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2026-03-25 - fun hacks

A lot of my JustForFun™ hacking happens late at night, often when I have trouble falling asleep. In a recent episode, I wanted to see if I could coax the C preprocessor into something JSX-like. Quite surprisingly, the Mastodon post got a lot of attention, so I wanted to preserve it here.

An xkcd-like cartoon with two characters having the following dialog. “I have an idea!” — “Is it bad? — “Extremely.”

#The monstrosity

Anyway, here’s what I came up with:

#define _(...) __VA_ARGS__

#define html_(x) "<html>" #x "</html>"
#define html(x) html_(x)

#define tag(name, x) <name>x</name>
#define tag_attr(name, attrs, x) <name attrs>x</name>
#define tag_void(name, attrs) <name attrs>
#define attr(name, value) name = value

#define head(x) tag(head, x)
#define link(attrs) tag_void(link, attrs)

#define body(x) tag(body, x)
#define div(attrs, x) tag_attr(div, attrs, x)
#define h1(attrs, x) tag_attr(h1, attrs, x)
#define hr <hr />
#define p(attrs, x) tag_attr(p, attrs, x)

As you can see, I got bored at one point and didn’t feel like adding any more tags, as they would all have followed the same pattern. We can use it like this:

#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#include "abuse.h"

// clang-format off
#define WATER_CSS "https://cdn.jsdelivr.net/npm/water.css@2/out/water.css"

#define HTML                                                              \
  head(                                                                   \
    tag(title, C Preprocessor Abuse)                                      \
    link(_(rel="stylesheet" href=WATER_CSS))                              \
  )                                                                       \
  body(                                                                   \
    div(attr(class, "container"),                                         \
      h1(, C Preprocessor Abuse)                                          \
      hr                                                                  \
      p(, This page was generated entirely by the C preprocessor.)        \
      p(, HTML tags are defined as function-like macros that expand to    \
          angle-bracket syntax. The entire page is a single string        \
          literal assembled at compile time via token stringification.)   \
      tag(blockquote, tag(em, The compiler is the template engine.))      \
      hr                                                                  \
    )                                                                     \
  )
// clang-format on

int main(void) {
  char *response = "HTTP/1.1 200 OK\r\n"
                   "Content-Type: text/html\r\n"
                   "\r\n" html(HTML);

  int fd = socket(AF_INET, SOCK_STREAM, 0);
  int opt = 1;
  setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

  struct sockaddr_in addr = {
      .sin_family = AF_INET,
      .sin_port = htons(8080),
      .sin_addr.s_addr = htonl(INADDR_LOOPBACK),
  };
  bind(fd, (struct sockaddr *)&addr, sizeof(addr));
  listen(fd, 1);

  puts("Listening on http://localhost:8080");

  while (true) {
    int client = accept(fd, NULL, NULL);
    char buf[1024];
    read(client, buf, sizeof(buf));
    write(client, response, strlen(response));
    close(client);
  }
}

Here’s the accompanying Makefile:

CC ?= clang
CFLAGS = -std=c23 -Wall -Werror

run: clean abuse
	@./abuse

abuse:
	@$(CC) $(CFLAGS) -I. -o abuse abuse.c

clean:
	@rm -f ./abuse

.PHONY: clean run

The rendered page:

A rendered version of the preprocessor-generated HTML with WaterCSS styling

You can find all three files in this gist.

#Summary

This was fun to do and deepened my understanding of the C preprocessor, so it wasn’t entirely pointless. I’m not gonna use this for anything, and you probably shouldn’t either, but I’m not gonna tell you what to do.

C Preprocessor Fun

2026-03-25 - fun hacks

#The monstrosity

#Summary

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