diff --git a/src/libstore/c/nix_api_store.cc b/src/libstore/c/nix_api_store.cc
index 199f5526a..93e1626a1 100644
--- a/src/libstore/c/nix_api_store.cc
+++ b/src/libstore/c/nix_api_store.cc
@@ -56,26 +56,24 @@ void nix_store_free(Store * store)
     delete store;
 }
 
-nix_err nix_store_get_uri(nix_c_context * context, Store * store, char * dest, unsigned int n)
+nix_err nix_store_get_uri(nix_c_context * context, Store * store, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto res = store->ptr->getUri();
-        return nix_export_std_string(context, res, dest, n);
+        return call_nix_observe_string(res, callback, user_data);
     }
     NIXC_CATCH_ERRS
 }
 
-nix_err nix_store_get_version(nix_c_context * context, Store * store, char * dest, unsigned int n)
+nix_err nix_store_get_version(nix_c_context * context, Store * store, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto res = store->ptr->getVersion();
-        if (!res)
-            res = "";
-        return nix_export_std_string(context, *res, dest, n);
+        return call_nix_observe_string(res.value_or(""), callback, user_data);
     }
     NIXC_CATCH_ERRS
 }
diff --git a/src/libstore/c/nix_api_store.h b/src/libstore/c/nix_api_store.h
index 28544fa90..25175de44 100644
--- a/src/libstore/c/nix_api_store.h
+++ b/src/libstore/c/nix_api_store.h
@@ -71,11 +71,12 @@ void nix_store_free(Store * store);
  * @brief get the URI of a nix store
  * @param[out] context Optional, stores error information
  * @param[in] store nix store reference
- * @param[out] dest The allocated area to write the string to.
- * @param[in] n Maximum size of the returned string.
+ * @param[in] callback Called with the URI.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
  * @return error code, NIX_OK on success.
  */
-nix_err nix_store_get_uri(nix_c_context * context, Store * store, char * dest, unsigned int n);
+nix_err nix_store_get_uri(nix_c_context * context, Store * store, void * callback, void * user_data);
 
 // returns: owned StorePath*
 /**
@@ -130,11 +131,12 @@ nix_err nix_store_realise(
  * If the store doesn't have a version (like the dummy store), returns an empty string.
  * @param[out] context Optional, stores error information
  * @param[in] store nix store reference
- * @param[out] dest The allocated area to write the string to.
- * @param[in] n Maximum size of the returned string.
+ * @param[in] callback Called with the version.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
  * @return error code, NIX_OK on success.
  */
-nix_err nix_store_get_version(nix_c_context *, Store * store, char * dest, unsigned int n);
+nix_err nix_store_get_version(nix_c_context * context, Store * store, void * callback, void * user_data);
 
 // cffi end
 #ifdef __cplusplus
diff --git a/src/libutil/c/nix_api_util.cc b/src/libutil/c/nix_api_util.cc
index 100e3b21d..ed542059d 100644
--- a/src/libutil/c/nix_api_util.cc
+++ b/src/libutil/c/nix_api_util.cc
@@ -63,16 +63,17 @@ const char * nix_version_get()
 }
 
 // Implementations
-nix_err nix_setting_get(nix_c_context * context, const char * key, char * value, int n)
+
+nix_err nix_setting_get(nix_c_context * context, const char * key, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         std::map<std::string, nix::AbstractConfig::SettingInfo> settings;
         nix::globalConfig.getSettings(settings);
-        if (settings.contains(key))
-            return nix_export_std_string(context, settings[key].value, value, n);
-        else {
+        if (settings.contains(key)) {
+            return call_nix_observe_string(settings[key].value, callback, user_data);
+        } else {
             return nix_set_err_msg(context, NIX_ERR_KEY, "Setting not found");
         }
     }
@@ -114,24 +115,24 @@ const char * nix_err_msg(nix_c_context * context, const nix_c_context * read_con
     return nullptr;
 }
 
-nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, char * value, int n)
+nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     if (read_context->last_err_code != NIX_ERR_NIX_ERROR) {
         return nix_set_err_msg(context, NIX_ERR_UNKNOWN, "Last error was not a nix error");
     }
-    return nix_export_std_string(context, read_context->name, value, n);
+    return call_nix_observe_string(read_context->name, callback, user_data);
 }
 
-nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, char * value, int n)
+nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     if (read_context->last_err_code != NIX_ERR_NIX_ERROR) {
         return nix_set_err_msg(context, NIX_ERR_UNKNOWN, "Last error was not a nix error");
     }
-    return nix_export_std_string(context, read_context->info->msg.str(), value, n);
+    return call_nix_observe_string(read_context->info->msg.str(), callback, user_data);
 }
 
 nix_err nix_err_code(const nix_c_context * read_context)
@@ -140,12 +141,8 @@ nix_err nix_err_code(const nix_c_context * read_context)
 }
 
 // internal
-nix_err nix_export_std_string(nix_c_context * context, const std::string_view str, char * dest, unsigned int n)
+nix_err call_nix_observe_string(const std::string str, void * callback, void * user_data)
 {
-    size_t i = str.copy(dest, n - 1);
-    dest[i] = 0;
-    if (i == n - 1) {
-        return nix_set_err_msg(context, NIX_ERR_OVERFLOW, "Provided buffer too short");
-    } else
-        return NIX_OK;
+    ((nix_observe_string) callback)(str.c_str(), str.size(), user_data);
+    return NIX_OK;
 }
diff --git a/src/libutil/c/nix_api_util.h b/src/libutil/c/nix_api_util.h
index c288654fd..fc6dc8655 100644
--- a/src/libutil/c/nix_api_util.h
+++ b/src/libutil/c/nix_api_util.h
@@ -119,6 +119,15 @@ typedef int nix_err;
  */
 typedef struct nix_c_context nix_c_context;
 
+/**
+ * @brief Called to get the value of a string owned by Nix.
+ *
+ * @param[in] start the string to copy.
+ * @param[in] n the string length.
+ * @param[in] user_data optional, arbitrary data, passed to the nix_observe_string when it's called.
+ */
+typedef void (*nix_observe_string)(const char * start, unsigned int n, void * user_data);
+
 // Function prototypes
 
 /**
@@ -160,14 +169,13 @@ nix_err nix_libutil_init(nix_c_context * context);
  *
  * @param[out] context optional, Stores error information
  * @param[in] key The key of the setting to retrieve.
- * @param[out] value A pointer to a buffer where the value of the setting will
- * be stored.
- * @param[in] n The size of the buffer pointed to by value.
- * @return NIX_ERR_KEY if the setting is unknown, NIX_ERR_OVERFLOW if the
- * provided buffer is too short, or NIX_OK if the setting was retrieved
+ * @param[in] callback Called with the setting value.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
+ * @return NIX_ERR_KEY if the setting is unknown, or NIX_OK if the setting was retrieved
  * successfully.
  */
-nix_err nix_setting_get(nix_c_context * context, const char * key, char * value, int n);
+nix_err nix_setting_get(nix_c_context * context, const char * key, void * callback, void * user_data);
 
 /**
  * @brief Sets a setting in the nix global configuration.
@@ -227,12 +235,14 @@ const char * nix_err_msg(nix_c_context * context, const nix_c_context * ctx, uns
  *
  * @param[out] context optional, the context to store errors in if this function
  * fails
- * @param[in] read_context the context to retrieve the error message from
- * @param[out] value The allocated area to write the error string to.
- * @param[in] n Maximum size of the returned string.
+ * @param[in] read_context the context to retrieve the error message from.
+ * @param[in] callback Called with the error message.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
  * @return NIX_OK if there were no errors, an error code otherwise.
  */
-nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, char * value, int n);
+nix_err
+nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data);
 
 /**
  * @brief Retrieves the error name from a context.
@@ -245,11 +255,12 @@ nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_con
  * @param context optional, the context to store errors in if this function
  * fails
  * @param[in] read_context the context to retrieve the error message from
- * @param[out] value The allocated area to write the error string to.
- * @param[in] n Maximum size of the returned string.
+ * @param[in] callback Called with the error name.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
  * @return NIX_OK if there were no errors, an error code otherwise.
  */
-nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, char * value, int n);
+nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data);
 
 /**
  * @brief Retrieves the most recent error code from a nix_c_context
diff --git a/src/libutil/c/nix_api_util_internal.h b/src/libutil/c/nix_api_util_internal.h
index 53c260e35..f91d8c118 100644
--- a/src/libutil/c/nix_api_util_internal.h
+++ b/src/libutil/c/nix_api_util_internal.h
@@ -20,16 +20,16 @@ nix_err nix_context_error(nix_c_context * context);
 /**
  * Internal use only.
  *
- * Export a std::string across the C api boundary
+ * Helper to invoke nix_observe_string
  * @param context optional, the context to store errors in if this function
  * fails
- * @param str The string to export
- * @param value The allocated area to write the string to.
- * @param n Maximum size of the returned string.
- * @return NIX_OK if there were no errors, NIX_ERR_OVERFLOW if the string length
- * exceeds `n`.
+ * @param str The string to observe
+ * @param callback Called with the observed string.
+ * @param user_data optional, arbitrary data, passed to the callback when it's called.
+ * @return NIX_OK if there were no errors.
+ * @see nix_observe_string
  */
-nix_err nix_export_std_string(nix_c_context * context, const std::string_view str, char * dest, unsigned int n);
+nix_err call_nix_observe_string(const std::string str, void * callback, void * user_data);
 
 #define NIXC_CATCH_ERRS \
     catch (...) \
diff --git a/tests/unit/libstore/nix_api_store.cc b/tests/unit/libstore/nix_api_store.cc
index dac7fa910..a31d66a4c 100644
--- a/tests/unit/libstore/nix_api_store.cc
+++ b/tests/unit/libstore/nix_api_store.cc
@@ -7,6 +7,11 @@
 
 namespace nixC {
 
+void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
+{
+    *user_data = std::string(start);
+}
+
 std::string PATH_SUFFIX = "/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-name";
 
 TEST_F(nix_api_util_context, nix_libstore_init)
@@ -17,10 +22,10 @@ TEST_F(nix_api_util_context, nix_libstore_init)
 
 TEST_F(nix_api_store_test, nix_store_get_uri)
 {
-    char value[256];
-    auto ret = nix_store_get_uri(ctx, store, value, 256);
+    std::string str;
+    auto ret = nix_store_get_uri(ctx, store, (void *) observe_string_cb, &str);
     ASSERT_EQ(NIX_OK, ret);
-    ASSERT_STREQ("local", value);
+    ASSERT_STREQ("local", str.c_str());
 }
 
 TEST_F(nix_api_store_test, InvalidPathFails)
@@ -50,10 +55,10 @@ TEST_F(nix_api_store_test, DoesNotCrashWhenContextIsNull)
 
 TEST_F(nix_api_store_test, get_version)
 {
-    char value[256];
-    auto ret = nix_store_get_version(ctx, store, value, 256);
+    std::string str;
+    auto ret = nix_store_get_version(ctx, store, (void *) observe_string_cb, &str);
     ASSERT_EQ(NIX_OK, ret);
-    ASSERT_STREQ(PACKAGE_VERSION, value);
+    ASSERT_STREQ(PACKAGE_VERSION, str.c_str());
 }
 
 TEST_F(nix_api_util_context, nix_store_open_dummy)
@@ -63,9 +68,9 @@ TEST_F(nix_api_util_context, nix_store_open_dummy)
     ASSERT_EQ(NIX_OK, ctx->last_err_code);
     ASSERT_STREQ("dummy", store->ptr->getUri().c_str());
 
-    char value[256];
-    nix_store_get_version(ctx, store, value, 256);
-    ASSERT_STREQ("", value);
+    std::string str;
+    nix_store_get_version(ctx, store, (void *) observe_string_cb, &str);
+    ASSERT_STREQ("", str.c_str());
 
     nix_store_free(store);
 }
diff --git a/tests/unit/libutil-support/tests/nix_api_util.hh b/tests/unit/libutil-support/tests/nix_api_util.hh
index 314ec70de..0dfb38f7b 100644
--- a/tests/unit/libutil-support/tests/nix_api_util.hh
+++ b/tests/unit/libutil-support/tests/nix_api_util.hh
@@ -5,6 +5,7 @@
 #include <gtest/gtest.h>
 
 namespace nixC {
+
 class nix_api_util_context : public ::testing::Test
 {
 protected:
diff --git a/tests/unit/libutil/nix_api_util.cc b/tests/unit/libutil/nix_api_util.cc
index a3ec5e0a1..20e46637c 100644
--- a/tests/unit/libutil/nix_api_util.cc
+++ b/tests/unit/libutil/nix_api_util.cc
@@ -9,6 +9,11 @@
 
 namespace nixC {
 
+void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
+{
+    *user_data = std::string(start);
+}
+
 TEST_F(nix_api_util_context, nix_context_error)
 {
     std::string err_msg_ref;
@@ -57,13 +62,13 @@ static nix::GlobalConfig::Register rs(&mySettings);
 TEST_F(nix_api_util_context, nix_setting_get)
 {
     ASSERT_EQ(ctx->last_err_code, NIX_OK);
-    char value[256];
-    nix_err result = nix_setting_get(ctx, "invalid-key", value, 256);
+    std::string setting_value;
+    nix_err result = nix_setting_get(ctx, "invalid-key", (void *) observe_string_cb, &setting_value);
     ASSERT_EQ(result, NIX_ERR_KEY);
 
-    result = nix_setting_get(ctx, "setting-name", value, 256);
+    result = nix_setting_get(ctx, "setting-name", (void *) observe_string_cb, &setting_value);
     ASSERT_EQ(result, NIX_OK);
-    ASSERT_STREQ("empty", value);
+    ASSERT_STREQ("empty", setting_value.c_str());
 }
 
 TEST_F(nix_api_util_context, nix_setting_set)
@@ -74,10 +79,10 @@ TEST_F(nix_api_util_context, nix_setting_set)
     result = nix_setting_set(ctx, "setting-name", "new-value");
     ASSERT_EQ(result, NIX_OK);
 
-    char value[256];
-    result = nix_setting_get(ctx, "setting-name", value, 256);
+    std::string setting_value;
+    result = nix_setting_get(ctx, "setting-name", (void *) observe_string_cb, &setting_value);
     ASSERT_EQ(result, NIX_OK);
-    ASSERT_STREQ("new-value", value);
+    ASSERT_STREQ("new-value", setting_value.c_str());
 }
 
 TEST_F(nix_api_util_context, nix_err_msg)
@@ -100,26 +105,26 @@ TEST_F(nix_api_util_context, nix_err_msg)
 
 TEST_F(nix_api_util_context, nix_err_info_msg)
 {
+    std::string err_info;
+
     // no error
-    EXPECT_THROW(nix_err_info_msg(NULL, ctx, NULL, 256), nix::Error);
+    EXPECT_THROW(nix_err_info_msg(NULL, ctx, (void *) observe_string_cb, &err_info), nix::Error);
 
     try {
         throw nix::Error("testing error");
     } catch (...) {
         nix_context_error(ctx);
     }
-    char buf[256];
-    nix_err_info_msg(nix_c_context_create(), ctx, buf, 256);
-    ASSERT_EQ(std::string(buf), "testing error");
-
-    // should overflow
-    EXPECT_THROW(nix_err_info_msg(NULL, ctx, buf, 1), nix::Error);
+    nix_err_info_msg(nix_c_context_create(), ctx, (void *) observe_string_cb, &err_info);
+    ASSERT_STREQ("testing error", err_info.c_str());
 }
 
 TEST_F(nix_api_util_context, nix_err_name)
 {
+    std::string err_name;
+
     // no error
-    EXPECT_THROW(nix_err_name(NULL, ctx, NULL, 256), nix::Error);
+    EXPECT_THROW(nix_err_name(NULL, ctx, (void *) observe_string_cb, &err_name), nix::Error);
 
     std::string err_msg_ref;
     try {
@@ -127,12 +132,8 @@ TEST_F(nix_api_util_context, nix_err_name)
     } catch (...) {
         nix_context_error(ctx);
     }
-    char err_name[32];
-    nix_err_name(nix_c_context_create(), ctx, err_name, 32);
+    nix_err_name(nix_c_context_create(), ctx, (void *) observe_string_cb, &err_name);
     ASSERT_EQ(std::string(err_name), "nix::Error");
-
-    // overflow
-    EXPECT_THROW(nix_err_name(NULL, ctx, err_name, 1), nix::Error);
 }
 
 TEST_F(nix_api_util_context, nix_err_code)
@@ -141,4 +142,5 @@ TEST_F(nix_api_util_context, nix_err_code)
     nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "unknown test error");
     ASSERT_EQ(nix_err_code(ctx), NIX_ERR_UNKNOWN);
 }
+
 }