added a simple test

Makefile

@@ -0,0 +1,16 @@
+CC?=gcc
+PROVE?=prove
+
+test: build_test
+	$(PROVE) -e '/bin/sh -c' ./test
+
+build_test: test.o
+	$(CC) $(LDFLAGS) -o test $^
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $<
+
+clean:
+	rm -f *.o
+	rm -f test_request
+	rm -f test_response

test.c

@@ -0,0 +1,46 @@
+#include <stdio.h>
+#include <string.h>
+
+#include "hex.h"
+
+static void plan(int num) {
+    printf("1..%d\n", num);
+}
+
+static void ok(int ok, const char* msg) {
+    static int testnum = 0;
+    printf("%s %d - %s\n", ok ? "ok" : "not ok", ++testnum, msg);
+}
+
+static void test_encode(void) {
+    unsigned char binary[] = {
+        0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef
+    };
+    unsigned char hexstring[sizeof(binary) * 2 + 1];
+
+    hex(binary, sizeof(binary), hexstring);
+
+    ok(0 == strncmp(hexstring, "0123456789abcdef", 16), "encode ok");
+}
+
+static void test_decode(void) {
+    unsigned char hexstring[] = "0123456789abcdef";
+    unsigned char binary[strlen(hexstring) / 2];
+    unsigned char expected[] = {
+        0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef
+    };
+
+    unhex(hexstring, strlen(hexstring), binary);
+
+    ok(0 == memcmp(binary, expected, strlen(hexstring)/2), "decode ok");
+}
+
+int main(void) {
+    plan(2);
+
+    test_encode();
+    test_decode();
+
+    return 0;
+}
+