slirp: document mbuf pointers and sizes

and fix confusing datasize name into gapsize in m_inc.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

slirp/mbuf.c

@@ -151,7 +151,7 @@ m_cat(struct mbuf *m, struct mbuf *n)
 void
 m_inc(struct mbuf *m, int size)
 {
-    int datasize;
+    int gapsize;
 
     /* some compilers throw up on gotos.  This one we can fake. */
     if (M_ROOM(m) > size) {
@@ -159,17 +159,17 @@ m_inc(struct mbuf *m, int size)
     }
 
     if (m->m_flags & M_EXT) {
-        datasize = m->m_data - m->m_ext;
-        m->m_ext = g_realloc(m->m_ext, size + datasize);
+        gapsize = m->m_data - m->m_ext;
+        m->m_ext = g_realloc(m->m_ext, size + gapsize);
     } else {
-        datasize = m->m_data - m->m_dat;
-        m->m_ext = g_malloc(size + datasize);
+        gapsize = m->m_data - m->m_dat;
+        m->m_ext = g_malloc(size + gapsize);
         memcpy(m->m_ext, m->m_dat, m->m_size);
         m->m_flags |= M_EXT;
     }
 
-    m->m_data = m->m_ext + datasize;
-    m->m_size = size + datasize;
+    m->m_data = m->m_ext + gapsize;
+    m->m_size = size + gapsize;
 }
 
 

slirp/mbuf.h

@@ -47,6 +47,19 @@
  * free the m_ext.  This is inefficient memory-wise, but who cares.
  */
 
+/*
+ * mbufs allow to have a gap between the start of the allocated buffer (m_ext if
+ * M_EXT is set, m_dat otherwise) and the in-use data:
+ *
+ *  |--gapsize----->|---m_len------->
+ *  |----------m_size------------------------------>
+ *                  |----M_ROOM-------------------->
+ *                                   |-M_FREEROOM-->
+ *
+ *  ^               ^                               ^
+ *  m_dat/m_ext     m_data                          end of buffer
+ */
+
 /*
  * How much room is in the mbuf, from m_data to the end of the mbuf
  */