ESP32 WiFiClient Library – write

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Description

The write method in the WiFiClient library enables your ESP32 to transmit data to a connected server or client over a Wi-Fi network. With multiple ways to use it—whether sending a single byte, a buffer, PROGMEM data, or streaming content—this method is a cornerstone for networked applications like IoT devices, web clients, or custom protocols. It empowers makers to communicate seamlessly with the digital world.

Syntax and Usage

The write method offers four distinct ways to send data, each suited to different scenarios. Here’s how you can use it:

– Writing a single byte:

client.write(data);

Send a single byte of data to the connected endpoint.

– Writing a buffer of bytes:

client.write(buf, size);

Send a sequence of bytes from a memory buffer, with a specified length.

– Writing PROGMEM data:

Send a buffer of bytes stored in PROGMEM (program memory), useful for conserving RAM.

client.write_P(buf, size);

– Writing from a Stream:

client.write(stream);

Send data directly from a Stream object, such as Serial or a file.

For practical applications and examples of this method, please consult the “Example Code” section on this page. This section provides comprehensive guidance to help you better understand and apply the method effectively.

Argument(s)

The write method’s arguments vary depending on the usage. Here’s a detailed list:

data: An unsigned 8-bit integer (uint8_t) representing a single byte to send. Used for single-byte writes.
buf: A pointer to an array of bytes (const uint8_t*) in RAM containing the data to send. Used with buffer writes.
size: An integer (size_t) specifying the number of bytes to send from the buffer. Used with buffer and PROGMEM writes.
buf (PROGMEM): A pointer to a PROGMEM-stored buffer (PGM_P) containing the data to send. Used for PROGMEM writes.
stream: A reference to a Stream object (e.g., Serial) from which data is read and sent. Used for stream writes.

Return Value

The write method returns a size_t value indicating the number of bytes successfully written to the connection. A return value of 0 typically means no data was sent, possibly due to a failed connection or empty input. Checking this value ensures your data transmission worked as intended.

Example Codes

Below are examples for each way to use the write method, connecting to www.httpbin.org where applicable. These snippets show practical applications for your ESP32 projects.

Example 1: Writing a Single Byte

This example connects to www.httpbin.org and sends a minimal HTTP GET request byte-by-byte using write. The server’s response is printed to the Serial monitor, proving the data was received.

/*
 * Author: Avant Maker
 * Date: February 24, 2025
 * Version: 1.0
 * Description: This example connects to www.httpbin.org
 * and sends a minimal HTTP GET request byte-by-byte using
 * ESP32 WiFiClient Library's write method.
 * The server’s response is printed to the Serial monitor,
 * proving the data was received.
 *
 * License: MIT 
 * 
 * Code Source: This example code is sourced from the Comprehensive 
 * Guide to the ESP32 Arduino Core Library, accessible on 
 * AvantMaker.com. For additional code examples and in-depth 
 * documentation related to the ESP32 Arduino Core Library, 
 * please visit:
 *
 * https://avantmaker.com/references/esp32-arduino-core-index/
 *
 * AvantMaker.com, your premier destination for all things 
 * DIY, AI, IoT, Smart Home, and STEM projects. We are dedicated 
 * to empowering makers, learners, and enthusiasts with the resources
 * they need to bring their innovative ideas to life.
 */

#include <WiFi.h>

const char* ssid = "your-SSID";          // Replace with your Wi-Fi SSID
const char* password = "your-PASSWORD"; // Replace with your Wi-Fi password
const char* host = "www.httpbin.org";
const int port = 80;

WiFiClient client;

void setup() {
    Serial.begin(115200);
    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("Connected to WiFi");

    if (client.connect(host, port)) {
        Serial.println("Connected to server");
        // Send HTTP GET request byte-by-byte
        const char* request = "GET /get HTTP/1.1\r\nHost: www.httpbin.org\r\nConnection: close\r\n\r\n";
        size_t bytesWritten = 0;
        for (int i = 0; i < strlen(request); i++) {
            bytesWritten += client.write(request[i]);
        }
        Serial.print("Total bytes written: ");
        Serial.println(bytesWritten);
        
        // Wait for and display server response
        delay(1000); // Give server time to respond
        Serial.println("Server response:");
        while (client.available()) {
            char c = client.read();
            Serial.write(c);
        }
        client.stop();
        Serial.println("\nConnection closed");
    } else {
        Serial.println("Connection failed");
    }
}

void loop() {
}

Example 2: Writing a Buffer of Bytes

This example sends a full HTTP GET request to www.httpbin.org using a buffer of bytes, showcasing how to transmit structured data.

/*
 * Author: Avant Maker
 * Date: February 24, 2025
 * Version: 1.0
 * Description: This example sends a full HTTP GET request 
 * to www.httpbin.org using a buffer of bytes, showcasing
 * how to transmit structured data with ESP32 WiFiClient
 * Library's write method.
 *
 * License: MIT 
 * 
 * Code Source: This example code is sourced from the Comprehensive 
 * Guide to the ESP32 Arduino Core Library, accessible on 
 * AvantMaker.com. For additional code examples and in-depth 
 * documentation related to the ESP32 Arduino Core Library, 
 * please visit:
 *
 * https://avantmaker.com/references/esp32-arduino-core-index/
 *
 * AvantMaker.com, your premier destination for all things 
 * DIY, AI, IoT, Smart Home, and STEM projects. We are dedicated 
 * to empowering makers, learners, and enthusiasts with the resources
 * they need to bring their innovative ideas to life.
 */

#include <WiFi.h>

const char* ssid = "your-SSID";          // Replace with your Wi-Fi SSID
const char* password = "your-PASSWORD";  // Replace with your Wi-Fi password
const char* host = "www.httpbin.org";
const int port = 80;

WiFiClient client;

void setup() {
    Serial.begin(115200);
    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("Connected to WiFi");

    if (client.connect(host, port)) {
        Serial.println("Connected to server");
        const char* request = "GET /get HTTP/1.1\r\nHost: www.httpbin.org\r\nConnection: close\r\n\r\n";
        size_t bytesWritten = client.write((const uint8_t*)request, strlen(request)); // Send buffer
        Serial.print("Bytes written: ");
        Serial.println(bytesWritten);
        delay(1000); // Wait for response
        while (client.available()) {
            Serial.write(client.read());
        }
        client.stop();
    } else {
        Serial.println("Connection failed");
    }
}

void loop() {
}

Example 3: Writing PROGMEM Data

This example stores an HTTP request in PROGMEM and sends it to www.httpbin.org, demonstrating RAM-efficient data transmission.

/*
 * Author: Avant Maker
 * Date: February 24, 2025
 * Version: 1.0
 * Description: This example stores an HTTP request in
 * PROGMEM and sends it to www.httpbin.org with
 * ESP32 WiFiClient Library's write method.
 * Demonstrating RAM-efficient data transmission.
 
 This example sends a full HTTP GET request 
 * to www.httpbin.org using a buffer of bytes, showcasing
 * how to transmit structured data with ESP32 WiFiClient
 * Library's write method.
 *
 * License: MIT 
 * 
 * Code Source: This example code is sourced from the Comprehensive 
 * Guide to the ESP32 Arduino Core Library, accessible on 
 * AvantMaker.com. For additional code examples and in-depth 
 * documentation related to the ESP32 Arduino Core Library, 
 * please visit:
 *
 * https://avantmaker.com/references/esp32-arduino-core-index/
 *
 * AvantMaker.com, your premier destination for all things 
 * DIY, AI, IoT, Smart Home, and STEM projects. We are dedicated 
 * to empowering makers, learners, and enthusiasts with the resources
 * they need to bring their innovative ideas to life.
 */

#include <WiFi.h>

const char* ssid = "your-SSID";          // Replace with your Wi-Fi SSID
const char* password = "your-PASSWORD";  // Replace with your Wi-Fi password
const char* host = "www.httpbin.org";
const int port = 80;

// Store request in PROGMEM
const char request[] PROGMEM = "GET /get HTTP/1.1\r\nHost: www.httpbin.org\r\nConnection: close\r\n\r\n";

WiFiClient client;

void setup() {
    Serial.begin(115200);
    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("Connected to WiFi");

    if (client.connect(host, port)) {
        Serial.println("Connected to server");
        size_t bytesWritten = client.write_P(request, strlen_P(request)); // Send PROGMEM data
        Serial.print("Bytes written: ");
        Serial.println(bytesWritten);
        delay(1000); // Wait for response
        while (client.available()) {
            Serial.write(client.read());
        }
        client.stop();
    } else {
        Serial.println("Connection failed");
    }
}

void loop() {
}

Example 4: Writing from a Stream

This example automatically sends a hardcoded HTTP POST request to www.httpbin.org/post with a key-value pair (message=Hello from ESP32) using a StringStream. The server’s JSON response, printed to the Serial monitor, proves the data was received.

/*
 * Author: Avant Maker
 * Date: February 24, 2025
 * Version: 1.0
 * Description: This example demonstrates how to use 
 * ESP32 WiFiClient Library's write method to
 * send a data directly from a Stream object. 
 * License: MIT 
 * 
 * Code Source: This example code is sourced from the Comprehensive 
 * Guide to the ESP32 Arduino Core Library, accessible on 
 * AvantMaker.com. For additional code examples and in-depth 
 * documentation related to the ESP32 Arduino Core Library, 
 * please visit:
 *
 * https://avantmaker.com/references/esp32-arduino-core-index/
 *
 * AvantMaker.com, your premier destination for all things 
 * DIY, AI, IoT, Smart Home, and STEM projects. We are dedicated 
 * to empowering makers, learners, and enthusiasts with the resources
 * they need to bring their innovative ideas to life.
 */

#include <WiFi.h>
#include <StreamString.h>

const char* ssid = "your-SSID";          // Replace with your Wi-Fi SSID
const char* password = "your-PASSWORD";  // Replace with your Wi-Fi password
const char* host = "www.httpbin.org";
const int port = 80;

WiFiClient client;

void setup() {
    Serial.begin(115200);
    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("Connected to WiFi");

    if (client.connect(host, port)) {
        Serial.println("Connected to server");
        
        // Define the HTTP POST request
        String request = "POST /post HTTP/1.1\r\n"
                        "Host: www.httpbin.org\r\n"
                        "Content-Type: application/x-www-form-urlencoded\r\n"
                        "Content-Length: 22\r\n"
                        "Connection: close\r\n"
                        "\r\n"
                        "message=Hello from ESP32";
        
        // Create a Stream from the String
        StreamString stream;
        stream.print(request);
        
        // Send the stream data
        size_t bytesWritten = client.write(stream);
        Serial.print("Bytes written: ");
        Serial.println(bytesWritten);
        
        // Wait for and display server response
        delay(1000); // Give server time to respond
        Serial.println("Server response:");
        while (client.available()) {
            Serial.write(client.read());
        }
        client.stop();
        Serial.println("\nConnection closed");
    } else {
        Serial.println("Connection failed");
    }
}

void loop() {
}

ESP32 Library Index

ESP32 Arduino Core Library

ESP32 WiFi Library
ESP32 HTTPClient Library
ESP32 WiFiClientSecure Library
ESP32 WebServer Library
ESP32 WiFiClient Library
- Connection
  - connect
  - stop
  - connected
- Send Data
  - print
  - println
  - write
- Receive Data
  - available
  - read
  - readBytes
  - readString
  - readStringUntil
  - peek
  - flush
- Config
  - setSocketOption
  - setNoDelay
- Status

FAQ

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