diff --git a/doc/educational_decoder/harness.c b/doc/educational_decoder/harness.c
index df4c0af94..338394916 100644
--- a/doc/educational_decoder/harness.c
+++ b/doc/educational_decoder/harness.c
@@ -21,10 +21,6 @@ typedef unsigned char u8;
 // Protect against allocating too much memory for output
 #define MAX_OUTPUT_SIZE ((size_t)1024 * 1024 * 1024)
 
-u8 *input;
-u8 *output;
-u8 *dict;
-
 static size_t read_file(const char *path, u8 **ptr)
 {
     FILE* const f = fopen(path, "rb");
@@ -61,6 +57,10 @@ static size_t read_file(const char *path, u8 **ptr)
 static void write_file(const char *path, const u8 *ptr, size_t size)
 {
     FILE* const f = fopen(path, "wb");
+    if (!f) {
+        fprintf(stderr, "failed to open file %s \n", path);
+        exit(1);
+    }
 
     size_t written = 0;
     while (written < size) {
@@ -73,7 +73,8 @@ static void write_file(const char *path, const u8 *ptr, size_t size)
     fclose(f);
 }
 
-int main(int argc, char **argv) {
+int main(int argc, char **argv)
+{
     if (argc < 3) {
         fprintf(stderr, "usage: %s <file.zst> <out_path> [dictionary] \n",
                 argv[0]);
@@ -81,27 +82,31 @@ int main(int argc, char **argv) {
         return 1;
     }
 
+    u8* input;
     size_t const input_size = read_file(argv[1], &input);
+
+    u8* dict;
     size_t dict_size = 0;
     if (argc >= 4) {
         dict_size = read_file(argv[3], &dict);
     }
 
-    size_t decompressed_size = ZSTD_get_decompressed_size(input, input_size);
-    if (decompressed_size == (size_t)-1) {
-        decompressed_size = MAX_COMPRESSION_RATIO * input_size;
+    size_t out_capacity = ZSTD_get_decompressed_size(input, input_size);
+    if (out_capacity == (size_t)-1) {
+        out_capacity = MAX_COMPRESSION_RATIO * input_size;
         fprintf(stderr, "WARNING: Compressed data does not contain "
                         "decompressed size, going to assume the compression "
                         "ratio is at most %d (decompressed size of at most "
                         "%u) \n",
-                MAX_COMPRESSION_RATIO, (unsigned)decompressed_size);
+                MAX_COMPRESSION_RATIO, (unsigned)out_capacity);
     }
-    if (decompressed_size > MAX_OUTPUT_SIZE) {
+    if (out_capacity > MAX_OUTPUT_SIZE) {
         fprintf(stderr,
                 "Required output size too large for this implementation \n");
         return 1;
     }
-    output = malloc(decompressed_size);
+
+    u8* const output = malloc(out_capacity);
     if (!output) {
         fprintf(stderr, "failed to allocate memory \n");
         return 1;
@@ -111,17 +116,17 @@ int main(int argc, char **argv) {
     if (dict) {
         parse_dictionary(parsed_dict, dict, dict_size);
     }
-    size_t decompressed =
-        ZSTD_decompress_with_dict(output, decompressed_size,
-                                  input, input_size, parsed_dict);
+    size_t const decompressed_size =
+        ZSTD_decompress_with_dict(output, out_capacity,
+                                  input, input_size,
+                                  parsed_dict);
 
     free_dictionary(parsed_dict);
 
-    write_file(argv[2], output, decompressed);
+    write_file(argv[2], output, decompressed_size);
 
     free(input);
     free(output);
     free(dict);
-    input = output = dict = NULL;
     return 0;
 }