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+.. _api_doc:
+
+Writing API handlers
+====================
+
+VPP provides a binary API scheme to allow a wide variety of client codes
+to program data-plane tables. As of this writing, there are hundreds of
+binary APIs.
+
+Messages are defined in ``*.api`` files. Today, there are about 50 api
+files, with more arriving as folks add programmable features. The API
+file compiler sources reside in @ref src/tools/vppapigen.
+
+From @ref src/vnet/interface.api, here’s a typical request/response
+message definition:
+
+.. code:: c
+
+ autoreply define sw_interface_set_flags
+ {
+ u32 client_index;
+ u32 context;
+ u32 sw_if_index;
+ /* 1 = up, 0 = down */
+ u8 admin_up_down;
+ };
+
+To a first approximation, the API compiler renders this definition into
+``build-root/.../vpp/include/vnet/interface.api.h`` as follows:
+
+.. code:: c
+
+ /****** Message ID / handler enum ******/
+ #ifdef vl_msg_id
+ vl_msg_id(VL_API_SW_INTERFACE_SET_FLAGS, vl_api_sw_interface_set_flags_t_handler)
+ vl_msg_id(VL_API_SW_INTERFACE_SET_FLAGS_REPLY, vl_api_sw_interface_set_flags_reply_t_handler)
+ #endif
+
+ /****** Message names ******/
+ #ifdef vl_msg_name
+ vl_msg_name(vl_api_sw_interface_set_flags_t, 1)
+ vl_msg_name(vl_api_sw_interface_set_flags_reply_t, 1)
+ #endif
+
+ /****** Message name, crc list ******/
+ #ifdef vl_msg_name_crc_list
+ #define foreach_vl_msg_name_crc_interface \
+ _(VL_API_SW_INTERFACE_SET_FLAGS, sw_interface_set_flags, f890584a) \
+ _(VL_API_SW_INTERFACE_SET_FLAGS_REPLY, sw_interface_set_flags_reply, dfbf3afa) \
+ #endif
+
+ /****** Typedefs *****/
+ #ifdef vl_typedefs
+ typedef VL_API_PACKED(struct _vl_api_sw_interface_set_flags {
+ u16 _vl_msg_id;
+ u32 client_index;
+ u32 context;
+ u32 sw_if_index;
+ u8 admin_up_down;
+ }) vl_api_sw_interface_set_flags_t;
+
+ typedef VL_API_PACKED(struct _vl_api_sw_interface_set_flags_reply {
+ u16 _vl_msg_id;
+ u32 context;
+ i32 retval;
+ }) vl_api_sw_interface_set_flags_reply_t;
+
+ ...
+ #endif /* vl_typedefs */
+
+To change the admin state of an interface, a binary api client sends a
+@ref vl_api_sw_interface_set_flags_t to VPP, which will respond with a
+@ref vl_api_sw_interface_set_flags_reply_t message.
+
+Multiple layers of software, transport types, and shared libraries
+implement a variety of features:
+
+- API message allocation, tracing, pretty-printing, and replay.
+- Message transport via global shared memory, pairwise/private shared
+ memory, and sockets.
+- Barrier synchronization of worker threads across thread-unsafe
+ message handlers.
+
+Correctly-coded message handlers know nothing about the transport used
+to deliver messages to/from VPP. It’s reasonably straightforward to use
+multiple API message transport types simultaneously.
+
+For historical reasons, binary api messages are (putatively) sent in
+network byte order. As of this writing, we’re seriously considering
+whether that choice makes sense.
+
+Message Allocation
+------------------
+
+Since binary API messages are always processed in order, we allocate
+messages using a ring allocator whenever possible. This scheme is
+extremely fast when compared with a traditional memory allocator, and
+doesn’t cause heap fragmentation. See @ref
+src/vlibmemory/memory_shared.c @ref vl_msg_api_alloc_internal().
+
+Regardless of transport, binary api messages always follow a @ref
+msgbuf_t header:
+
+.. code:: c
+
+ typedef struct msgbuf_
+ {
+ unix_shared_memory_queue_t *q;
+ u32 data_len;
+ u32 gc_mark_timestamp;
+ u8 data[0];
+ } msgbuf_t;
+
+This structure makes it easy to trace messages without having to decode
+them - simply save data_len bytes - and allows @ref vl_msg_api_free() to
+rapidly dispose of message buffers:
+
+.. code:: c
+
+ void
+ vl_msg_api_free (void *a)
+ {
+ msgbuf_t *rv;
+ api_main_t *am = &api_main;
+
+ rv = (msgbuf_t *) (((u8 *) a) - offsetof (msgbuf_t, data));
+
+ /*
+ * Here's the beauty of the scheme. Only one proc/thread has
+ * control of a given message buffer. To free a buffer, we just
+ * clear the queue field, and leave. No locks, no hits, no errors...
+ */
+ if (rv->q)
+ {
+ rv->q = 0;
+ rv->gc_mark_timestamp = 0;
+ return;
+ }
+ <snip>
+ }
+
+Message Tracing and Replay
+--------------------------
+
+It’s extremely important that VPP can capture and replay sizeable binary
+API traces. System-level issues involving hundreds of thousands of API
+transactions can be re-run in a second or less. Partial replay allows
+one to binary-search for the point where the wheels fall off. One can
+add scaffolding to the data plane, to trigger when complex conditions
+obtain.
+
+With binary API trace, print, and replay, system-level bug reports of
+the form “after 300,000 API transactions, the VPP data-plane stopped
+forwarding traffic, FIX IT!” can be solved offline.
+
+More often than not, one discovers that a control-plane client
+misprograms the data plane after a long time or under complex
+circumstances. Without direct evidence, “it’s a data-plane problem!”
+
+See @ref src/vlibmemory/memory_vlib.c @ref vl_msg_api_process_file(),
+and @ref src/vlibapi/api_shared.c. See also the debug CLI command “api
+trace”
+
+Client connection details
+-------------------------
+
+Establishing a binary API connection to VPP from a C-language client is
+easy:
+
+.. code:: c
+
+ int
+ connect_to_vpe (char *client_name, int client_message_queue_length)
+ {
+ vat_main_t *vam = &vat_main;
+ api_main_t *am = &api_main;
+
+ if (vl_client_connect_to_vlib ("/vpe-api", client_name,
+ client_message_queue_length) < 0)
+ return -1;
+
+ /* Memorize vpp's binary API message input queue address */
+ vam->vl_input_queue = am->shmem_hdr->vl_input_queue;
+ /* And our client index */
+ vam->my_client_index = am->my_client_index;
+ return 0;
+ }
+
+32 is a typical value for client_message_queue_length. VPP cannot block
+when it needs to send an API message to a binary API client, and the
+VPP-side binary API message handlers are very fast. When sending
+asynchronous messages, make sure to scrape the binary API rx ring with
+some enthusiasm.
+
+binary API message RX pthread
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Calling @ref vl_client_connect_to_vlib spins up a binary API message RX
+pthread:
+
+.. code:: c
+
+ static void *
+ rx_thread_fn (void *arg)
+ {
+ unix_shared_memory_queue_t *q;
+ memory_client_main_t *mm = &memory_client_main;
+ api_main_t *am = &api_main;
+
+ q = am->vl_input_queue;
+
+ /* So we can make the rx thread terminate cleanly */
+ if (setjmp (mm->rx_thread_jmpbuf) == 0)
+ {
+ mm->rx_thread_jmpbuf_valid = 1;
+ while (1)
+ {
+ vl_msg_api_queue_handler (q);
+ }
+ }
+ pthread_exit (0);
+ }
+
+To handle the binary API message queue yourself, use @ref
+vl_client_connect_to_vlib_no_rx_pthread.
+
+In turn, vl_msg_api_queue_handler(…) uses mutex/condvar signalling to
+wake up, process VPP -> client traffic, then sleep. VPP supplies a
+condvar broadcast when the VPP -> client API message queue transitions
+from empty to nonempty.
+
+VPP checks its own binary API input queue at a very high rate. VPP
+invokes message handlers in “process” context [aka cooperative
+multitasking thread context] at a variable rate, depending on data-plane
+packet processing requirements.
+
+Client disconnection details
+----------------------------
+
+To disconnect from VPP, call @ref vl_client_disconnect_from_vlib. Please
+arrange to call this function if the client application terminates
+abnormally. VPP makes every effort to hold a decent funeral for dead
+clients, but VPP can’t guarantee to free leaked memory in the shared
+binary API segment.
+
+Sending binary API messages to VPP
+----------------------------------
+
+The point of the exercise is to send binary API messages to VPP, and to
+receive replies from VPP. Many VPP binary APIs comprise a client request
+message, and a simple status reply. For example, to set the admin status
+of an interface, one codes:
+
+.. code:: c
+
+ vl_api_sw_interface_set_flags_t *mp;
+
+ mp = vl_msg_api_alloc (sizeof (*mp));
+ memset (mp, 0, sizeof (*mp));
+ mp->_vl_msg_id = clib_host_to_net_u16 (VL_API_SW_INTERFACE_SET_FLAGS);
+ mp->client_index = api_main.my_client_index;
+ mp->sw_if_index = clib_host_to_net_u32 (<interface-sw-if-index>);
+ vl_msg_api_send (api_main.shmem_hdr->vl_input_queue, (u8 *)mp);
+
+Key points:
+
+- Use @ref vl_msg_api_alloc to allocate message buffers
+
+- Allocated message buffers are not initialized, and must be presumed
+ to contain trash.
+
+- Don’t forget to set the \_vl_msg_id field!
+
+- As of this writing, binary API message IDs and data are sent in
+ network byte order
+
+- The client-library global data structure @ref api_main keeps track of
+ sufficient pointers and handles used to communicate with VPP
+
+Receiving binary API messages from VPP
+--------------------------------------
+
+Unless you’ve made other arrangements (see @ref
+vl_client_connect_to_vlib_no_rx_pthread), *messages are received on a
+separate rx pthread*. Synchronization with the client application main
+thread is the responsibility of the application!
+
+Set up message handlers about as follows:
+
+.. code:: c
+
+ #define vl_typedefs /* define message structures */
+ #include <vpp/api/vpe_all_api_h.h>
+ #undef vl_typedefs
+
+ /* declare message handlers for each api */
+
+ #define vl_endianfun /* define message structures */
+ #include <vpp/api/vpe_all_api_h.h>
+ #undef vl_endianfun
+
+ /* instantiate all the print functions we know about */
+ #define vl_print(handle, ...)
+ #define vl_printfun
+ #include <vpp/api/vpe_all_api_h.h>
+ #undef vl_printfun
+
+ /* Define a list of all message that the client handles */
+ #define foreach_vpe_api_reply_msg \
+ _(SW_INTERFACE_SET_FLAGS_REPLY, sw_interface_set_flags_reply)
+
+ static clib_error_t *
+ my_api_hookup (vlib_main_t * vm)
+ {
+ api_main_t *am = &api_main;
+
+ #define _(N,n) \
+ vl_msg_api_set_handlers(VL_API_##N, #n, \
+ vl_api_##n##_t_handler, \
+ vl_noop_handler, \
+ vl_api_##n##_t_endian, \
+ vl_api_##n##_t_print, \
+ sizeof(vl_api_##n##_t), 1);
+ foreach_vpe_api_msg;
+ #undef _
+
+ return 0;
+ }
+
+The key API used to establish message handlers is @ref
+vl_msg_api_set_handlers , which sets values in multiple parallel vectors
+in the @ref api_main_t structure. As of this writing: not all vector
+element values can be set through the API. You’ll see sporadic API
+message registrations followed by minor adjustments of this form:
+
+.. code:: c
+
+ /*
+ * Thread-safe API messages
+ */
+ am->is_mp_safe[VL_API_IP_ADD_DEL_ROUTE] = 1;
+ am->is_mp_safe[VL_API_GET_NODE_GRAPH] = 1;