-code (c-gethostname name 256))
         (hostname (when (zerop result-code)
                     (uffi:convert-from-foreign-string name))))
    ;; UFFI does not yet provide a universal way to free
    ;; memory allocated by C's malloc. At this point, a program
    ;; needs to call C's free function to free such memory.
    (unless (zerop result-code)
      (error "gethostname() failed."))))
    </screen>

    <para>
      Foreign functions that return pointers to freshly allocated
      strings should in general not return cstrings, but foreign
      strings. (There is no portable way to release such cstrings from
      Lisp.) The following is an example of handling such a function.
    </para>

    <screen>
(uffi:def-function ("readline" c-readline)
    ((prompt :cstring))
  :returning (* :char))

(defun readline (prompt)
  "Reads a string from console with line-editing."
  (with-cstring (c-prompt prompt)
      (let* ((c-str (c-readline c-prompt))
             (str (convert-from-foreign-string c-str)))
        (uffi:free-foreign-object c-str)
        str)))
    </screen>
    
  </partintro>
  
    <refentry id="convert-from-cstring">
      <refnamediv>
	<refname>convert-from-cstring</refname>
	<refpurpose>Converts a cstring to a Lisp string.</refpurpose>
	<refclass>Macro</refclass>
      </refnamediv>
      <refsynopsisdiv>
	<title>Syntax</title>
	<synopsis>
	  <function>convert-from-cstring</function>
	  <replaceable>cstring</replaceable> 
	  => 
	  <returnvalue>string</returnvalue>
	</synopsis>
      </refsynopsisdiv>
      <refsect1>
	<title>Arguments and Values</title>
	<variablelist>
	  <varlistentry>
	    <term><parameter>cstring</parameter></term>
	    <listitem>
	      <para>A cstring.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term><returnvalue>string</returnvalue></term>
	    <listitem>
	      <para>A Lisp string.
	      </para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </refsect1>
      <refsect1>
	<title>Description</title>
	<para>
	  Converts a Lisp string to a <constant>cstring</constant>. This is
	  most often used when processing the results of a foreign function
	  that returns a cstring.
	</para>
      </refsect1>
      <refsect1>
	<title>Side Effects</title>
	<para>None.</para>
      </refsect1>
      <refsect1>
	<title>Affected by</title>
	<para>None.</para>
      </refsect1>
      <refsect1>
	<title>Exceptional Situations</title>
	<para>None.</para>
      </refsect1>
  </refentry>
  
  
  <refentry id="convert-to-cstring">
    <refnamediv>
      <refname>convert-to-cstring</refname>
      <refpurpose>Converts a Lisp string to a cstring.</refpurpose>
      <refclass>Macro</refclass>
    </refnamediv>
    <refsynopsisdiv>
      <title>Syntax</title>
      <synopsis>
	<function>convert-to-cstring</function> 
	<replaceable>string</replaceable>
	=>
	<returnvalue>cstring</returnvalue>
      </synopsis>
    </refsynopsisdiv>
    <refsect1>
      <title>Arguments and Values</title>
      <variablelist>
	<varlistentry>
	  <term><parameter>string</parameter></term>
	  <listitem>
	    <para>A Lisp string.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><returnvalue>cstring</returnvalue></term>
	  <listitem>
	    <para>A cstring.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </refsect1>
    <refsect1>
      <title>Description</title>
      <para>
	Converts a Lisp string to a <varname>cstring</varname>. The
	<varname>cstring</varname> should be freed with
	<function>free-cstring</function>.
      </para>
    </refsect1>
    <refsect1>
      <title>Side Effects</title>
      <para>On some implementations, this function allocates memory.</para>
    </refsect1>
    <refsect1>
      <title>Affected by</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Exceptional Situations</title>
      <para>None.</para>
    </refsect1>
  </refentry>
  
  
  <refentry id="free-cstring">
    <refnamediv>
      <refname>free-cstring</refname>
      <refpurpose>Free memory used by cstring.
      </refpurpose>
      <refclass>Macro</refclass>
    </refnamediv>
    <refsynopsisdiv>
      <title>Syntax</title>
      <synopsis>
	<function>free-cstring</function> <replaceable>cstring</replaceable>
      </synopsis>
    </refsynopsisdiv>
    <refsect1>
      <title>Arguments and Values</title>
      <variablelist>
	<varlistentry>
	  <term><parameter>cstring</parameter></term>
	  <listitem>
	    <para>A cstring.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </refsect1>
    <refsect1>
      <title>Description</title>
      <para>
	Frees any memory possibly allocated by
	<function>convert-to-cstring</function>. On some implementions, a cstring is just the Lisp string itself.
      </para>
    </refsect1>
    <refsect1>
      <title>Side Effects</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Affected by</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Exceptional Situations</title>
      <para>None.</para>
    </refsect1>
  </refentry>
  
  
  <refentry id="with-cstring">
    <refnamediv>
      <refname>with-cstring</refname>
      <refpurpose>Binds a newly created cstring.</refpurpose>
      <refclass>Macro</refclass>
    </refnamediv>
    <refsynopsisdiv>
      <title>Syntax</title>
      <synopsis>
	<function>with-cstring</function>
	<replaceable>(cstring string) {body}</replaceable>
      </synopsis>
    </refsynopsisdiv>
    <refsect1>
      <title>Arguments and Values</title>
      <variablelist>
	<varlistentry>
	  <term><parameter>cstring</parameter></term>
	  <listitem>
	    <para>A symbol naming the cstring to be created.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><parameter>string</parameter></term>
	  <listitem>
	    <para>A Lisp string that will be translated to a cstring.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><parameter>body</parameter></term>
	  <listitem>
	    <para>The body of where the cstring will be bound.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </refsect1>
    <refsect1>
      <title>Description</title>
      <para>
	Binds a symbol to a cstring created from conversion of a
	string. Automatically frees the <varname>cstring</varname>.
      </para>
    </refsect1>
    <refsect1>
      <title>Examples</title>
      <para>
	<screen>
(def-function ("getenv" c-getenv) 
   ((name :cstring))
   :returning :cstring)

(defun getenv (key)
  "Returns an environment variable, or NIL if it does not exist"
  (check-type key string)
  (with-cstring (key-cstring key)
    (convert-from-cstring (c-getenv key-cstring))))
	</screen>
      </para>
    </refsect1>
    <refsect1>
      <title>Side Effects</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Affected by</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Exceptional Situations</title>
      <para>None.</para>
    </refsect1>
  </refentry>
  
  
  <refentry id="convert-from-foreign-string">
    <refnamediv>
      <refname>convert-from-foreign-string</refname>
      <refpurpose>Converts a foreign string into a Lisp string.</refpurpose>
      <refclass>Macro</refclass>
    </refnamediv>
    <refsynopsisdiv>
      <title>Syntax</title>
      <synopsis>
	<function>convert-from-foreign-string</function>
	<replaceable>foreign-string &amp;key length null-terminated-p</replaceable>
	=>
	<returnvalue>string</returnvalue>
      </synopsis>
    </refsynopsisdiv>
    <refsect1>
      <title>Arguments and Values</title>
      <variablelist>
	<varlistentry>
	  <term><parameter>foreign-string</parameter></term>
	  <listitem>
	    <para>A foreign string.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><parameter>length</parameter></term>
	  <listitem>
	    <para>The length of the foreign string to convert. The
	    default is the length of the string until a &null;
	    character is reached.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><parameter>null-terminated-p</parameter></term>
	  <listitem>
	    <para>A boolean flag with a default value of &t; When true,
	    the string is converted until the first &null; character is reached.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><returnvalue>string</returnvalue></term>
	  <listitem>
	    <para>A Lisp string.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </refsect1>
    <refsect1>
      <title>Description</title>
      <para>
	Returns a Lisp string from a foreign string. 
	Can translated ASCII and binary strings.
      </para>
    </refsect1>
    <refsect1>
      <title>Side Effects</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Affected by</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Exceptional Situations</title>
      <para>None.</para>
    </refsect1>
  </refentry>
  
  
  <refentry id="convert-to-foreign-string">
    <refnamediv>
      <refname>convert-to-foreign-string</refname>
      <refpurpose>Converts a Lisp string to a foreign string.
      </refpurpose>
      <refclass>Macro</refclass>
    </refnamediv>
    <refsynopsisdiv>
      <title>Syntax</title>
      <synopsis>
	<function>convert-to-foreign-string</function>
	<replaceable>string</replaceable> =>
	<returnvalue>foreign-string</returnvalue>
      </synopsis>
    </refsynopsisdiv>
    <refsect1>
      <title>Arguments and Values</title>
      <variablelist>
	<varlistentry>
	  <term><parameter>string</parameter></term>
	  <listitem>
	    <para>A Lisp string.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><returnvalue>foreign-string</returnvalue></term>
	  <listitem>
	    <para>A foreign string.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </refsect1>
    <refsect1>
      <title>Description</title>
      <para>
	Converts a Lisp string to a foreign string. Memory should be
	freed with <function>free-foreign-object</function>.
      </para>
    </refsect1>
    <refsect1>
      <title>Side Effects</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Affected by</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Exceptional Situations</title>
      <para>None.</para>
    </refsect1>
  </refentry>
  
  <refentry id="allocate-foreign-string">
    <refnamediv>
      <refname>allocate-foreign-string</refname>
      <refpurpose>Allocates space for a foreign string.
      </refpurpose>
      <refclass>Macro</refclass>
    </refnamediv>
    <refsynopsisdiv>
      <title>Syntax</title>
      <synopsis>
	<function>allocate-foreign-string</function> <replaceable>size
	&amp;key unsigned</replaceable> =>
	<returnvalue>foreign-string</returnvalue>
      </synopsis>
    </refsynopsisdiv>
    <refsect1>
      <title>Arguments and Values</title>
      <variablelist>
	<varlistentry>
	  <term><parameter>size</parameter></term>
	  <listitem>
	    <para>The size of the space to be allocated in bytes.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><parameter>unsigned</parameter></term>
	  <listitem>
	    <para>A boolean flag with a default value of &t;. When true,
	    marks the pointer as an <constant>:unsigned-char</constant>.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><returnvalue>foreign-string</returnvalue></term>
	  <listitem>
	    <para>A foreign string which has undefined contents.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </refsect1>
    <refsect1>
      <title>Description</title>
      <para>
	Allocates space for a foreign string. Memory should
	be freed with <function>free-foreign-object</function>.
      </para>
    </refsect1>
    <refsect1>
      <title>Side Effects</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Affected by</title>
      <para>None.</para>
    </refsect1>
    <refsect1>
      <title>Exceptional Situations</title>
      <para>None.</para>
    </refsect1>
  </refentry>
  
</reference>