diff --git a/src/libexpr/c/nix_api_value.cc b/src/libexpr/c/nix_api_value.cc
index 608a625cb..9a313d8fd 100644
--- a/src/libexpr/c/nix_api_value.cc
+++ b/src/libexpr/c/nix_api_value.cc
@@ -34,18 +34,39 @@ static nix::Value & check_value_not_null(Value * value)
     return *((nix::Value *) value);
 }
 
+/**
+ * Helper function to convert calls from nix into C API.
+ *
+ * Deals with errors and converts arguments from C++ into C types.
+ */
+static void nix_c_primop_wrapper(
+    PrimOpFun f, void * userdata, nix::EvalState & state, const nix::PosIdx pos, nix::Value ** args, nix::Value & v)
+{
+    f(userdata, (State *) &state, *reinterpret_cast<const int *>(&pos), (Value **) args, (Value *) &v);
+}
+
 PrimOp * nix_alloc_primop(
-    nix_c_context * context, PrimOpFun fun, int arity, const char * name, const char ** args, const char * doc)
+    nix_c_context * context,
+    PrimOpFun fun,
+    int arity,
+    const char * name,
+    const char ** args,
+    const char * doc,
+    void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        auto fun2 = (nix::PrimOpFun) fun;
         auto p = new
 #ifdef HAVE_BOEHMGC
             (GC)
 #endif
-                nix::PrimOp{.name = name, .args = {}, .arity = (size_t) arity, .doc = doc, .fun = fun2};
+                nix::PrimOp{
+                    .name = name,
+                    .args = {},
+                    .arity = (size_t) arity,
+                    .doc = doc,
+                    .fun = std::bind_front(nix_c_primop_wrapper, fun, user_data)};
         if (args)
             for (size_t i = 0; args[i]; i++)
                 p->args.emplace_back(*args);
diff --git a/src/libexpr/c/nix_api_value.h b/src/libexpr/c/nix_api_value.h
index 21647552b..ffba4c097 100644
--- a/src/libexpr/c/nix_api_value.h
+++ b/src/libexpr/c/nix_api_value.h
@@ -65,6 +65,7 @@ typedef struct ExternalValue ExternalValue;
  * @{
  */
 /** @brief Function pointer for primops
+ * @param[in] user_data Arbitrary data, passed to nix_alloc_primop and stored.
  * @param[in] state Evaluator state
  * @param[in] pos Call position, opaque index into the state's position table.
  * @param[in] args list of arguments. Note that these can be thunks and should be forced using nix_value_force before
@@ -72,7 +73,7 @@ typedef struct ExternalValue ExternalValue;
  * @param[out] ret return value
  * @see nix_alloc_primop, nix_set_primop
  */
-typedef void (*PrimOpFun)(State * state, int pos, Value ** args, Value * ret);
+typedef void (*PrimOpFun)(void * user_data, State * state, int pos, Value ** args, Value * ret);
 
 /** @brief Allocate a PrimOp
  *
@@ -85,19 +86,27 @@ typedef void (*PrimOpFun)(State * state, int pos, Value ** args, Value * ret);
  * @param[in] name function name
  * @param[in] args array of argument names, NULL-terminated
  * @param[in] doc optional, documentation for this primop
+ * @param[in] user_data optional, arbitrary data, passed to the function when it's called
  * @return primop, or null in case of errors
  * @see nix_set_primop
  */
 PrimOp * nix_alloc_primop(
-    nix_c_context * context, PrimOpFun fun, int arity, const char * name, const char ** args, const char * doc);
+    nix_c_context * context,
+    PrimOpFun fun,
+    int arity,
+    const char * name,
+    const char ** args,
+    const char * doc,
+    void * user_data);
 
 /** @brief add a primop to the `builtins` attribute set
  *
  * Only applies to States created after this call.
  *
- * Moves your PrimOp into the global evaluator
- * registry, meaning your input PrimOp pointer is no longer usable
- * (but still possibly subject to garbage collection).
+ * Moves your PrimOp content into the global evaluator
+ * registry, meaning your input PrimOp pointer is no longer usable.
+ * You are free to remove your references to it,
+ * after which it will be garbage collected.
  *
  * @param[out] context Optional, stores error information
  * @return primop, or null in case of errors