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()