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Run a C++ app

In this guide we create and deploy a simple C++-based HTTP web server. To run this example, follow these steps:

  1. Install the kraft CLI tool and a container runtime engine, e.g. Docker.

  2. Clone the example repository and cd into the examples/http-cpp/ directory:

Terminal window
git clone
cd examples/http-cpp/

Make sure to log into KraftCloud by setting your token and a metro close to you. We use fra0 (Frankfurt, 🇩🇪) in this guide:

Terminal window
# Set KraftCloud access token
# Set metro to Frankfurt, DE

When done, invoke the following command to deploy this application on KraftCloud:

Terminal window
kraft cloud deploy -p 443:8080 .

The output shows the instance URL and other details:

Terminal window
[] Deployed successfully!
────────── name: http-cpp-jzbuo
────────── uuid: b8e015fd-d006-49d5-849e-3fd497c9159a
───────── state: running
─────────── url:
───────── image: http-cpp@sha256:a58873987104b52c13b79168a2e2f1a81876ba6efacd6dbc98e996afe5c09699
───── boot time: 15.61 ms
──────── memory: 128 MiB
service group: throbbing-wave-grxjih4t
── private fqdn: http-cpp-jzbuo.internal
──── private ip:
────────── args: /http_server

In this case, the instance name is http-cpp-jzbuo and the URL is They are different for each run.

Use curl to query the KraftCloud instance of the C++ HTTP web server:

Terminal window
Hello, World!

At any point in time, you can list information about the instance:

Terminal window
kraft cloud instance list
http-cpp-jzbuo running 1 minute ago http-cpp@sha256:a58873987104b52c13b79168a2e2f1a81876ba6efac... 128 MiB /http_server 15614us

When done, you can remove the instance:

Terminal window
kraft cloud instance remove http-cpp-jzbuo

Customize your Application

To customize the application, update the files in the repository, listed below:

  • http_server.cpp: the actual C++ HTTP server
  • Kraftfile: the KraftCloud specification
  • Dockerfile: the Docker-specified application filesystem
/* SPDX-License-Identifier: BSD-3-Clause */
* Copyright (c) 2023, Unikraft GmbH and the Unikraft Authors.
#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>
#define LISTEN_PORT 8080
static std::string reply = "HTTP/1.1 200 OK\r\n" \
"Content-Type: text/html\r\n" \
"Content-Length: 14\r\n" \
"Connection: close\r\n" \
"\r\n" \
"Hello, World!\n";
#define BUFLEN 2048
static char recvbuf[BUFLEN];
int main(int argc __attribute__((unused)),
char *argv[] __attribute__((unused)))
int rc = 0;
int srv, client;
ssize_t n;
struct sockaddr_in srv_addr;
srv = socket(AF_INET, SOCK_STREAM, 0);
if (srv < 0) {
std::cerr << "Failed to create socket: " << errno << std::endl;
goto out;
srv_addr.sin_family = AF_INET;
srv_addr.sin_addr.s_addr = INADDR_ANY;
srv_addr.sin_port = htons(LISTEN_PORT);
rc = bind(srv, (struct sockaddr *) &srv_addr, sizeof(srv_addr));
if (rc < 0) {
std::cerr << "Failed to bind socket: " << errno << std::endl;
goto out;
/* Accept one simultaneous connection */
rc = listen(srv, 1);
if (rc < 0) {
std::cerr << "Failed to listen on socket: " << errno << std::endl;
goto out;
std::cout << "Listening on port " << LISTEN_PORT << std::endl;
while (1) {
client = accept(srv, NULL, 0);
if (client < 0) {
std::cerr << "Failed to accept incoming connection: " << errno << std::endl;
goto out;
/* Receive some bytes (ignore errors) */
read(client, recvbuf, BUFLEN);
/* Send reply */
n = write(client, reply.c_str(), reply.length());
if (n < 0)
std::cerr << "Failed to send reply" << std::endl;
std::cout << "Sent a reply" << std::endl;
/* Close connection */
return rc;

Lines in the Kraftfile have the following roles:

  • spec: v0.6: The current Kraftfile specification version is 0.6.

  • runtime: base: The Unikraft runtime kernel to use is its base one.

  • rootfs: ./Dockerfile: Build the application root filesystem using the Dockerfile.

  • cmd: ["/http_server"]: Use /http_server as the starting command of the instance.

Lines in the Dockerfile have the following roles:

  • FROM --platform=linux/x86_64 gcc:13.2.0-bookworm AS build: Build the filesystem from the bookworm gcc container image, to create a base image.

  • COPY ./http_server.cpp /src/http_server.cpp: Copy the server implementation file (http_server.cpp) in the Docker filesystem (in /src/http_server.cpp).

The following options are available for customizing the application:

  • If only updating the implementation in the http_server.cpp source file, no other change is required.

  • If new files are added, these have to be copied in the application filesystem, using the COPY command in the Dockerfile.

  • If a new C++ source files is added, update the cmd line in the Kraftfile and replace http_server.cpp to run that file when creating the instance.

  • More extensive changes may require expanding the Dockerfile with additional Dockerfile commands.

Learn More

Use the --help option for detailed information on using KraftCloud:

Terminal window
kraft cloud --help

Or visit the CLI Reference.