General • Need advice on what to read to understand httpd server.

Code:

/** * Copyright (c) 2022 Raspberry Pi (Trading) Ltd. * * SPDX-License-Identifier: BSD-3-Clause */#ifndef __unused#define __unused __attribute__((unused))#endif#include "pico/cyw43_arch.h"#include "pico/stdlib.h"#include "lwip/ip4_addr.h"#include "lwip/init.h"#include "lwip/apps/httpd.h"//#include "motor_control.h"// breaker added for testingvoid decode_country_code(uint32_t code);// LwIP: Initialize the httpd: set up a listening PCB and bind it to the defined portvoid httpd_init(void);static const char *cgi_handler_test(int iIndex, int iNumParams, char *pcParam[], char *pcValue[]);// Note that the buffer size is limited by LWIP_HTTPD_MAX_TAG_INSERT_LEN, so use LWIP_HTTPD_SSI_MULTIPART to return larger amounts of datau16_t ssi_example_ssi_handler(int iIndex, char *pcInsert, int iInsertLen#if LWIP_HTTPD_SSI_MULTIPART    , uint16_t current_tag_part, uint16_t *next_tag_part#endif);err_t httpd_post_begin(void *connection, const char *uri, const char *http_request,        u16_t http_request_len, int content_len, char *response_uri,        u16_t response_uri_len, u8_t *post_auto_wnd);// Return a value for a parameterchar *httpd_param_value(struct pbuf *p, const char *param_name, char *value_buf, size_t value_buf_len);err_t httpd_post_receive_data(void *connection, struct pbuf *p);void httpd_post_finished(void *connection, char *response_uri, u16_t response_uri_len);static absolute_time_t wifi_connected_time;static bool led_on = false;static tCGI cgi_handlers[] = {    { "/", cgi_handler_test },    { "/index.shtml", cgi_handler_test },};// Be aware of LWIP_HTTPD_MAX_TAG_NAME_LENstatic const char *ssi_tags[] = {    "status",   // case 0    "welcome",  // case 1    "uptime",   // case 2    "ledstate", // case 3    "ledinv",   // case 4    "table",    // case 5};int main(void) {    stdio_init_all();    if (cyw43_arch_init_with_country(CYW43_COUNTRY_UK)) {        printf("failed to initialise\n");        return 1;    }    cyw43_arch_enable_sta_mode();    printf("\nConnecting to WiFi...\n");    if (cyw43_arch_wifi_connect_timeout_ms(WIFI_SSID, WIFI_PASSWORD, CYW43_AUTH_WPA2_AES_PSK, 30000)) {        printf("failed to connect.\n");        exit(1);    } else {        printf("Connected\n");        // Decode and print the country code        decode_country_code(cyw43_arch_get_country_code());    }    printf("Ready, running httpd at %s\n", ip4addr_ntoa(netif_ip4_addr(netif_list)));    // start http server    wifi_connected_time = get_absolute_time();    // setup http server    cyw43_arch_lwip_begin();    httpd_init();    http_set_cgi_handlers(cgi_handlers, LWIP_ARRAYSIZE(cgi_handlers));    http_set_ssi_handler(ssi_example_ssi_handler, ssi_tags, LWIP_ARRAYSIZE(ssi_tags));    cyw43_arch_lwip_end();    while(true) {#if PICO_CYW43_ARCH_POLL        cyw43_arch_poll();        cyw43_arch_wait_for_work_until(led_time);#else        sleep_ms(1000);#endif    }    cyw43_arch_deinit();} // end main()// Function to decode the country codevoid decode_country_code(uint32_t code) {    char a = (char)(code & 0xFF);        // Extract the first character    char b = (char)((code >> 8) & 0xFF); // Extract the second character    int rev = (code >> 16) & 0xFF;       // Extract the revision number    printf("country code: %c%c, Revision: %d\n", a, b, rev); // Print the decoded characters and revision}static const char *cgi_handler_test(int iIndex, int iNumParams, char *pcParam[], char *pcValue[]) {    if (iNumParams > 0) {        if (strcmp(pcParam[0], "test") == 0) {            return "/test.shtml";        }    }    return "/index.shtml";}// Note that the buffer size is limited by LWIP_HTTPD_MAX_TAG_INSERT_LEN, so use LWIP_HTTPD_SSI_MULTIPART to return larger amounts of datau16_t ssi_example_ssi_handler(int iIndex, char *pcInsert, int iInsertLen#if LWIP_HTTPD_SSI_MULTIPART    , uint16_t current_tag_part, uint16_t *next_tag_part#endif) {    size_t printed;    switch (iIndex) {        case 0: { // "status"            printed = snprintf(pcInsert, iInsertLen, "Pass");            break;        }        case 1: { // "welcome"            printed = snprintf(pcInsert, iInsertLen, "Hello from Pico");            break;        }        case 2: { // "uptime"            uint64_t uptime_s = absolute_time_diff_us(wifi_connected_time, get_absolute_time()) / 1e6;            printed = snprintf(pcInsert, iInsertLen, "%"PRIu64, uptime_s);            break;        }        case 3: { // "ledstate"            printed = snprintf(pcInsert, iInsertLen, "%s", led_on ? "ON" : "OFF");            break;        }        case 4: { // "ledinv"            printed = snprintf(pcInsert, iInsertLen, "%s", led_on ? "OFF" : "ON");            break;        }#if LWIP_HTTPD_SSI_MULTIPART        case 5: { /* "table" */            printed = snprintf(pcInsert, iInsertLen, "<tr><td>This is table row number %d</td></tr>", current_tag_part + 1);            // Leave "next_tag_part" unchanged to indicate that all data has been returned for this tag            if (current_tag_part < 9) {                *next_tag_part = current_tag_part + 1;            }            break;        }#endif        default: { // unknown tag            printed = 0;            break;        }    }  return (u16_t)printed;}#if LWIP_HTTPD_SUPPORT_POST#define LED_STATE_BUFSIZE 4static void *current_connection;err_t httpd_post_begin(void *connection, const char *uri, const char *http_request,        u16_t http_request_len, int content_len, char *response_uri,        u16_t response_uri_len, u8_t *post_auto_wnd) {    if (memcmp(uri, "/led.cgi", 8) == 0 && current_connection != connection) {        current_connection = connection;        snprintf(response_uri, response_uri_len, "/ledfail.shtml");        *post_auto_wnd = 1;        return ERR_OK;    }    return ERR_VAL;}// Return a value for a parameterchar *httpd_param_value(struct pbuf *p, const char *param_name, char *value_buf, size_t value_buf_len) {    size_t param_len = strlen(param_name);    u16_t param_pos = pbuf_memfind(p, param_name, param_len, 0);    if (param_pos != 0xFFFF) {        u16_t param_value_pos = param_pos + param_len;        u16_t param_value_len = 0;        u16_t tmp = pbuf_memfind(p, "&", 1, param_value_pos);        if (tmp != 0xFFFF) {            param_value_len = tmp - param_value_pos;        } else {            param_value_len = p->tot_len - param_value_pos;        }        if (param_value_len > 0 && param_value_len < value_buf_len) {            char *result = (char *)pbuf_get_contiguous(p, value_buf, value_buf_len, param_value_len, param_value_pos);            if (result) {                result[param_value_len] = 0;                return result;            }        }    }    return NULL;}err_t httpd_post_receive_data(void *connection, struct pbuf *p) {    err_t ret = ERR_VAL;    LWIP_ASSERT("NULL pbuf", p != NULL);    if (current_connection == connection) {        char buf[LED_STATE_BUFSIZE];        char *val = httpd_param_value(p, "led_state=", buf, sizeof(buf));        if (val) {            led_on = (strcmp(val, "ON") == 0) ? true : false;            cyw43_gpio_set(&cyw43_state, 0, led_on);            ret = ERR_OK;        }    }    pbuf_free(p);    return ret;}void httpd_post_finished(void *connection, char *response_uri, u16_t response_uri_len) {    snprintf(response_uri, response_uri_len, "/ledfail.shtml");    if (current_connection == connection) {        snprintf(response_uri, response_uri_len, "/ledpass.shtml");    }    current_connection = NULL;}#endif