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>

2 files changed, 20 insertions, 7 deletions

diff --git a/mbuf.c b/mbuf.c
index 9335f9e..79abd18 100644
--- a/mbuf.c
+++ b/mbuf.c
@@ -145,7 +145,7 @@ void m_cat(struct mbuf *m, struct mbuf *n)
 /* make m 'size' bytes large from m_data */
 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) {
@@ -153,17 +153,17 @@ void 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;
 }
 
 
diff --git a/mbuf.h b/mbuf.h
index ec396c8..5aaf2b4 100644
--- a/mbuf.h
+++ b/mbuf.h
@@ -48,6 +48,19 @@
  */
 
 /*
+ * 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
  */
 #define M_ROOM(m)                                                        \