r4817: Auto commit for Debian build

[uffi.git] / doc / ref.sgml

diff --git a/doc/ref.sgml b/doc/ref.sgml
index 39ae02ebaf9a920fceb1b76cef1a1671bd0f70fb..510dd4ca3a64351ce68ad4c22e25e95db7680d9a 100644 (file)
--- a/doc/ref.sgml
+++ b/doc/ref.sgml
@@ -1498,21 +1498,70 @@ if a cstring returned by a function is &null;.
 
          &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 and Lispworks, a cstring may not be a
-foreign type but rather the Lisp string itself while on other
-platforms a cstring is a newly allocated foreign vector for storing
-characters. Thus, it is not possible to portably
-<emphasis>allocate</emphasis> a cstring. 
+         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>
-<para>
-In contrast, foreign strings
-are always a foreign vector of characters which have a memory
-allocated to hold them. Because of this, if you need to allocate memory to
-hold the return value of a string, use a foreign string and not a cstring.
-       </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">
@@ -1655,7 +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>