diff --git a/2017-03-28/Makefile b/2017-03-28/Makefile
index c8afa26..be456f6 100644
--- a/2017-03-28/Makefile
+++ b/2017-03-28/Makefile
@@ -2,7 +2,8 @@
 
 CFLAGS=-g -O2 -fPIC -fPIE -Wall -Wextra -Wformat=2 -Wunused -Wlogical-op -Wundef \
 	-Wswitch-enum -Wpointer-arith -Wcast-qual -Wcast-align -Wwrite-strings \
-	-Wconversion -D_FORTIFY_SOURCE=2 -fstack-protector -Wshadow -Wnested-externs
+	-Wconversion -D_FORTIFY_SOURCE=2 -fstack-protector -Wshadow -Wnested-externs \
+	-Wmissing-prototypes
 
 LDFLAGS=-lm
 
diff --git a/2017-03-28/jo-so_euler87.c b/2017-03-28/jo-so_euler87.c
index ef461f9..5c0a6d7 100644
--- a/2017-03-28/jo-so_euler87.c
+++ b/2017-03-28/jo-so_euler87.c
@@ -12,7 +12,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-int gen_prime_numbers(uint limit, uint* pn, uint** pn_end)
+static int gen_prime_numbers(uint limit, uint* pn, uint** pn_end)
 {
     pn[0] = 2u;
     pn[1] = 3u;
@@ -43,14 +43,14 @@ typedef struct {
     uint* mem_end;
 } vector_uint;
 
-void vector_init(vector_uint* vec)
+static void vector_init(vector_uint* vec)
 {
     vec->begin = NULL;
     vec->end = NULL;
     vec->mem_end = NULL;
 }
 
-int cmp_uint(const void* p1, const void* p2)
+static int cmp_uint(const void* p1, const void* p2)
 {
     const uint a = *(const uint*)p1;
     const uint b = *(const uint*)p2;
@@ -61,7 +61,7 @@ int cmp_uint(const void* p1, const void* p2)
     return 1;
 }
 
-int vector_add(vector_uint* vec, uint el)
+static int vector_add(vector_uint* vec, uint el)
 {
     if (vec->end >= vec->mem_end)
     {