diff --git a/skeletons/constraints.h b/skeletons/constraints.h
new file mode 100644
index 0000000..255b6ae
--- /dev/null
+++ b/skeletons/constraints.h
@@ -0,0 +1,57 @@
+/*-
+ * Copyright (c) 2004 Lev Walkin <vlm@lionet.info>. All rights reserved.
+ * Redistribution and modifications are permitted subject to BSD license.
+ */
+#ifndef	_ASN1_CONSTRAINTS_VALIDATOR_H_
+#define	_ASN1_CONSTRAINTS_VALIDATOR_H_
+
+#include <asn_types.h>	/* System-dependent types */
+
+struct asn1_TYPE_descriptor_s;		/* Forward declaration */
+
+/*
+ * Validate the structure according to the ASN.1 constraints.
+ * If errbuf and errlen are given, they shall be pointing to the appropriate
+ * buffer space and its length before calling this function. Alternatively,
+ * they could be passed as NULL's. If constraints validation fails,
+ * errlen will contain the actual number of bytes taken from the errbuf
+ * to encode an error message (properly 0-terminated).
+ */
+int
+asn_check_constraints(struct asn1_TYPE_descriptor_s *type_descriptor,
+	const void *struct_ptr,	/* Target language's structure */
+	char *errbuf,		/* Returned error description */
+	size_t *errlen		/* Length of the error description */
+	);
+
+/*
+ * Generic type for constraint checking callback,
+ * associated with every type descriptor.
+ */
+typedef int (asn_constr_check_f)(
+	struct asn1_TYPE_descriptor_s *type_descriptor,
+	const void *struct_ptr,
+	asn_app_consume_bytes_f *optional_app_errlog,	/* Log the error */
+	void *optional_app_key		/* Opaque key passed to app_errlog */
+	);
+
+/*******************************
+ * INTERNALLY USEFUL FUNCTIONS *
+ *******************************/
+
+asn_constr_check_f asn_generic_no_constraint;	/* No constraint whatsoever */
+asn_constr_check_f asn_generic_unknown_constraint; /* Not fully supported */
+
+/*
+ * Invoke the callback with a complete error message.
+ */
+#define	_ASN_ERRLOG(fmt, args...)	do {	\
+	if(app_errlog)				\
+		_asn_i_log_error(app_errlog,	\
+			app_key, fmt, ##args);	\
+		break;				\
+	} while(0);
+void _asn_i_log_error(asn_app_consume_bytes_f *, void *key,
+	const char *fmt, ...) __attribute__ ((format(printf, 3, 4)));
+
+#endif	/* _ASN1_CONSTRAINTS_VALIDATOR_H_ */