Here is an example of C code for a basic HTTP3 client using the quiche library:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <quiche.h>

#define MAX_DATAGRAM_SIZE 1350
#define SERVER_NAME "example.com"

int main() {
    // Create socket and connect to server
    struct addrinfo hints;
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_protocol = IPPROTO_UDP;

    struct addrinfo *result;
    int error = getaddrinfo(SERVER_NAME, "443", &hints, &result);
    if (error != 0) {
        printf("Error: %s\n", gai_strerror(error));
        return 1;
    }

    int sock = socket(result->ai_family, result->ai_socktype, result->ai_protocol);
    if (sock == -1) {
        perror("socket");
        return 1;
    }

    if (connect(sock, result->ai_addr, result->ai_addrlen) == -1) {
        perror("connect");
        return 1;
    }

    freeaddrinfo(result);

    // Initialize quiche
    uint8_t scid[QUICHE_MAX_CONN_ID_LEN];
    size_t scid_len = QUICHE_MAX_CONN_ID_LEN;

    uint8_t odcid[QUICHE_MAX_CONN_ID_LEN];
    size_t odcid_len = 0;

    quiche_config *config = quiche_config_new(QUICHE_PROTOCOL_VERSION_DRAFT29);
    quiche_config_load_cert_chain_from_pem_file(config, "cert.pem");
    quiche_config_load_priv_key_from_pem_file(config, "key.pem");

    quiche_conn *conn = quiche_connect(SERVER_NAME, scid, scid_len, odcid, odcid_len, config);
    if (conn == NULL) {
        fprintf(stderr, "failed to create connection\n");
        return 1;
    }

    // Send HTTP request
    const char *request = "GET /index.html HTTP/3\r\nHost: example.com\r\n\r\n";
    ssize_t written = quiche_conn_stream_send(conn, 0, request, strlen(request), true);
    if (written != strlen(request)) {
        fprintf(stderr, "failed to send request\n");
        return 1;
    }

    // Receive response
    uint8_t buf[MAX_DATAGRAM_SIZE];
    memset(buf, 0, MAX_DATAGRAM_SIZE);

    ssize_t recvlen = recv(sock, buf, MAX_DATAGRAM_SIZE, 0);
    if (recvlen < 0) {
        perror("recv");
        return 1;
    }

    quiche_recv_info recv_info = { 0 };
    ssize_t processed = quiche_conn_recv(conn, buf, recvlen, &recv_info);
    if (processed < 0) {
        fprintf(stderr, "failed to process packet\n");
        return 1;
    }

    if (quiche_conn_is_established(conn)) {
        bool closed = false;

        while (!closed) {
            quiche_stream_iter *readable = quiche_conn_readable(conn);

            while (quiche_stream_iter_next(readable)) {
                int64_t stream_id = quiche_stream_iter_get(readable);
                uint8_t data[MAX_DATAGRAM_SIZE];
                size_t datalen = sizeof(data);

                ssize_t recvlen = quiche_conn_stream_recv(conn, stream_id, data, datalen, false);
                if (recvlen == QUICHE_ERR_DONE) {
                    continue;
                }

                fwrite(data, 1, recvlen, stdout);

                if (quiche_conn_stream_finished(conn, stream_id)) {
                    quiche_conn_stream_shutdown(conn, stream_id, true, 0);
                }
            }

            closed = quiche_conn_is_closed(conn);

            if (!closed) {
                ssize_t written = quiche_conn_send(conn, buf, MAX_DATAGRAM_SIZE);
                if (written < 0) {
                    fprintf(stderr, "failed to send packet\n");
                    return 1;
                }
            }
        }
    }

    quiche_conn_free(conn);
    quiche_config_free(config);
    close(sock);

    return 0;
}

This code creates a UDP socket and connects to the specified server using the QUIC protocol. It then initializes the quiche library with the appropriate certificate and private key files, and sends an HTTP request to the server.

The code then receives the response from the server and processes it using quiche’s stream API, which allows you to read and write data on individual streams within the QUIC connection. Finally, the code closes the connection and cleans up memory and resources used by the quiche library.