Telegram bot in C using json-c & libcurl

main.c

#include <assert.h>
#include <curl/curl.h>
#include <json.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static const char *API_URL = "https://api.telegram.org";

struct response {
	char *body;
	size_t bodylen;
};

void tg_url(const char *meth, char *out_url) {
	char *token = getenv("BOT_TOKEN");
	assert(token && strlen(token) > 0);
	sprintf(out_url, "%s/bot%s/%s", API_URL, token, meth);
}

size_t
curl_on_write(const void *data, size_t size, size_t nmemb, void *userdata) {
	char **body = userdata;
	size_t total = size * nmemb;
	if (*body == NULL) {
		*body = malloc(total + 1);
		memcpy(*body, data, total);
		(*body)[total] = '\0';
	} else {
		size_t bodylen = strlen(*body);
		*body = realloc(*body, bodylen + total + 1);
		memcpy(*body + bodylen, data, total);
		(*body)[bodylen + total] = '\0';
	}
	return total;
}

json_object *wait_for_updates(long timeout, long offset) {
	CURL *curl = curl_easy_init();
	assert(curl);
	char url[1024] = {0};
	char *token = getenv("BOT_TOKEN");
	sprintf(
		url, "%s/bot%s/getUpdates?offset=%ld&timeout=%ld", API_URL, token,
		offset, timeout);
	char *raw_response = NULL;
	curl_easy_setopt(curl, CURLOPT_URL, url);
	curl_easy_setopt(curl, CURLOPT_WRITEDATA, &raw_response);
	curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_on_write);
	curl_easy_perform(curl);
	json_object *res_json = json_tokener_parse(raw_response);
	free(raw_response);
	curl_easy_cleanup(curl);
	return res_json;
}

long update_offset(const json_object *updates) {
	size_t nupdates = json_object_array_length(updates);
	json_object *last = json_object_array_get_idx(updates, nupdates - 1);
	json_object *uid_json = json_object_object_get(last, "update_id");
	long uid = json_object_get_int64(uid_json) + 1;
	return uid;
}

void tg_html_escape(const char *s, char *out_s) {
	for (size_t i = 0; s[i]; i++) {
		switch (s[i]) {
		case '<':
			strcpy(out_s, "&lt;");
			out_s += 4;
			break;
		case '>':
			strcpy(out_s, "&gt;");
			out_s += 4;
			break;
		case '&':
			strcpy(out_s, "&amp;");
			out_s += 5;
			break;
		default:
			out_s[0] = s[i];
			out_s++;
			break;
		}
	}
}

void send_raw_update(
	const long chat_id, const long message_id, json_object *update) {
	CURL *curl = curl_easy_init();
	char url[1024] = {0};
	char *token = getenv("BOT_TOKEN");
	sprintf(url, "%s/bot%s/sendMessage", API_URL, token);
	json_object *req_json = json_object_new_object();
	json_object_object_add(req_json, "chat_id", json_object_new_int64(chat_id));
	json_object_object_add(
		req_json, "parse_mode", json_object_new_string("HTML"));
	json_object_object_add(
		req_json, "reply_to_message_id", json_object_new_int64(message_id));
	char text[4096] = {0};
	char escaped_text[4096] = {0};
	const char *update_str =
		json_object_to_json_string_ext(update, JSON_C_TO_STRING_PRETTY);
	tg_html_escape(update_str, escaped_text);
	sprintf(text, "<code>%s</code>", escaped_text);
	json_object_object_add(req_json, "text", json_object_new_string(text));
	fprintf(
		stderr, "%s\n",
		json_object_to_json_string_ext(req_json, JSON_C_TO_STRING_PRETTY));
	struct curl_slist *req_headers = NULL;
	req_headers =
		curl_slist_append(req_headers, "Content-Type: application/json");
	curl_easy_setopt(curl, CURLOPT_URL, url);
	curl_easy_setopt(curl, CURLOPT_HTTPHEADER, req_headers);
	curl_easy_setopt(curl, CURLOPT_HTTPPOST, 1);
	curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
	curl_easy_setopt(
		curl, CURLOPT_POSTFIELDS, json_object_to_json_string(req_json));
	curl_easy_perform(curl);
	curl_slist_free_all(req_headers);
	json_object_put(req_json);
	curl_easy_cleanup(curl);
}

void process_update(json_object *update) {
	json_object *msg_json = json_object_object_get(update, "message");
	if (json_object_get_type(msg_json) != json_type_object)
		return;
	long msg_id =
		json_object_get_int64(json_object_object_get(msg_json, "message_id"));
	json_object *chat_json = json_object_object_get(msg_json, "chat");
	if (json_object_get_type(chat_json) != json_type_object)
		return;
	long chat_id =
		json_object_get_int64(json_object_object_get(chat_json, "id"));
	send_raw_update(chat_id, msg_id, update);
}

int main(int argc, char *argv[]) {
	char *token = getenv("BOT_TOKEN");
	if (!token || strlen(token) == 0) {
		fprintf(stderr, "A BOT_TOKEN environment variable is required\n");
		return 1;
	}
	long offset = 0;
	int i = 0;
	for (;;) {
		i++;
		json_object *updates_json = wait_for_updates(30, offset);
		json_object *updates = json_object_object_get(updates_json, "result");
		size_t nupdates = json_object_array_length(updates);
		json_object *update = NULL;
		for (size_t i = 0; i < nupdates; i++) {
			update = json_object_array_get_idx(updates, i);
			process_update(update);
		}
		if (update) {
			offset = 1 + json_object_get_int64(
							 json_object_object_get(update, "update_id"));
		}
		json_object_put(updates_json);
	}
	return 0;
}