diff --git a/CMakeLists.txt b/CMakeLists.txt
index fdfc1e6..4cadfd8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,6 +2,10 @@ cmake_minimum_required(VERSION 3.17)
 project(flint C)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
+if ((${CMAKE_SYSTEM_NAME} STREQUAL "Linux"))
+    add_compile_definitions(flint __USE_XOPEN_EXTENDED)
+endif()
+
 set(CMAKE_C_STANDARD 99)
 include_directories(include)
 
diff --git a/Makefile b/Makefile
index 5aa38ee..2821104 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 .PHONY : clean tests all manual
 
-CFLAGS =		-std=c99 -Iinclude -pedantic
+CFLAGS =		-std=c99 -Iinclude -pedantic -D_POSIX_C_SOURCE=200112L
 WARNINGS= 	-Wall -Wextra
 LDFLAGS =		-fPIC -shared
 MACROS= 		-D_POSIX_C_SOURCE=1