diff --git a/meson.build b/meson.build
index 0a2cc04..fff2c0d 100644
--- a/meson.build
+++ b/meson.build
@@ -17,7 +17,8 @@
   add_languages('cpp')
 endif
 
-add_project_arguments('-D_DEFAULT_SOURCE', language: ['c'])
+# For memmem() in src/msgbuf.h
+add_project_arguments('-D_GNU_SOURCE', language: ['c'])
 
 compiler = meson.get_compiler('c')
 conf = configuration_data()
diff --git a/src/msgbuf.h b/src/msgbuf.h
index 1d06a75..c2c02fe 100644
--- a/src/msgbuf.h
+++ b/src/msgbuf.h
@@ -2,13 +2,6 @@
 #ifndef PLDM_MSGBUF_H
 #define PLDM_MSGBUF_H
 
-// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp)
-#ifndef _GNU_SOURCE
-/* For memmem(3) */
-#define _GNU_SOURCE
-#endif
-// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp)
-
 /*
  * Historically, many of the structs exposed in libpldm's public headers are
  * defined with __attribute__((packed)). This is unfortunate: it gives the
@@ -1140,7 +1133,7 @@
 	 * not form a UTF16 NUL _code-point_ due to alignment with respect to the
 	 * start of the string
 	 */
-	end = (char *)ctx->cursor;
+	end = ctx->cursor;
 	do {
 		if (end != ctx->cursor) {
 			/*
@@ -1150,8 +1143,8 @@
 			end = (char *)end + 1;
 		}
 		measured = (char *)end - (char *)ctx->cursor;
-		end = (char *)memmem(end, ctx->remaining - measured, &term,
-				     sizeof(term));
+		end = memmem(end, ctx->remaining - measured, &term,
+			     sizeof(term));
 	} while (end && ((uintptr_t)end & 1) != ((uintptr_t)ctx->cursor & 1));
 
 	if (!end) {
diff --git a/src/requester/instance-id.c b/src/requester/instance-id.c
index a2700af..d891a48 100644
--- a/src/requester/instance-id.c
+++ b/src/requester/instance-id.c
@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
 // NOLINTNEXTLINE(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp)
-#define _GNU_SOURCE
 #include <libpldm/instance-id.h>
 #include <libpldm/pldm.h>
 
diff --git a/src/transport/test.c b/src/transport/test.c
index 2a929cc..1260031 100644
--- a/src/transport/test.c
+++ b/src/transport/test.c
@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
 /* NOLINTNEXTLINE(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp) */
-#define _GNU_SOURCE
 #include "array.h"
 #include "container-of.h"
 #include "transport.h"