dfilter: Use a constant to declare function return type

Simplify the interface to declare the function. This also makes
it easier to infer the function return type from a compiled
filter.

Print the function return type when dumping the compiled filter
or <***> if the type is unknown from the function name.

epan/dfilter/dfunctions.c

@@ -430,45 +430,19 @@ ul_semcheck_absolute_value(dfwork_t *dfw, const char *func_name, ftenum_t logica
     return ftype;
 }
 
-static ftenum_t
-return_string(GSList *param_list _U_)
-{
-    return FT_STRING;
-}
-
-static ftenum_t
-return_unsigned(GSList *param_list _U_)
-{
-    return FT_UINT32;
-}
-
-static ftenum_t
-return_compare(GSList *param_list)
-{
-    GSList *l;
-    ftenum_t ftype;
-
-    for (l = param_list; l != NULL; l = l->next) {
-        ftype = get_logical_ftype(l->data);
-        if (ftype != FT_NONE)
-            return ftype;
-    }
-    return FT_NONE;
-}
-
 /* The table of all display-filter functions */
 static df_func_def_t
 df_functions[] = {
-    { "lower",  df_func_lower,  1, 1, return_string, ul_semcheck_is_field_string },
-    { "upper",  df_func_upper,  1, 1, return_string, ul_semcheck_is_field_string },
+    { "lower",  df_func_lower,  1, 1, FT_STRING, ul_semcheck_is_field_string },
+    { "upper",  df_func_upper,  1, 1, FT_STRING, ul_semcheck_is_field_string },
     /* Length function is implemented as a DFVM instruction. */
-    { "len",    NULL,           1, 1, return_unsigned, ul_semcheck_can_length },
-    { "count",  df_func_count,  1, 1, return_unsigned, ul_semcheck_is_field },
-    { "string", df_func_string, 1, 1, return_string, ul_semcheck_string_param },
-    { "max",    df_func_max,    1, 0, return_compare, ul_semcheck_compare },
-    { "min",    df_func_min,    1, 0, return_compare, ul_semcheck_compare },
-    { "abs",    df_func_abs,    1, 1, return_compare, ul_semcheck_absolute_value },
-    { NULL, NULL, 0, 0, NULL, NULL }
+    { "len",    NULL,           1, 1, FT_UINT32, ul_semcheck_can_length },
+    { "count",  df_func_count,  1, 1, FT_UINT32, ul_semcheck_is_field },
+    { "string", df_func_string, 1, 1, FT_STRING, ul_semcheck_string_param },
+    { "max",    df_func_max,    1, 0, FT_NONE, ul_semcheck_compare },
+    { "min",    df_func_min,    1, 0, FT_NONE, ul_semcheck_compare },
+    { "abs",    df_func_abs,    1, 1, FT_NONE, ul_semcheck_absolute_value },
+    { NULL, NULL, 0, 0, FT_NONE, NULL }
 };
 
 /* Lookup a display filter function record by name */

epan/dfilter/dfunctions.h

@@ -20,9 +20,6 @@
 /* The run-time logic of the dfilter function */
 typedef bool (*DFFuncType)(GSList *stack, uint32_t arg_count, df_cell_t *retval);
 
-/* The return type for the dfilter function */
-typedef ftenum_t (*DFReturnType)(GSList *param_list);
-
 /* The semantic check for the dfilter function */
 typedef ftenum_t (*DFSemCheckType)(dfwork_t *dfw, const char *func_name, ftenum_t lhs_ftype,
                                 GSList *param_list, df_loc_t func_loc);
@@ -34,7 +31,8 @@ typedef struct {
     DFFuncType      function;
     unsigned        min_nargs;
     unsigned        max_nargs; /* 0 for no limit */
-    DFReturnType    return_type;
+    ftenum_t        return_ftype; /* Can be FT_NONE if the function returns the same type
+                                   * as its arguments. */
     DFSemCheckType  semcheck_param_function;
 } df_func_def_t;
 

epan/dfilter/dfvm.c

@@ -281,6 +281,12 @@ value_type_tostr(dfvm_value_t *v, bool show_ftype)
 		case FVALUE:
 			s = fvalue_type_name(dfvm_value_get_fvalue(v));
 			break;
+		case FUNCTION_DEF:
+			if (v->value.funcdef->return_ftype != FT_NONE)
+				s = ftype_name(v->value.funcdef->return_ftype);
+			else
+				s = "***";
+			break;
 		default:
 			return ws_strdup("");
 			break;
@@ -305,8 +311,8 @@ dump_str_stack_pop(GSList *stack, uint32_t count)
 }
 
 static void
-append_call_function(wmem_strbuf_t *buf, const char *func, uint32_t nargs,
-					GSList *stack_print)
+append_call_function(wmem_strbuf_t *buf, const char *func, const char *func_type,
+			uint32_t nargs, GSList *stack_print)
 {
 	uint32_t idx;
 	GString	*gs;
@@ -324,7 +330,7 @@ append_call_function(wmem_strbuf_t *buf, const char *func, uint32_t nargs,
 		wmem_strbuf_append(buf, gs->str);
 		g_string_free(gs, true);
 	}
-	wmem_strbuf_append(buf, ")");
+	wmem_strbuf_append_printf(buf, ")%s", func_type);
 }
 
 static void
@@ -412,7 +418,8 @@ append_op_args(wmem_strbuf_t *buf, dfvm_insn_t *insn, GSList **stack_print,
 			break;
 
 		case DFVM_CALL_FUNCTION:
-			append_call_function(buf, arg1_str, arg3->value.numeric, *stack_print);
+			append_call_function(buf, arg1_str, arg1_str_type,
+						arg3->value.numeric, *stack_print);
 			indent2(buf, col_start);
 			append_to_register(buf, arg2_str);
 			break;

epan/dfilter/semcheck.c

@@ -674,9 +674,18 @@ get_function_ftype(stnode_t *st_node)
 	params   = sttype_function_params(st_node);
 	nparams  = g_slist_length(params);
 
+	if (funcdef->return_ftype != FT_NONE)
+		return funcdef->return_ftype;
 	if (nparams < 1)
 		return FT_NONE;
-	return funcdef->return_type(params);
+
+	for (GSList *l = params; l != NULL; l = l->next) {
+		ftenum_t ftype = get_logical_ftype(params->data);
+		if (ftype != FT_NONE) {
+			return ftype;
+		}
+	}
+	return FT_NONE;
 }
 
 ftenum_t