diff --git a/CMakeLists.txt b/CMakeLists.txt
index fdfc1e6..06e6043 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,6 +3,7 @@ project(flint C)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 set(CMAKE_C_STANDARD 99)
+add_definitions(-D_POSIX_C_SOURCE=200112L)
 include_directories(include)
 
 set(SOURCES
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