diff --git a/database/DBcellsrch.c b/database/DBcellsrch.c
index 2c14301a..354eadc3 100644
--- a/database/DBcellsrch.c
+++ b/database/DBcellsrch.c
@@ -677,7 +677,7 @@ dbCellLabelSrFunc(scx, fp)
 {
     Label *lab;
     Rect *r = &scx->scx_area;
-    TileTypeBitMask *mask = fp->tf_mask;
+    const TileTypeBitMask *mask = fp->tf_mask;
     CellDef *def = scx->scx_use->cu_def;
     char *tnext;
     int result;
diff --git a/database/DBconnect.c b/database/DBconnect.c
index 74af4e4f..9ab280f8 100644
--- a/database/DBconnect.c
+++ b/database/DBconnect.c
@@ -474,7 +474,7 @@ dbSrConnectFunc(tile, csa)
     int i, pNum;
     int result = 0;
     bool callClient;
-    TileTypeBitMask *connectMask;
+    const TileTypeBitMask *connectMask;
     TileType loctype, checktype;
     PlaneMask planes;
 
@@ -826,7 +826,7 @@ dbcConnectLabelFunc(scx, lab, tpath, csa2)
 	CellDef *orig_def = scx->scx_use->cu_def;
 	Label *slab;
 	int lidx = lab->lab_port;
-	TileTypeBitMask *connectMask;
+	const TileTypeBitMask *connectMask;
 
 	/* Check for equivalent ports. For any found, call	*/
 	/* DBTreeSrTiles recursively on the type and area	*/
@@ -940,7 +940,8 @@ dbcConnectFunc(tile, cx)
 {
     struct conSrArg2 *csa2;
     Rect tileArea, newarea;
-    TileTypeBitMask *connectMask, notConnectMask;
+    const TileTypeBitMask *connectMask;
+    TileTypeBitMask notConnectMask;
     Rect *srArea;
     SearchContext *scx = cx->tc_scx;
     SearchContext scx2;
@@ -1155,7 +1156,7 @@ DBTreeCopyConnect(scx, mask, xMask, connect, area, doLabels, destUse)
 					 */
 {
     struct conSrArg2 csa2;
-    TileTypeBitMask *newmask;
+    const TileTypeBitMask *newmask;
     TileType newtype;
     unsigned char searchtype;
 
diff --git a/database/database.h.in b/database/database.h.in
index 299754ef..0a0b16a8 100644
--- a/database/database.h.in
+++ b/database/database.h.in
@@ -647,7 +647,7 @@ typedef struct treeFilter
 {
     int (*tf_func)();		/* Client's filter function */
     ClientData tf_arg;		/* Client's argument to pass to filter */
-    TileTypeBitMask *tf_mask;	/* Only process tiles with these types */
+    const TileTypeBitMask *tf_mask;	/* Only process tiles with these types */
     int tf_xmask;		/* Expand mask */
     PlaneMask tf_planes;	/* Mask of planes which will be visited */
     TileType tf_dinfo;		/* Info for processing triangular areas */
@@ -685,7 +685,7 @@ struct conSrArg
 {
     CellDef *csa_def;                   /* Definition being searched. */
     int csa_pNum;			/* Index of plane being searched */
-    TileTypeBitMask *csa_connect;       /* Table indicating what connects
+    const TileTypeBitMask *csa_connect; /* Table indicating what connects
                                          * to what.
                                          */
     int (*csa_clientFunc)();            /* Client function to call. */
@@ -700,19 +700,19 @@ struct conSrArg
 typedef struct
 {
     Rect                area;           /* Area to process */
-    TileTypeBitMask     *connectMask;   /* Connection mask for search */
+    const TileTypeBitMask *connectMask; /* Connection mask for search */
     TileType            dinfo;          /* Info about triangular search areas */
 } conSrArea;
 
 struct conSrArg2
 {
     CellUse             *csa2_use;      /* Destination use */
-    TileTypeBitMask     *csa2_connect;  /* Table indicating what connects
+    const TileTypeBitMask *csa2_connect;/* Table indicating what connects
                                          * to what.
                                          */
     SearchContext       *csa2_topscx;   /* Original top-level search context */
     int                  csa2_xMask;    /* Cell window mask for search */
-    Rect                *csa2_bounds;   /* Area that limits the search */
+    const Rect          *csa2_bounds;   /* Area that limits the search */
 
     Stack               *csa2_stack;    /* Stack of full csa2_list entries */
     conSrArea           *csa2_list;     /* List of areas to process */