diff --git a/flake.nix b/flake.nix
index e158b50..0bef8c3 100644
--- a/flake.nix
+++ b/flake.nix
@@ -114,7 +114,7 @@
             inherit (pkgs)
               # passthru.buildInputs
               go
-              gcc
+              clang
 
               # nativeBuildInputs
               pkg-config
@@ -129,6 +129,10 @@
               zstd
               gnutar
               coreutils
+
+              # for check
+              util-linux
+              nettools
               ;
           };
           hsu = pkgs.callPackage ./cmd/hsu/package.nix { inherit (self.packages.${system}) hakurei; };
diff --git a/internal/outcome/shim-signal.c b/internal/outcome/shim-signal.c
index 7e56206..0e04abb 100644
--- a/internal/outcome/shim-signal.c
+++ b/internal/outcome/shim-signal.c
@@ -38,7 +38,7 @@ static void hakurei_shim_sigaction(int sig, siginfo_t *si, void *ucontext) {
 
 void hakurei_shim_setup_cont_signal(pid_t ppid, int fd) {
   if (hakurei_shim_param_ppid != -1 || hakurei_shim_fd != -1)
-    *(int *)NULL = 0; /* unreachable */
+    *(volatile int *)NULL = 0; /* unreachable */
 
   struct sigaction new_action = {0}, old_action = {0};
   if (sigaction(SIGCONT, NULL, &old_action) != 0)
diff --git a/package.nix b/package.nix
index 21aee35..fb449c1 100644
--- a/package.nix
+++ b/package.nix
@@ -20,7 +20,7 @@
 
   # for passthru.buildInputs
   go,
-  gcc,
+  clang,
 
   # for check
   util-linux,
@@ -81,8 +81,13 @@ buildGoModule rec {
         hsuPath = "/run/wrappers/bin/hsu";
       };
 
-  # nix build environment does not allow acls
-  env.GO_TEST_SKIP_ACL = 1;
+  env = {
+    # use clang instead of gcc
+    CC = "clang -O3 -Werror";
+
+    # nix build environment does not allow acls
+    GO_TEST_SKIP_ACL = 1;
+  };
 
   buildInputs = [
     libffi
@@ -135,7 +140,7 @@ buildGoModule rec {
 
   passthru.targetPkgs = [
     go
-    gcc
+    clang
     xorg.xorgproto
     util-linux