libutil: add C bindings

src/libutil/nix_api_util_internal.h

@@ -0,0 +1,61 @@
+#ifndef NIX_API_UTIL_INTERNAL_H
+#define NIX_API_UTIL_INTERNAL_H
+
+#include <string>
+
+// forward declaration
+namespace nix {
+class Error;
+};
+
+struct nix_c_context {
+  nix_err last_err_code = NIX_OK;
+  std::optional<std::string> last_err = {};
+  std::optional<nix::ErrorInfo> info = {};
+  std::string name = "";
+};
+
+nix_err nix_context_error(nix_c_context *context);
+
+/**
+ * Internal use only.
+ *
+ * Sets the most recent error message.
+ *
+ * @param context context to write the error message to, or NULL
+ * @param err The error code to set and return
+ * @param msg The error message to set.
+ * @returns the error code set
+ */
+nix_err nix_set_err_msg(nix_c_context *context, nix_err err, const char *msg);
+
+/**
+ * Internal use only.
+ *
+ * Export a std::string across the C api boundary
+ * @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`.
+ */
+nix_err nix_export_std_string(nix_c_context *context,
+                              const std::string_view str, char *dest,
+                              unsigned int n);
+
+#define NIXC_CATCH_ERRS                                                        \
+  catch (...) {                                                                \
+    return nix_context_error(context);                                         \
+  }                                                                            \
+  return NIX_OK;
+
+#define NIXC_CATCH_ERRS_RES(def)                                               \
+  catch (...) {                                                                \
+    nix_context_error(context);                                                \
+    return def;                                                                \
+  }
+#define NIXC_CATCH_ERRS_NULL NIXC_CATCH_ERRS_RES(nullptr)
+
+#endif // NIX_API_UTIL_INTERNAL_H