I: Program causes undefined operation
   (likely same variable used twiceand post/pre incremented in the same expression).
   e.g. x = x++; Split it in two operations.
W: ruby sequence-point yaml2byte.c:67, 104

yaml2byte.c: In function 'bytestring_append':
yaml2byte.c:67:21: warning: operation on 'str->buffer' may be undefined
yaml2byte.c: In function 'bytestring_extend':
yaml2byte.c:104:25: warning: operation on 'str->buffer' may be undefined

#define S_REALLOC_N(var,type,n) (var)=(type*)realloc((char*)(var),sizeof(type)*(n))

so the old code expanded to:
str->buffer = str->buffer = (char*)realloc((char*)str->buffer, sizeof(char)*str->length + 1)

Index: ext/syck/yaml2byte.c
===================================================================
--- ext/syck/yaml2byte.c	(revision 27446)
+++ ext/syck/yaml2byte.c	(working copy)
@@ -64,7 +64,7 @@
         grow = (length - str->remaining) + CHUNKSIZE;
         str->remaining += grow;
         str->length    += grow; 
-        str->buffer = S_REALLOC_N( str->buffer, char, str->length + 1 );
+        S_REALLOC_N( str->buffer, char, str->length + 1 );
         assert(str->buffer);
     }
     curr = str->buffer + (str->length - str->remaining);
@@ -101,7 +101,7 @@
             grow = (length - str->remaining) + CHUNKSIZE;
             str->remaining += grow;
             str->length    += grow; 
-            str->buffer = S_REALLOC_N( str->buffer, char, str->length + 1 );
+            S_REALLOC_N( str->buffer, char, str->length + 1 );
         }
         curr = str->buffer + (str->length - str->remaining);
         from = ext->buffer;