diff --git a/rif.h b/rif.h
index 27f2ccd..1f6a85a 100644
--- a/rif.h
+++ b/rif.h
@@ -13,15 +13,18 @@ typedef struct rif {
 #define RIF_FORMAT_R8G8B8   0
 #define RIF_FORMAT_R8G8B8A8 1
 
+// TODO detect endianess - this breaks on big endian systems.
 void write_rif_little(char* path, uint32_t width, uint32_t size, uint8_t format, void* data) {
-#define IMG_SIZE 17 + size // 17 is the size of raw_img excluding the data field
-  rif_t* img = malloc(IMG_SIZE);
+  // TODO is this better than just hardcoding 17?
+  const uint32_t header_size = sizeof(rif_t) - sizeof(uint8_t*);
+  const uint64_t img_size = header_size + size;
+  rif_t* img = malloc(img_size);
   char magic[] = RIF_MAGIC_LITTLE;
-  memcpy(((char*)img)+0, magic, 8);
+  memcpy(&img->magic_num, magic, 8);
   img->width = width;
   img->format = format;
-  memcpy(((char*)img)+17, data, size);
+  memcpy(&img->data, data, size);
   FILE* output_file = fopen(path, "w");
-  fwrite(img, 1, IMG_SIZE, output_file);
+  fwrite(img, 1, img_size, output_file);
   free(img);
 }