feat(examples): Add C ZeroMQ server example

examples/zmq-server-c/Dockerfile

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+FROM debian:bookworm AS build
+
+RUN set -ex; \
+    apt-get update; \
+    apt-get install -y build-essential libzmq3-dev; \
+    ldconfig
+
+WORKDIR /src
+
+COPY main.c .
+
+RUN gcc main.c -o zmq-server -Wall -Os -lzmq
+
+FROM scratch
+
+COPY --from=build /src/zmq-server /usr/bin/zmq-server
+COPY --from=build /lib/x86_64-linux-gnu/libzmq.so.5 /lib/x86_64-linux-gnu/libzmq.so.5
+COPY --from=build /lib/x86_64-linux-gnu/libc.so.6 /lib/x86_64-linux-gnu/libc.so.6
+COPY --from=build /lib/x86_64-linux-gnu/libbsd.so.0 /lib/x86_64-linux-gnu/libbsd.so.0
+COPY --from=build /lib/x86_64-linux-gnu/libsodium.so.23 /lib/x86_64-linux-gnu/libsodium.so.23
+COPY --from=build /lib/x86_64-linux-gnu/libpgm-5.3.so.0 /lib/x86_64-linux-gnu/libpgm-5.3.so.0
+COPY --from=build /lib/x86_64-linux-gnu/libnorm.so.1 /lib/x86_64-linux-gnu/libnorm.so.1
+COPY --from=build /lib/x86_64-linux-gnu/libgssapi_krb5.so.2 /lib/x86_64-linux-gnu/libgssapi_krb5.so.2
+COPY --from=build /lib/x86_64-linux-gnu/libstdc++.so.6 /lib/x86_64-linux-gnu/libstdc++.so.6
+COPY --from=build /lib/x86_64-linux-gnu/libgcc_s.so.1 /lib/x86_64-linux-gnu/libgcc_s.so.1
+COPY --from=build /lib64/ld-linux-x86-64.so.2 /lib64/ld-linux-x86-64.so.2
+COPY --from=build /lib/x86_64-linux-gnu/libmd.so.0 /lib/x86_64-linux-gnu/libmd.so.0
+COPY --from=build /lib/x86_64-linux-gnu/libpthread.so.0 /lib/x86_64-linux-gnu/libpthread.so.0
+COPY --from=build /lib/x86_64-linux-gnu/libm.so.6 /lib/x86_64-linux-gnu/libm.so.6
+COPY --from=build /lib/x86_64-linux-gnu/libkrb5.so.3 /lib/x86_64-linux-gnu/libkrb5.so.3
+COPY --from=build /lib/x86_64-linux-gnu/libk5crypto.so.3 /lib/x86_64-linux-gnu/libk5crypto.so.3
+COPY --from=build /lib/x86_64-linux-gnu/libcom_err.so.2 /lib/x86_64-linux-gnu/libcom_err.so.2
+COPY --from=build /lib/x86_64-linux-gnu/libkrb5support.so.0 /lib/x86_64-linux-gnu/libkrb5support.so.0
+COPY --from=build /lib/x86_64-linux-gnu/libkeyutils.so.1 /lib/x86_64-linux-gnu/libkeyutils.so.1
+COPY --from=build /lib/x86_64-linux-gnu/libresolv.so.2 /lib/x86_64-linux-gnu/libresolv.so.2

examples/zmq-server-c/Kraftfile

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+spec: v0.6
+
+runtime: base:latest
+
+rootfs: ./Dockerfile
+
+cmd: ["/usr/bin/zmq-server"]

examples/zmq-server-c/README.md

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+# C ZeroMQ Request/Reply Server
+
+This directory contains a C ZeroMQ Request/Reply server running on Unikraft.
+
+## Set Up
+
+To run this example, [install Unikraft's companion command-line toolchain `kraft`](https://unikraft.org/docs/cli), clone this repository and `cd` into this directory.
+
+## Run and Use
+
+Use `kraft` to run the image and start a Unikraft instance:
+
+```bash
+kraft run --rm -p 5000:5000 --plat qemu --arch x86_64 .
+```
+
+If the `--plat` argument is left out, it defaults to `qemu`.
+If the `--arch` argument is left out, it defaults to your system's CPU architecture.
+
+Once executed, it will open port `5000` and wait for connections. To test it you
+can install the `pyzmq` Pythoon package and use the following Python code to
+connect:
+
+```python
+import zmq
+
+ctx = zmq.Context()
+sock = ctx.socket(zmq.REQ)
+sock.connect("tcp://localhost:5000")
+sock.send(b'Hello, World!")
+print(sock.recv())
+```
+
+You should see a "Hello, World!" message in the ZeroMQ console and an "ACK"
+message in your Python script output.
+
+## Inspect and Close
+
+To list information about the Unikraft instance, use:
+
+```bash
+kraft ps
+```
+
+```text
+NAME              KERNEL                          ARGS                 CREATED        STATUS   MEM  PORTS                   PLAT
+infallible_massa  oci://unikraft.org/base:latest  /usr/bin/zmq-server  1 seconds ago  running  64M  0.0.0.0:5000->5000/tcp  qemu/x86_64
+```
+
+The instance name is `infallible_massa`.
+To close the Unikraft instance, close the `kraft` process (e.g., via `Ctrl+c`) or run:
+
+```bash
+kraft rm infallible_massa
+```
+
+Note that depending on how you modify this example your instance **may** need more memory to run.
+To do so, use the `kraft run`'s `-M` flag, for example:
+
+```bash
+kraft run --rm -p 5000:5000 --plat qemu --arch x86_64 -M 256M .
+```
+
+## `kraft` and `sudo`
+
+Mixing invocations of `kraft` and `sudo` can lead to unexpected behavior.
+Read more about how to start `kraft` without `sudo` at [https://unikraft.org/sudoless](https://unikraft.org/sudoless).
+
+## Learn More
+
+- [How to run unikernels locally](https://unikraft.org/docs/cli/running)
+- [Building `Dockerfile` Images with `BuildKit`](https://unikraft.org/guides/building-dockerfile-images-with-buildkit)

examples/zmq-server-c/main.c

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+#include <stdio.h>
+#include <string.h>
+#include <zmq.h>
+
+#define BUF_SIZE 256
+
+int main()
+{
+	void *ctx, *sock;
+	char buff[BUF_SIZE];
+
+	ctx = zmq_ctx_new();
+	sock = zmq_socket(ctx, ZMQ_REP);
+
+	if (zmq_bind(sock, "tcp://*:5000")) {
+		fputs("could not bind to port", stderr);
+		return -1;
+	}
+
+	while (1) {
+		memset(buff, 0, BUF_SIZE);
+		zmq_recv(sock, buff, BUF_SIZE - 1, 0);
+		printf("Received message: %s\n", buff);
+		zmq_send(sock, "ACK", 3, 0);
+	}
+
+	return 0;
+}