diff --git a/README b/README
index d3cfc33..ff2e1cd 100644
--- a/README
+++ b/README
@@ -45,7 +45,7 @@ effort, ease of customization, and long-term maintenance.
 - a C toolchain, consisting of
     - a C compiler such as gcc, clang, or [cparser];
     - the compiler should support -std=c99;
-    - the system's libc should support -D_XOPEN_SOURCE=500.
+    - the system's libc should support -D_XOPEN_SOURCE=600.
 
 ## Usage
 
diff --git a/config.project b/config.project
index e9e8761..6144f6d 100644
--- a/config.project
+++ b/config.project
@@ -19,7 +19,7 @@ mb_default_shell=sh
 
 
 # switches
-mb_default_cflags_common="-std=c99 -D_XOPEN_SOURCE=500"
+mb_default_cflags_common="-std=c99 -D_XOPEN_SOURCE=600"
 mb_default_cflags_common="$mb_default_cflags_common -D_DEFAULT_SOURCE -D_BSD_SOURCE"
 mb_default_cflags_common="$mb_default_cflags_common -I\$(PROJECT_DIR)/src/internal"
 mb_default_cflags_common="$mb_default_cflags_common -I\$(PROJECT_DIR)/include"