add check for return values of openssl functions

Makefile

@@ -2,7 +2,7 @@
 CC = cl
 
 # Flags for the compiler
-CFLAGS = /W4 /MT
+CFLAGS = /W4 /MT /O2
 
 # Include and lib directories for OpenSSL
 OPENSSL_INCLUDE_DIR = include
@@ -28,6 +28,14 @@ $(OUTFILE): $(OBJECTS)
 .c.obj:
     $(CC) $(CFLAGS) /c $< /Fo$@ /I "$(OPENSSL_INCLUDE_DIR)"
 
+# Assembly output with optimization
+# asm-optimized: $(SOURCES)
+#     $(CC) /FAs /O2 /c $(SOURCES) /Foasm-optimized.obj /I "$(OPENSSL_INCLUDE_DIR)"
+
+# Assembly output without optimization
+# asm-unoptimized: $(SOURCES)
+#     $(CC) /FAs /c $(SOURCES) /Foasm-unoptimized.obj /I "$(OPENSSL_INCLUDE_DIR)"
+
 # Clean rule
 clean:
     del *.obj $(OUTFILE)

main.c

@@ -4,39 +4,60 @@
 
 int main(int argc, char *argv[])
 {
+    // args
     if (argc < 2)
     {
-        printf("missing file parameter");
+        printf("missing file parameter\n");
         return 1;
     }
 
+    // open file
     FILE *file = NULL;
     errno_t err = fopen_s(&file, argv[1], "rb");
 
     if (err != 0)
     {
-        printf("Failed to open the file. Error code: %d\n", err);
+        printf("failed to open the file, err code: %d\n", err);
         return 1;
     }
 
+    // hash
     unsigned char hash[EVP_MAX_MD_SIZE];
     unsigned int hash_len;
-
     EVP_MD_CTX *ctx = EVP_MD_CTX_new();
-    EVP_DigestInit_ex(ctx, EVP_sha256(), NULL);
+    if (!EVP_DigestInit_ex(ctx, EVP_sha256(), NULL))
+    {
+        printf("failed to init digest.\n");
+        EVP_MD_CTX_free(ctx);
+        fclose(file);
+        return 1;
+    }
 
     char buf[1024];
     size_t nread;
     while ((nread = fread(buf, 1, sizeof buf, file)) > 0)
     {
-        EVP_DigestUpdate(ctx, buf, nread);
+        if (!EVP_DigestUpdate(ctx, buf, nread))
+        {
+            printf("failed to update digest\n");
+            EVP_MD_CTX_free(ctx);
+            fclose(file);
+            return 1;
+        }
+    }
+
+    if (!EVP_DigestFinal_ex(ctx, hash, &hash_len))
+    {
+        printf("failed to finalize digest\n");
+        EVP_MD_CTX_free(ctx);
+        fclose(file);
+        return 1;
     }
 
-    EVP_DigestFinal_ex(ctx, hash, &hash_len);
     EVP_MD_CTX_free(ctx);
 
-    unsigned int i;
-    for (i = 0; i < hash_len; i++)
+    // print hash
+    for (unsigned int i = 0; i < hash_len; i++)
     {
         printf("%02x", hash[i]);
     }