dnn/dnn_backend_native: Avoid allocation for checking file magic

Reviewed-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/dnn/dnn_backend_native.c

@@ -114,10 +114,10 @@ static DNNReturnType get_output_native(void *model, const char *input_name, int
 // For DEPTH_TO_SPACE layer: block_size
 DNNModel *ff_dnn_load_model_native(const char *model_filename, DNNFunctionType func_type, const char *options, AVFilterContext *filter_ctx)
 {
+#define DNN_NATIVE_MAGIC "FFMPEGDNNNATIVE"
     DNNModel *model = NULL;
-    char header_expected[] = "FFMPEGDNNNATIVE";
-    char *buf;
-    size_t size;
+    // sizeof - 1 to skip the terminating '\0' which is not written in the file
+    char buf[sizeof(DNN_NATIVE_MAGIC) - 1];
     int version, header_size, major_version_expected = 1;
     NativeModel *native_model = NULL;
     AVIOContext *model_file_context;
@@ -138,20 +138,10 @@ DNNModel *ff_dnn_load_model_native(const char *model_filename, DNNFunctionType f
     /**
      * check file header with string and version
      */
-    size = sizeof(header_expected);
-    buf = av_malloc(size);
-    if (!buf) {
+    if (avio_read(model_file_context, buf, sizeof(buf)) != sizeof(buf) ||
+        memcmp(buf, DNN_NATIVE_MAGIC, sizeof(buf)))
         goto fail;
-    }
-
-    // size - 1 to skip the ending '\0' which is not saved in file
-    avio_get_str(model_file_context, size - 1, buf, size);
-    dnn_size = size - 1;
-    if (strncmp(buf, header_expected, size) != 0) {
-        av_freep(&buf);
-        goto fail;
-    }
-    av_freep(&buf);
+    dnn_size = sizeof(buf);
 
     version = (int32_t)avio_rl32(model_file_context);
     dnn_size += 4;