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ESP32 WiFiClientSecure Library – peek

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Description

The peek method in the WiFiClientSecure library allows you to look at the next byte of data in a secure network stream (e.g., an HTTPS connection) without removing it from the buffer. This is particularly useful for checking incoming data before deciding how to process it, such as verifying a header or delimiter in a secure communication. Unlike reading methods, peek leaves the data intact for subsequent operations.

Syntax and Usage

The peek method has a single usage with no arguments. Below is the syntax:

int nextByte = client.peek();
  • Peeking at the next byte: This method returns the next byte from the secure stream as an integer without consuming it. It allows you to preview the data, making it useful for conditional processing or parsing without advancing the stream position.

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)

This method does not require any arguments. It simply inspects the next available byte in the secure stream and returns its value, providing a straightforward way to preview data.

Return Value

The peek method returns an int representing the next byte in the secure stream (0 to 255). If no data is available (e.g., the stream is empty or the connection is closed), it returns -1. The returned value is the ASCII code of the character or the raw byte value, depending on the context.

Example Codes

Below is an example demonstrating the use of the peek method, corresponding to the usage described in Section 2.

Example: Previewing Data in an HTTPS Response

This example connects to a secure server (e.g., “www.httpbin.org” over HTTPS), sends a POST request, and uses peek to check the first character of the response before reading it. The peeked value and subsequent read data are printed to the Serial Monitor. Replace the Wi-Fi credentials and certificate with your own for testing.

ATTENTION: The Root CA certificate has an expiration date. If the code fails to execute, it may be due to the Root CA certificate has expired. Try obtaining the latest Root CA certificate and replacing the expired one in this code. For detailed instructions on acquiring a website’s Root CA certificate, check out the link below:
How to Acquire the Root CA Certificate

/*
 * Author: Avant Maker
 * Date: February 24, 2025
 * Version: 1.0
 *
 * Description: This example demostrate how to use 
 * ESP32 WiFiClientSecure Libary's 
 * readStringUntil method to retrieve the entire response 'line-by-line'.  
 * 
 * ATTENTION: The Root CA certificate has an expiration date. 
 * If the code fails to execute, it may be due to the Root CA
 * certificate has expired. Try obtaining the latest 
 * Root CA certificate and replacing the expired one in this code.
 * For detailed instructions on acquiring a website's
 * Root CA certificate, check out the link below:
 *
 * https://avantmaker.com/references/esp32-arduino-core-index/esp32-wificlientsecure-library/how-to-acquire-the-root-ca-certificate/
 *
 * 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 <NetworkClientSecure.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 = 443;

// Root CA certificate for www.httpbin.org (update with your server's CA if different)
const char* rootCACert = R"literal(
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
)literal";

// Optional: Add client certificate and key for mutual TLS if needed
// const char* clientCertKey = "";
// const char* clientCert = "";

NetworkClientSecure secureClient;

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

    // Set the Root CA Cert
    secureClient.setCACert(rootCACert);

    // Establish secure connection to server
    if (secureClient.connect(host, port)) {
        Serial.println("Connected to server");

        // Create the POST data
        String postData = "AvantMaker=HelloFromESP32";

        // Send the POST request
        secureClient.print("POST /post HTTP/1.1\r\n");
        secureClient.print("Host: www.httpbin.org\r\n");
        secureClient.print("Content-Type: application/x-www-form-urlencoded\r\n");
        secureClient.print("Content-Length: ");
        secureClient.print(postData.length());
        secureClient.print("\r\nConnection: close\r\n\r\n");
        secureClient.print(postData);

        // Wait for server response
        while (secureClient.connected() && !secureClient.available()) {
            delay(10);
        }

        // Read response
        if (secureClient.available()) {
            int lineNumber = 0;
            Serial.println("Server response:");
            while (secureClient.available()) {
                Serial.print("Line ");
                lineNumber++;
                Serial.print(lineNumber);
                Serial.print(" - ");
                String line = secureClient.readStringUntil('\n');
                Serial.println(line);
            }
        } else {
            Serial.println("No data available");
        }

        secureClient.stop();

    } else {
        Serial.println("Failed to connect to server! Check the code description for solution.");
        return;
    }     
}

void loop() {
}

Explanation: In this code, the ESP32 connects to a Wi-Fi network and establishes a secure connection to “www.httpbin.org” using WiFiClientSecure. After sending an HTTP POST request, peek is used to preview the first byte of the response without removing it from the stream. The peeked value is printed, followed by reading the first line with readStringUntil to show that the peeked byte remains available. This demonstrates how peek can be used to inspect data before processing, such as checking for specific characters or patterns in a secure stream.

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