diff --git a/Makefile b/Makefile
index 2849938..6b283bb 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,28 @@
 
-CC := gcc
-CFLAGS := -g -O3 -std=c2x -Wall -Wextra -Wpedantic -Werror -Iinclude
+all:
+
+BUILD ?= debug
+COMPILER ?= gcc
+
+# ==== set compiler flags ====
+# credits Maxim Egorushkin:
+# https://stackoverflow.com/questions/48791883/best-practice-for-building-a-make-file/48793058#48793058
+
+CC := $(COMPILER)
+
+# -fsanitize={address,undefined}
+CFLAGS.gcc.debug := -Og -ggdb -pg -fsanitize=address -fno-omit-frame-pointer
+CFLAGS.gcc.release := -O3 -g -march=native -DNDEBUG
+CFLAGS.gcc := -fanalyzer
+
+CFLAGS.clang.debug :=-O0 -g3 -DBACKTRACE -rdynamic
+CFLAGS.clang.release :=-O3 -g -march=native -DNDEBUG
+CFLAGS.clang :=
+
+CFLAGS := ${CFLAGS.${COMPILER}} ${CFLAGS.${COMPILER}.${BUILD}} \
+		  -Iinclude \
+		  -std=c2x \
+		  -Wall -Wextra -Wpedantic -Werror
 
 BUILD_DIR := build
 BIN_DIR := lib
@@ -9,6 +31,8 @@ TEST_DIR := test
 OBJS := $(BUILD_DIR)/arena.o
 FPIC_OBJS := $(BUILD_DIR)/fpic/arena.o
 
+all : static
+
 static : $(BIN_DIR)/libarena.a
 
 dynamic : $(BIN_DIR)/libarena.so
@@ -16,13 +40,13 @@ dynamic : $(BIN_DIR)/libarena.so
 tests : test/test_arena
 
 $(TEST_DIR)/test_arena : src/test_arena.c $(BIN_DIR)/libarena.a | $(TEST_DIR)
-	$(CC) -o $@ -O0 -Iinclude -ggdb $^
+	$(CC) -o $@ $(CFLAGS) $^
 
 $(BIN_DIR)/libarena.a : $(OBJS) | $(BIN_DIR)
 	ar cr $@ $^
 
 $(BIN_DIR)/libarena.so : $(FPIC_OBJS) | $(BIN_DIR)
-	$(CC) -shared -o $@ $^
+	$(CC) -o $@ -shared $(CFLAGS) $^
 
 $(BUILD_DIR)/%.o : src/%.c | $(BUILD_DIR)
 	$(CC) -o $@ -c $(CFLAGS) $<
diff --git a/src/arena.c b/src/arena.c
index a72cbf4..5af820e 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -58,11 +58,13 @@ void _arena_new_page(arena_t* a, size_t size)
         return;
     }
 
-    a->head->next = calloc(1, sizeof *(a->head->next));
+    void* tmp = calloc(1, sizeof *(a->head->next));
 
-    if (a->head->next == NULL)
+    if (tmp == NULL)
         exit(errno);
 
+    a->head->next = tmp;
+
     a->head = a->head->next;
     a->head->data = malloc(size);
 
@@ -115,12 +117,14 @@ void* arena_calloc(arena_t* a, size_t nmemb, size_t size)
 void* arena_realloc_tail(arena_t* a, size_t len)
 {
     if (a->head->offset == BIG_PAGE) {
-        a->head->data = realloc(a->head->data, len);
+        void* tmp = realloc(a->head->data, len);
 
-        if (a->head->data == NULL)
+        if (tmp == NULL)
             exit(errno);
 
-        return a->head->data;
+        a->head->data = tmp;
+
+        return tmp;
     }
 
     a->head->offset = a->head->prev_offset;
diff --git a/src/test_arena.c b/src/test_arena.c
index d44125e..55d34fe 100644
--- a/src/test_arena.c
+++ b/src/test_arena.c
@@ -17,10 +17,10 @@ int main()
     for (size_t i = 0; i < 1024; i++) {
         char* c = arena_alloc(&default_arena, sizeof *c);
 
-        *c = i & 0xFF;
-
         if (c == NULL)
             err(EXIT_FAILURE, "failed to allocate memory");
+
+        *c = i & 0xFF;
     }
     printf("\n    OK!\n");
 
@@ -36,5 +36,7 @@ int main()
 
     printf("\n    OK!\n");
 
+    arena_delete(&default_arena);
+
     return 0;
 }