diff --git a/common/jpeg/jmemsys.h b/common/jpeg/jmemsys.h
index 6c3c6d3..b190945 100644
--- a/common/jpeg/jmemsys.h
+++ b/common/jpeg/jmemsys.h
@@ -100,10 +100,10 @@
  * Conversely, zero may be returned to always use the minimum amount of memory.
  */
 
-EXTERN(long) jpeg_mem_available JPP((j_common_ptr cinfo,
-				     long min_bytes_needed,
-				     long max_bytes_needed,
-				     long already_allocated));
+EXTERN(size_t) jpeg_mem_available JPP((j_common_ptr cinfo,
+				     size_t min_bytes_needed,
+				     size_t max_bytes_needed,
+				     size_t already_allocated));
 
 
 /*