* NEW [docs] add SQLite APIs description

docs/man/libnng.3.adoc

@@ -490,6 +490,7 @@ These functions are intended for use with MQTT client applications.
 |xref:nng_mqttv5_client_open.3.adoc[nng_mqttv5_client_open]|open mqtt V5 client socket
 |xref:nng_mqtt_set_cb.3.adoc[nng_mqtt_set_connect_cb]| set mqtt connect callback function
 |xref:nng_mqtt_set_cb.3.adoc[nng_mqtt_set_disconnect_cb]| set mqtt disconnect callback function
+|xref:nng_mqtt_set_sqlite.3.adoc[nng_mqtt_sqlite]|set SQLite options
 |===
 
 == SEE ALSO

docs/man/nng_mqtt_set_sqlite.3.adoc

@@ -0,0 +1,94 @@
+= nng_mqtt_sqlite_functions(3)
+//
+// Copyright 2018 Staysail Systems, Inc. <[email protected]>
+// Copyright 2018 Capitar IT Group BV <[email protected]>
+//
+// This document is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+== NAME
+
+nng_mqtt_alloc_sqlite_opt - allocate a nng_mqtt_sqlite_option object
+
+nng_mqtt_free_sqlite_opt - free a nng_mqtt_sqlite_option object 
+
+nng_mqtt_set_sqlite_enable - enable or disable sqlite caching   
+
+nng_mqtt_set_sqlite_db_dir - specify a mount path for SQLite DB file
+
+nng_mqtt_set_sqlite_max_rows - specify Max message limitation for caching
+
+nng_mqtt_set_sqlite_flush_threshold - specify a threshold of flushing messages to flash
+
+nng_mqtt_sqlite_db_init - initialize sqlite options and create or open a database file
+
+nng_mqtt_sqlite_db_fini - deinit `nng_mqtt_sqlite_option` and close database file
+
+nng_mqtt_sqlite_db_get_cached_size - get cached message size
+
+== SYNOPSIS
+
+[source, c]
+----
+#include <nng/mqtt/mqtt_client.h>
+
+typedef struct nng_mqtt_sqlite_option nng_mqtt_sqlite_option;
+
+int  nng_mqtt_alloc_sqlite_opt(nng_mqtt_sqlite_option **opt);
+int  nng_mqtt_free_sqlite_opt(nng_mqtt_sqlite_option *opt);
+void nng_mqtt_set_sqlite_enable(nng_mqtt_sqlite_option *opt, bool enable);
+void nng_mqtt_set_sqlite_db_dir(
+    nng_mqtt_sqlite_option *opt, const char *dir_path);
+void nng_mqtt_set_sqlite_max_rows(nng_mqtt_sqlite_option *opt, size_t max_rows);
+void nng_mqtt_set_sqlite_flush_threshold(
+    nng_mqtt_sqlite_option *opt, size_t threshold);
+void nng_mqtt_sqlite_db_init(
+    nng_mqtt_sqlite_option *opt, const char *db_name, uint8_t mqtt_proto);
+void   nng_mqtt_sqlite_db_fini(nng_mqtt_sqlite_option *opt);
+size_t nng_mqtt_sqlite_db_get_cached_size(nng_mqtt_sqlite_option *opt);
+
+
+----
+
+== DESCRIPTION
+
+SQLite is disabled by default.To enable SQLite, it's necessary to compile with cmake option `-DNNG_ENABLE_SQLITE=ON`.
+
+
+|===
+|Function                           | Description                               | Return 
+|`nng_mqtt_alloc_sqlite_opt()`      | allocate a nng_mqtt_sqlite_option object  | 0: success  +
+nonzero: error code 
+|`nng_mqtt_free_sqlite_opt()`       | free a nng_mqtt_sqlite_option object      | 0
+|`nng_mqtt_set_sqlite_enable()`     | enable or disable sqlite caching          | non
+|`nng_mqtt_set_sqlite_db_dir()`     | specify a mount path for SQLite DB file  +
+(default: client running path)  | non
+|`nng_mqtt_set_sqlite_max_rows()`   | specify Max message limitation for caching +
+0 means ineffective +
+range: 1-infinity +
+(default: 102400) | non
+|`nng_mqtt_set_sqlite_flush_threshold()` |specify a threshold of flushing messages to flash +
+1-infinity +
+(default: 100)
+ | non
+|`nng_mqtt_sqlite_db_init()`            | initialize sqlite options and create or open a database file | non
+|`nng_mqtt_sqlite_db_fini()`            | deinit `nng_mqtt_sqlite_option` and close database file | non
+|`nng_mqtt_sqlite_db_get_cached_size()` | get cached message size | message size 
+|===
+
+== ERRORS
+
+none
+
+== SEE ALSO
+
+[.text-left]
+xref:nng_mqtt_msg_alloc.3.adoc[nng_mqtt_msg_alloc()],
+xref:nng_mqtt_msg_set_connect.3.adoc[nng_mqtt_msg_set_connect()],
+xref:xref:nng_pipe_notify.3.adoc[nng_pipe_notify()],
+xref:nng_strerror.3.adoc[nng_strerror(3)],
+xref:nng_url_parse.3.adoc[nng_url_parse(3)],
+xref:nng.7.adoc[nng(7)]