r4817: Auto commit for Debian build

[uffi.git] / doc / ref.sgml

diff --git a/doc/ref.sgml b/doc/ref.sgml
index 827ac079f6e3e39f5102540b8a8964bc3d1c2c59..510dd4ca3a64351ce68ad4c22e25e95db7680d9a 100644 (file)
--- a/doc/ref.sgml
+++ b/doc/ref.sgml
@@ -1495,13 +1495,73 @@ if a cstring returned by a function is &null;.
 <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 as parameters to and from functions. An implementation, such as CMUCL,
-may not convert these to a foreign type for efficiency sake. Thus, it is not
-possible to "allocate" a cstring. In contrast, foreign strings
-always need to have memory for them.
-       </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>
+
+<programlisting>
+(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))))
+</programlisting>
+
+<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>
+
+<programlisting>
+(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:free-foreign-object name)
+    (unless (zerop result-code)
+      (error "gethostname() failed."))))
+</programlisting>
+
+<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>
+
+<programlisting>
+(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)))
+</programlisting>
+
 </partintro>
 
     <refentry id="convert-from-cstring">
@@ -1602,7 +1662,7 @@ that returns a cstring.
       </refsect1>
       <refsect1>
        <title>Side Effects</title>
-       <para>None.</para>
+       <para>On some implementations, this function allocates memory.</para>
       </refsect1>
       <refsect1>
        <title>Affected by</title>
@@ -1644,7 +1704,7 @@ that returns a cstring.
        <title>Description</title>
        <para>
          Frees any memory possibly allocated by
-         <function>convert-to-cstring</function>.
+         <function>convert-to-cstring</function>. On some implementions, a cstring is just the Lisp string itself.
        </para>
       </refsect1>
       <refsect1>
@@ -1704,7 +1764,7 @@ that returns a cstring.
       <refsect1>
        <title>Description</title>
        <para>
-         Binds a lexical variable to a newly allocated <varname>cstring</varname>. Automatically frees <varname>cstring</varname>.
+         Binds a symbol to a cstring created from conversion of a string. Automatically frees the <varname>cstring</varname>.
        </para>
       </refsect1>
       <refsect1>