Sending SMS in C++: LibCurl API Library and REST Requests

Sending SMS in C++: A Complete Guide to Using the API with the LibCurl Library

Sending SMS messages is one of the key functions of modern applications: from two-factor authentication to order status notifications. The C++ ecosystem does not have built-in tools for working with SMS gateways, however, using the universal libcurl library and standard HTTP requests, you can easily integrate any REST API of an SMS provider. In this article, we will analyze how to write a reliable function for sending SMS in C++ using the LibCurl library, and look at typical examples for popular services (Twilio, SMS Aero, Bytehand).

Why LibCurl?

LibCurl is a cross-platform library for data transfer over various protocols (HTTP, HTTPS, FTP, etc.). It supports:

• GET, POST, PUT, DELETE requests
• SSL/TLS (HTTPS)
• Working with request headers and body
• Processing server responses

To send an SMS, it is enough to make a POST request to the API of the selected provider, passing the necessary parameters (phone number, text, API key). LibCurl handles all the low-level work with sockets and encryption.

Installing the Library

In most Linux distributions, LibCurl can be installed via the package manager:

sudo apt-get install libcurl4-openssl-dev  # Ubuntu/Debiansudo yum install libcurl-devel             # CentOS/RHEL

For Windows, you can download pre-built DLLs from the official website or use vcpkg:

vcpkg install curl

Including in code:

#include <curl/curl.h>#include <string>#include <json/json.h>  // if using JSON (nlohmann/json)

Basic Structure of the SMS Function

Let's create a universal function sendSms that takes the API URL, key, recipient number, and message text. For working with JSON, it is convenient to use the nlohmann/json library (header-only, no build required).

#include <iostream>#include <string>#include <curl/curl.h>#include <nlohmann/json.hpp>

using json = nlohmann::json;

// Callback for writing the server responsestatic size_t WriteCallback(void *contents, size_t size, size_t nmemb, void *userp) {    ((std::string*)userp)->append((char*)contents, size * nmemb);    return size * nmemb;}

bool sendSms(const std::string& url, const std::string& apiKey,              const std::string& phone, const std::string& message) {    CURL *curl;    CURLcode res;    std::string readBuffer;

    curl_global_init(CURL_GLOBAL_DEFAULT);    curl = curl_easy_init();

    if(!curl) return false;

    // Form the JSON request body    json payload;    payload["phone"] = phone;    payload["text"] = message;    payload["api_key"] = apiKey;

    std::string jsonStr = payload.dump();

    // HTTP request setup    struct curl_slist *headers = NULL;    headers = curl_slist_append(headers, "Content-Type: application/json");    headers = curl_slist_append(headers, "Accept: application/json");

    curl_easy_setopt(curl, CURLOPT_URL, url.c_str());    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, jsonStr.c_str());    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &readBuffer);

    // Execute the request    res = curl_easy_perform(curl);

    // Cleanup    curl_easy_cleanup(curl);    curl_global_cleanup();

    if(res != CURLE_OK) {        std::cerr << "Curl error: " << curl_easy_strerror(res) << std::endl;        return false;    }

    // Parse the response (example for Twilio)    try {        json response = json::parse(readBuffer);        if(response.co