diff --git a/nixide-sys/include/nix-expr.h b/nixide-sys/include/nix-expr.h
index b20d63b..02f48ea 100644
--- a/nixide-sys/include/nix-expr.h
+++ b/nixide-sys/include/nix-expr.h
@@ -1,3 +1,6 @@
+#ifndef NIXIDE_EXPR
+#define NIXIDE_EXPR
+
 // Nix C API for the Nix expressions evaluator.
 #include <nix_api_expr.h>
 
@@ -8,3 +11,5 @@
 // Nix C API for external values.
 //
 #include <nix_api_external.h>
+
+#endif
diff --git a/nixide-sys/include/nix-fetchers.h b/nixide-sys/include/nix-fetchers.h
index 4e80a28..805a84b 100644
--- a/nixide-sys/include/nix-fetchers.h
+++ b/nixide-sys/include/nix-fetchers.h
@@ -1,3 +1,8 @@
+#ifndef NIXIDE_FETCHERS
+#define NIXIDE_FETCHERS
+
 // Nix C API for fetcher operations.
 //
 #include <nix_api_fetchers.h>
+
+#endif
diff --git a/nixide-sys/include/nix-flake.h b/nixide-sys/include/nix-flake.h
index e07cfab..5bd3d3b 100644
--- a/nixide-sys/include/nix-flake.h
+++ b/nixide-sys/include/nix-flake.h
@@ -1,3 +1,8 @@
+#ifndef NIXIDE_FLAKE
+#define NIXIDE_FLAKE
+
 // Nix C API for flake support.
 //
 #include <nix_api_flake.h>
+
+#endif
diff --git a/nixide-sys/include/nix-main.h b/nixide-sys/include/nix-main.h
index 7815125..b2320d9 100644
--- a/nixide-sys/include/nix-main.h
+++ b/nixide-sys/include/nix-main.h
@@ -1,3 +1,8 @@
+#ifndef NIXIDE_MAIN
+#define NIXIDE_MAIN
+
 // Nix C API for CLI support.
 //
 #include <nix_api_main.h>
+
+#endif
diff --git a/nixide-sys/include/nix-store.h b/nixide-sys/include/nix-store.h
index 239bde5..8ad9413 100644
--- a/nixide-sys/include/nix-store.h
+++ b/nixide-sys/include/nix-store.h
@@ -1,3 +1,8 @@
+#ifndef NIXIDE_STORE
+#define NIXIDE_STORE
+
 // Nix C API for store operations.
 //
 #include <nix_api_store.h>
+
+#endif
diff --git a/nixide-sys/include/nix-util.h b/nixide-sys/include/nix-util.h
index 8d60eed..59690ee 100644
--- a/nixide-sys/include/nix-util.h
+++ b/nixide-sys/include/nix-util.h
@@ -1,3 +1,6 @@
+#ifndef NIXIDE_UTIL
+#define NIXIDE_UTIL
+
 // Nix C API for utilities.
 //
 // Most notably containing functions for handling
@@ -5,3 +8,5 @@
 // the Nix C APIs for error handling.
 //
 #include <nix_api_util.h>
+
+#endif