diff --git a/frontends/ast/genrtlil.cc b/frontends/ast/genrtlil.cc
index 5fd478ff3..84f2f9d73 100644
--- a/frontends/ast/genrtlil.cc
+++ b/frontends/ast/genrtlil.cc
@@ -1539,7 +1539,7 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)
 				if (dynamic_cast<RTLIL::Binding*>(current_module)) {
 					/* nothing to do here */
 				} else if (flag_autowire)
-					log_file_warning(*location.begin.filename, location.begin.line, "Identifier `%s' is implicitly declared.\n", str.c_str());
+					log_file_warning(*location.begin.filename, location.begin.line, "Identifier `%s' is implicitly declared.\n", str);
 				else
 					input_error("Identifier `%s' is implicitly declared and `default_nettype is set to none.\n", str);
 			}
@@ -2200,7 +2200,7 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)
 					log_file_info(*location.begin.filename, location.begin.line, "\n");
 			} else if (str == "$warning") {
 				if (sz > 0)
-					log_file_warning(*location.begin.filename, location.begin.line, "%s.\n", children[0]->str.c_str());
+					log_file_warning(*location.begin.filename, location.begin.line, "%s.\n", children[0]->str);
 				else
 					log_file_warning(*location.begin.filename, location.begin.line, "\n");
 			} else if (str == "$error") {
diff --git a/frontends/ast/simplify.cc b/frontends/ast/simplify.cc
index 9b679d0aa..d4e78b730 100644
--- a/frontends/ast/simplify.cc
+++ b/frontends/ast/simplify.cc
@@ -1032,7 +1032,7 @@ bool AstNode::simplify(bool const_fold, int stage, int width_hint, bool sign_hin
 	// note that $display, $finish, and $stop are used for synthesis-time DRC so they're not in this list
 	if ((type == AST_FCALL || type == AST_TCALL) && (str == "$strobe" || str == "$monitor" || str == "$time" ||
 			str == "$dumpfile" || str == "$dumpvars" || str == "$dumpon" || str == "$dumpoff" || str == "$dumpall")) {
-		log_file_warning(*location.begin.filename, location.begin.line, "Ignoring call to system %s %s.\n", type == AST_FCALL ? "function" : "task", str.c_str());
+		log_file_warning(*location.begin.filename, location.begin.line, "Ignoring call to system %s %s.\n", type == AST_FCALL ? "function" : "task", str);
 		delete_children();
 		str = std::string();
 	}
@@ -1042,7 +1042,7 @@ bool AstNode::simplify(bool const_fold, int stage, int width_hint, bool sign_hin
 		 str == "$write"   || str == "$writeb"   || str == "$writeh"   || str == "$writeo"))
 	{
 		if (!current_always) {
-			log_file_warning(*location.begin.filename, location.begin.line, "System task `%s' outside initial or always block is unsupported.\n", str.c_str());
+			log_file_warning(*location.begin.filename, location.begin.line, "System task `%s' outside initial or always block is unsupported.\n", str);
 			delete_children();
 			str = std::string();
 		} else {
diff --git a/frontends/verilog/const2ast.cc b/frontends/verilog/const2ast.cc
index 661b4ef96..573af336b 100644
--- a/frontends/verilog/const2ast.cc
+++ b/frontends/verilog/const2ast.cc
@@ -49,7 +49,7 @@ void ConstParser::log_maybe_loc_error(std::string msg) {
 }
 
 void ConstParser::log_maybe_loc_warn(std::string msg) {
-	log_file_warning(*loc.begin.filename, loc.begin.line, "%s", msg.c_str());
+	log_file_warning(*loc.begin.filename, loc.begin.line, "%s", msg);
 }
 
 // divide an arbitrary length decimal number by two and return the rest