diff --git a/SLS_C/Makefile b/SLS_C/Makefile
index e73a172..5ab1a65 100644
--- a/SLS_C/Makefile
+++ b/SLS_C/Makefile
@@ -1,9 +1,9 @@
 # Makefile for SLS project with automatic header dependencies
 
 CC ?= gcc
-CFLAGS ?= -std=c11 -Wall -Wextra -g -Iinclude -MMD -MP
+CFLAGS ?= -std=c99 -Wall -Wextra -g -Iinclude -MMD -MP
 LDFLAGS ?=
-CTESTFLAGS ?= -std=c11 -Wall -Wextra -Wno-unused-function -g -O0 -Iinclude -MMD -MP
+CTESTFLAGS ?= -std=c99 -Wall -Wextra -Wno-unused-function -g -O0 -Iinclude -MMD -MP
 
 SRCDIR := src
 OBJDIR := obj