The `buildNimPackage` function generates flags and additional build dependencies from the `lockFile` parameter passed to `buildNimPackage`. Using [`overrideAttrs`](#sec-pkg-overrideAttrs) on the final package will apply after this has already been generated, so this can't be used to override the `lockFile` in a package built with `buildNimPackage`. To be able to override parameters before flags and build dependencies are generated from the `lockFile`, use `overrideNimAttrs` instead with the same syntax as `overrideAttrs`:
```nix
pkgs.nitter.overrideNimAttrs {
# using a different source which has different dependencies from the standard package
The `buildNimPackage` function matches the libraries specified by `lockFile` to attrset of override functions that are then applied to the package derivation.
The default overrides are maintained as the top-level `nimOverrides` attrset at `pkgs/top-level/nim-overrides.nix`.
For example, to propagate a dependency on SDL2 for lockfiles that select the Nim `sdl2` library, an overlay is added to the set in the `nim-overrides.nix` file:
The annotations in the `nim-overrides.nix` set are functions that take two arguments and return a new attrset to be overlayed on the package being built.
- lockAttrs: the attrset for this library from within a lockfile. This can be used to implement library version constraints, such as marking libraries as broken or insecure.
- prevAttrs: the attrset produced by initial arguments to `buildNimPackage` and any preceding lockfile overlays.
### Overriding an Nim library override {#nim-lock-overrides-overrides}
The `nimOverrides` attrset makes it possible to modify overrides in a few different ways.