added lwip doc

_posts/2023-10-09-lwip-tcp.markdown

@@ -0,0 +1,58 @@
+---
+layout: post
+title:  "LwIP TCP API"
+date:   2023-10-09 00:00:00 -0400
+---
+
+# Minimal LwIP TCP API (as used on a Raspberry PI pico-w)
+
+### tcp_new()
+```
+struct tcp_pcb* tcp_new	(	void 		)	
+struct tcp_pcb* tcp_new_ip_type	(	u8_t 	type	)
+```
+Creates a new TCP protocol control block but doesn't place it on any of the TCP PCB lists until binding using tcp_bind().
+
+Parameters
+- IPADDR_TYPE_ANY|
+Returns
+- a new tcp_pcb that initially is in state CLOSED
+
+### tcp_bind()
+
+### tcp_connect()
+```
+err_t tcp_connect ( struct tcp_pcb * pcb, const ip_addr_t * ipaddr, u16_t port, tcp_connected_fn connected)
+```
+Parameters
+- pcb	the tcp_pcb used to establish the connection
+- ipaddr	the remote ip address to connect to
+- port	the remote tcp port to connect to
+- connected	callback function to call when connected (on error, the err calback will be called)
+Returns
+- ERR_VAL if invalid arguments are given ERR_OK if connect request has been sent other err_t values if connect request couldn't be sent
+
+### tcp_write()
+```
+err_t tcp_write	( struct tcp_pcb * pcb, const void * arg, u16_t len, u8_t apiflags)
+```
+Parameters
+- pcb   Protocol control block for the TCP connection to enqueue data for.
+- arg	Pointer to the data to be enqueued for sending.
+- len	Data length in bytes
+- apiflags	combination of following flags :
+    - TCP_WRITE_FLAG_COPY (0x01) data will be copied into memory belonging to the stack
+    - TCP_WRITE_FLAG_MORE (0x02) for TCP connection, PSH flag will not be set on last segment sent,
+Returns
+- ERR_OK if enqueued, another err_t on error
+
+### tcp_output()
+```
+err_t tcp_output ( struct tcp_pcb * pcb )
+```
+Parameters
+- pcb	Protocol control block for the TCP connection to send data
+Returns
+- ERR_OK if data has been sent or nothing to send another err_t on error
+
+