Add two macros for char*-based pointer arithmetic and use it in font.c

This fixes errornous pointer addition (+ on a short*), which crashed in some situation.
Fixes FS#12317 and should hopefully get the clips booting again.

Thanks to Jonathan Gordon for spotting the bad pointer arithmetic.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30724 a1c6a512-1295-4272-9138-f99709370657

2 files changed, 15 insertions, 16 deletions

diff --git a/firmware/export/system.h b/firmware/export/system.h
index cec47f1..aa07994 100644
--- a/firmware/export/system.h
+++ b/firmware/export/system.h
@@ -124,6 +124,8 @@ int get_cpu_boost_counter(void);
     ptr = (typeof(ptr))tmp_ptr1; \
 }
 
+#define PTR_ADD(ptr, x) ((typeof(ptr))((char*)(ptr) + (x)))
+#define PTR_SUB(ptr, x) ((typeof(ptr))((char*)(ptr) - (x)))
 
 /* newer? SDL includes endian.h, So we ignore it */
 #if (CONFIG_PLATFORM & PLATFORM_HOSTED) || defined(__PCTOOL__)
diff --git a/firmware/font.c b/firmware/font.c
index 0546061..8cd9be1 100644
--- a/firmware/font.c
+++ b/firmware/font.c
@@ -88,26 +88,23 @@ static int buflibmove_callback(int handle, void* current, void* new)
 {
     (void)handle;
     struct buflib_alloc_data *alloc = (struct buflib_alloc_data*)current;
-    size_t diff = new - current;
+    ptrdiff_t diff = new - current;
 
     if (alloc->handle_locked)
         return BUFLIB_CB_CANNOT_MOVE;
 
-    if (alloc->font.bits)
-        alloc->font.bits += diff;
-    if (alloc->font.offset)
-        alloc->font.offset += diff;
-    if (alloc->font.width)
-        alloc->font.width += diff;
-
-    alloc->font.buffer_start += diff;
-    alloc->font.buffer_end += diff;
-    alloc->font.buffer_position += diff;
-
-    if (alloc->font.cache._index)
-        alloc->font.cache._index += diff;
-    if (alloc->font.cache._lru._base)
-        alloc->font.cache._lru._base += diff;
+#define UPDATE(x) if (x) { x = PTR_ADD(x, diff); }
+
+    UPDATE(alloc->font.bits);
+    UPDATE(alloc->font.offset);
+    UPDATE(alloc->font.width);
+
+    UPDATE(alloc->font.buffer_start);
+    UPDATE(alloc->font.buffer_end);
+    UPDATE(alloc->font.buffer_position);
+
+    UPDATE(alloc->font.cache._index);
+    UPDATE(alloc->font.cache._lru._base);
 
     return BUFLIB_CB_OK;
 }