diff --git a/src/include/gpxe/ramdisk.h b/src/include/gpxe/ramdisk.h
index 738c6705..4a77f05e 100644
--- a/src/include/gpxe/ramdisk.h
+++ b/src/include/gpxe/ramdisk.h
@@ -16,7 +16,7 @@ struct ramdisk {
 	userptr_t data;
 };
 
-int init_ramdisk ( struct ramdisk *ramdisk, userptr_t data, size_t len,
-		   unsigned int blksize );
+extern int init_ramdisk ( struct ramdisk *ramdisk, userptr_t data, size_t len,
+			  unsigned int blksize );
 
 #endif /* _GPXE_RAMDISK_H */