Use TwiBuffers class for all buffers needed by Wire and twi. Allows

optionally tweaking Wire buffer and twi buffer sizes at compile time.

enum TWI_STATE : uint8_t becomes uint8_t.
twi_state type changed form uint8_t to TWI_STATE.

Added examples for customized buffers.

Do println() after all received characters have been printed in
master_reader.ino.

Corrected setting of transmit buffer size in example sketch.
Use template struct for wire WireBuffers instead of namespace.
Use only a single macro SET_WIRE_BUFFERS().

Author: dac1e

libraries/Wire/examples/master_reader/master_reader.ino

@@ -24,6 +24,7 @@ void loop() {
     char c = Wire.read(); // receive a byte as character
     Serial.print(c);         // print the character
   }
+  Serial.println();
 
   delay(500);
 }

libraries/Wire/examples/master_reader_custombuffer/master_reader_custombuffer.ino

@@ -0,0 +1,62 @@
+// Wire Master Reader Custom Buffer
+
+// Demonstrates use of the Wire library
+// Reads data from an I2C/TWI slave device
+// Refer to the "Wire Slave Sender Custom Buffer" example for use with this
+
+// Created 31 Dec 2024
+
+// This example code is in the public domain.
+
+
+#include <Wire.h>
+#include <TwoWireBuffers.h>
+#include "Arduino.h"
+
+// request 6 bytes from slave device #8
+constexpr size_t REQUESTED_BYTE_COUNT = 6;
+
+constexpr size_t RECEIVE_BUFFER_SIZE  = REQUESTED_BYTE_COUNT;
+constexpr size_t TRANSMIT_BUFFER_SIZE = 0; // There is no transmit in this sketch.
+
+SET_WIRE_BUFFERS(RECEIVE_BUFFER_SIZE, TRANSMIT_BUFFER_SIZE,
+    true /* master buffers needed */, false /* no slave buffers needed */ );
+
+void setup() {
+  Wire.begin();        // join I2C bus (address optional for master)
+  Serial.begin(9600);  // start serial for output
+
+  // This is just for curiosity and can be removed
+  printWireBuffersCapacity(Serial);
+}
+
+void loop() {
+  Wire.requestFrom(8, REQUESTED_BYTE_COUNT);
+
+  while (Wire.available()) { // slave may send less than requested
+    char c = Wire.read(); // receive a byte as character
+    Serial.print(c);         // print the character
+  }
+  Serial.println();
+
+  delay(500);
+}
+
+void printWireBuffersCapacity(Stream& stream) {
+  const auto& buffers = GET_WIRE_BUFFERS();
+
+  stream.print("Wire transmit buffer size is ");
+  stream.println(buffers.txWireBufferCapacity());
+
+  stream.print("Wire receive buffer size is ");
+  stream.println(buffers.rxWireBufferCapacity());
+
+  stream.print("twi_masterBuffer size is ");
+  stream.println(buffers.twi_masterBufferCapacity());
+
+  stream.print("twi_rxBuffer size is ");
+  stream.println(buffers.twi_rxBufferCapacity());
+
+  stream.print("twi_txBuffer size is ");
+  stream.println(buffers.twi_txBufferCapacity());
+}

libraries/Wire/examples/master_writer/master_writer.ino

@@ -16,7 +16,7 @@ void setup() {
   Wire.begin(); // join I2C bus (address optional for master)
 }
 
-byte x = 0;
+static byte x = 0;
 
 void loop() {
   Wire.beginTransmission(8); // transmit to device #8

libraries/Wire/examples/master_writer_custombuffer/master_writer_custombuffer.ino

@@ -0,0 +1,62 @@
+// Wire Master Writer Custom Buffer
+
+// Demonstrates use of the Wire library
+// Writes data to an I2C/TWI slave device
+// Refer to the "Wire Slave Receiver Custom Buffer" example for use with this
+
+// Created 31 Dec 2024
+
+// This example code is in the public domain.
+
+
+#include <Wire.h>
+#include <TwoWireBuffers.h>
+#include "Arduino.h"
+
+// The following text will not fit into the default buffer of 32 bytes.
+static const char text[] = "You really won't believe it, but x is ";
+
+size_t constexpr RECEIVE_BUFFER_SIZE  = 0;  // There is no receive in this sketch.
+size_t constexpr TRANSMIT_BUFFER_SIZE = 42; // Enhance the buffer to 42 characters.
+
+SET_WIRE_BUFFERS(RECEIVE_BUFFER_SIZE, TRANSMIT_BUFFER_SIZE,
+    true /* master buffers needed */, false /* no slave buffers needed */ );
+
+void setup() {
+  Wire.begin(); // join I2C bus (address optional for master)
+
+  // This is just for curiosity and can be removed
+  Serial.begin(9600);   // start serial for output
+  printWireBuffersCapacity(Serial);
+}
+
+static byte x = 0;
+
+void loop() {
+  Wire.beginTransmission(8); // transmit to device #8
+  Wire.write(text);        // sends five bytes
+  Wire.write(x);              // sends one byte
+  Wire.endTransmission();    // stop transmitting
+
+  x++;
+  delay(500);
+}
+
+void printWireBuffersCapacity(Stream& stream) {
+  const auto& buffers = GET_WIRE_BUFFERS();
+
+  stream.print("Wire transmit buffer size is ");
+  stream.println(buffers.txWireBufferCapacity());
+
+  stream.print("Wire receive buffer size is ");
+  stream.println(buffers.rxWireBufferCapacity());
+
+  stream.print("twi_masterBuffer size is ");
+  stream.println(buffers.twi_masterBufferCapacity());
+
+  stream.print("twi_rxBuffer size is ");
+  stream.println(buffers.twi_rxBufferCapacity());
+
+  stream.print("twi_txBuffer size is ");
+  stream.println(buffers.twi_txBufferCapacity());
+}

libraries/Wire/examples/slave_receiver_custombuffer/slave_receiver_custombuffer.ino

@@ -0,0 +1,63 @@
+// Wire Slave Receiver Custom Buffer
+
+// Demonstrates use of the Wire library
+// Receives data as an I2C/TWI slave device
+// Refer to the "Wire Master Writer Custom Buffer" example for use with this
+
+// Created 31 Dec 2024
+
+// This example code is in the public domain.
+
+
+#include <Wire.h>
+#include <TwoWireBuffers.h>
+#include "Arduino.h"
+
+size_t constexpr RECEIVE_BUFFER_SIZE  = 42; // Be able receive up to 42 characters in one message.
+size_t constexpr TRANSMIT_BUFFER_SIZE = 0;  // There is no transmit in this sketch.
+
+SET_WIRE_BUFFERS(RECEIVE_BUFFER_SIZE, TRANSMIT_BUFFER_SIZE,
+    true /* master buffers needed */, false /* no slave buffers needed */ );
+
+void setup() {
+  Wire.begin(8);                // join I2C bus with address #8
+  Wire.onReceive(receiveEvent); // register event
+  Serial.begin(9600);           // start serial for output
+
+  // This is just for curiosity and can be removed
+  printWireBuffersCapacity(Serial);
+}
+
+void loop() {
+  delay(100);
+}
+
+// function that executes whenever data is received from master
+// this function is registered as an event, see setup()
+void receiveEvent(int howMany) {
+  while (1 < Wire.available()) { // loop through all but the last
+    char c = Wire.read(); // receive byte as a character
+    Serial.print(c);         // print the character
+  }
+  int x = Wire.read();    // receive byte as an integer
+  Serial.println(x);         // print the integer
+}
+
+void printWireBuffersCapacity(Stream& stream) {
+  const auto& buffers = GET_WIRE_BUFFERS();
+
+  stream.print("Wire transmit buffer size is ");
+  stream.println(buffers.txWireBufferCapacity());
+
+  stream.print("Wire receive buffer size is ");
+  stream.println(buffers.rxWireBufferCapacity());
+
+  stream.print("twi_masterBuffer size is ");
+  stream.println(buffers.twi_masterBufferCapacity());
+
+  stream.print("twi_rxBuffer size is ");
+  stream.println(buffers.twi_rxBufferCapacity());
+
+  stream.print("twi_txBuffer size is ");
+  stream.println(buffers.twi_txBufferCapacity());
+}

libraries/Wire/examples/slave_sender_custombuffer/slave_sender_custombuffer.ino

@@ -0,0 +1,61 @@
+// Wire Slave Sender Custom Buffer
+
+// Demonstrates use of the Wire library
+// Sends data as an I2C/TWI slave device
+// Refer to the "Wire Master Reader Custom Buffer" example for use with this
+
+// Created 31 Dec 2024
+
+// This example code is in the public domain.
+
+
+#include <Wire.h>
+#include <TwoWireBuffers.h>
+#include "Arduino.h"
+
+static const char text[] = "hello "; // respond with message of 6 bytes
+
+size_t constexpr RECEIVE_BUFFER_SIZE  = 0; // There is no receive in this sketch.
+size_t constexpr TRANSMIT_BUFFER_SIZE = sizeof(text)-1; // Don't need a byte for the \0
+
+SET_WIRE_BUFFERS(RECEIVE_BUFFER_SIZE, TRANSMIT_BUFFER_SIZE,
+    true /* master buffers needed */, false /* no slave buffers needed */ );
+
+void setup() {
+  Wire.begin(8);                // join I2C bus with address #8
+  Wire.onRequest(requestEvent); // register event
+
+  // This is just for curiosity and can be removed
+  Serial.begin(9600);
+  printWireBuffersCapacity(Serial);
+}
+
+void loop() {
+  delay(100);
+}
+
+// function that executes whenever data is requested by master
+// this function is registered as an event, see setup()
+void requestEvent() {
+  Wire.write(text);
+  // as expected by master
+}
+
+void printWireBuffersCapacity(Stream& stream) {
+  const auto& buffers = GET_WIRE_BUFFERS();
+
+  stream.print("Wire transmit buffer size is ");
+  stream.println(buffers.txWireBufferCapacity());
+
+  stream.print("Wire receive buffer size is ");
+  stream.println(buffers.rxWireBufferCapacity());
+
+  stream.print("twi_masterBuffer size is ");
+  stream.println(buffers.twi_masterBufferCapacity());
+
+  stream.print("twi_rxBuffer size is ");
+  stream.println(buffers.twi_rxBufferCapacity());
+
+  stream.print("twi_txBuffer size is ");
+  stream.println(buffers.twi_txBufferCapacity());
+}

libraries/Wire/src/TwoWireBuffers.cpp

@@ -0,0 +1,29 @@
+/*
+  twi.c - TWI/I2C library for Wiring & Arduino
+  Copyright (c) 2006 Nicholas Zambetti.  All right reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#include "TwoWireBuffers.h"
+
+constexpr size_t RX_BUFFER_DEFAULT_LENGTH = 32;
+constexpr size_t TX_BUFFER_DEFAULT_LENGTH = 32;
+
+// Default buffers for the one and only Wire object
+template<> __attribute__((weak)) TwoWireBuffers::Interface& WireBuffers<0>::instance() { \
+  static TwoWireBuffers::Impl<RX_BUFFER_DEFAULT_LENGTH, TX_BUFFER_DEFAULT_LENGTH, true, true> buffers; \
+  return buffers; \
+}