text: // Copyright 2018 Espressif Systems (Shanghai) PTE LTD
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #ifndef _ESP_HTTP_SERVER_H_
  #define _ESP_HTTP_SERVER_H_
  
  #include <stdio.h>
  #include <string.h>
  #include <freertos/FreeRTOS.h>
  #include <freertos/task.h>
  #include <http_parser.h>
  #include <sdkconfig.h>
  #include <esp_err.h>
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  /*
  note: esp_https_server.h includes a customized copy of this
  initializer that should be kept in sync
  */
  #define HTTPD_DEFAULT_CONFIG() {                        \
          .task_priority      = tskIDLE_PRIORITY+5,       \
          .stack_size         = 40960,                     \
          .core_id            = tskNO_AFFINITY,           \
          .server_port        = 80,                       \
          .ctrl_port          = 32768,                    \
          .max_open_sockets   = 7,                        \
          .max_uri_handlers   = 16,                        \
          .max_resp_headers   = 8,                        \
          .backlog_conn       = 5,                        \
          .lru_purge_enable   = false,                    \
          .recv_wait_timeout  = 5,                        \
          .send_wait_timeout  = 5,                        \
          .global_user_ctx = NULL,                        \
          .global_user_ctx_free_fn = NULL,                \
          .global_transport_ctx = NULL,                   \
          .global_transport_ctx_free_fn = NULL,           \
          .open_fn = NULL,                                \
          .close_fn = NULL,                               \
          .uri_match_fn = NULL                            \
  }
  
  #define ESP_ERR_HTTPD_BASE              (0x8000)                    /*!< Starting number of HTTPD error codes */
  #define ESP_ERR_HTTPD_HANDLERS_FULL     (ESP_ERR_HTTPD_BASE +  1)   /*!< All slots for registering URI handlers have been consumed */
  #define ESP_ERR_HTTPD_HANDLER_EXISTS    (ESP_ERR_HTTPD_BASE +  2)   /*!< URI handler with same method and target URI already registered */
  #define ESP_ERR_HTTPD_INVALID_REQ       (ESP_ERR_HTTPD_BASE +  3)   /*!< Invalid request pointer */
  #define ESP_ERR_HTTPD_RESULT_TRUNC      (ESP_ERR_HTTPD_BASE +  4)   /*!< Result string truncated */
  #define ESP_ERR_HTTPD_RESP_HDR          (ESP_ERR_HTTPD_BASE +  5)   /*!< Response header field larger than supported */
  #define ESP_ERR_HTTPD_RESP_SEND         (ESP_ERR_HTTPD_BASE +  6)   /*!< Error occured while sending response packet */
  #define ESP_ERR_HTTPD_ALLOC_MEM         (ESP_ERR_HTTPD_BASE +  7)   /*!< Failed to dynamically allocate memory for resource */
  #define ESP_ERR_HTTPD_TASK              (ESP_ERR_HTTPD_BASE +  8)   /*!< Failed to launch server task/thread */
  
  /* Symbol to be used as length parameter in httpd_resp_send APIs
   * for setting buffer length to string length */
  #define HTTPD_RESP_USE_STRLEN -1
  
  /* ************** Group: Initialization ************** */
  /** @name Initialization
   * APIs related to the Initialization of the web server
   * @{
   */
  
  /**
   * @brief   HTTP Server Instance Handle
   *
   * Every instance of the server will have a unique handle.
   */
  typedef void* httpd_handle_t;
  
  /**
   * @brief   HTTP Method Type wrapper over "enum http_method"
   *          available in "http_parser" library
   */
  typedef enum http_method httpd_method_t;
  
  /**
   * @brief  Prototype for freeing context data (if any)
   * @param[in] ctx   object to free
   */
  typedef void (*httpd_free_ctx_fn_t)(void *ctx);
  
  /**
   * @brief  Function prototype for opening a session.
   *
   * Called immediately after the socket was opened to set up the send/recv functions and
   * other parameters of the socket.
   *
   * @param[in] hd       server instance
   * @param[in] sockfd   session socket file descriptor
   * @return
   *  - ESP_OK   : On success
   *  - Any value other than ESP_OK will signal the server to close the socket immediately
   */
  typedef esp_err_t (*httpd_open_func_t)(httpd_handle_t hd, int sockfd);
  
  /**
   * @brief  Function prototype for closing a session.
   *
   * @note   It's possible that the socket descriptor is invalid at this point, the function
   *         is called for all terminated sessions. Ensure proper handling of return codes.
   *
   * @param[in] hd   server instance
   * @param[in] sockfd   session socket file descriptor
   */
  typedef void (*httpd_close_func_t)(httpd_handle_t hd, int sockfd);
  
  /**
   * @brief  Function prototype for URI matching.
   *
   * @param[in] reference_uri   URI/template with respect to which the other URI is matched
   * @param[in] uri_to_match    URI/template being matched to the reference URI/template
   * @param[in] match_upto      For specifying the actual length of `uri_to_match` up to
   *                            which the matching algorithm is to be applied (The maximum
   *                            value is `strlen(uri_to_match)`, independent of the length
   *                            of `reference_uri`)
   * @return true on match
   */
  typedef bool (*httpd_uri_match_func_t)(const char *reference_uri,
                                         const char *uri_to_match,
                                         size_t match_upto);
  
  /**
   * @brief   HTTP Server Configuration Structure
   *
   * @note    Use HTTPD_DEFAULT_CONFIG() to initialize the configuration
   *          to a default value and then modify only those fields that are
   *          specifically determined by the use case.
   */
  typedef struct httpd_config {
      unsigned    task_priority;      /*!< Priority of FreeRTOS task which runs the server */
      size_t      stack_size;         /*!< The maximum stack size allowed for the server task */
      BaseType_t  core_id;            /*!< The core the HTTP server task will run on */
  
      /**
       * TCP Port number for receiving and transmitting HTTP traffic
       */
      uint16_t    server_port;
  
      /**
       * UDP Port number for asynchronously exchanging control signals
       * between various components of the server
       */
      uint16_t    ctrl_port;
  
      uint16_t    max_open_sockets;   /*!< Max number of sockets/clients connected at any time*/
      uint16_t    max_uri_handlers;   /*!< Maximum allowed uri handlers */
      uint16_t    max_resp_headers;   /*!< Maximum allowed additional headers in HTTP response */
      uint16_t    backlog_conn;       /*!< Number of backlog connections */
      bool        lru_purge_enable;   /*!< Purge "Least Recently Used" connection */
      uint16_t    recv_wait_timeout;  /*!< Timeout for recv function (in seconds)*/
      uint16_t    send_wait_timeout;  /*!< Timeout for send function (in seconds)*/
  
      /**
       * Global user context.
       *
       * This field can be used to store arbitrary user data within the server context.
       * The value can be retrieved using the server handle, available e.g. in the httpd_req_t struct.
       *
       * When shutting down, the server frees up the user context by
       * calling free() on the global_user_ctx field. If you wish to use a custom
       * function for freeing the global user context, please specify that here.
       */
      void * global_user_ctx;
  
      /**
       * Free function for global user context
       */
      httpd_free_ctx_fn_t global_user_ctx_free_fn;
  
      /**
       * Global transport context.
       *
       * Similar to global_user_ctx, but used for session encoding or encryption (e.g. to hold the SSL context).
       * It will be freed using free(), unless global_transport_ctx_free_fn is specified.
       */
      void * global_transport_ctx;
  
      /**
       * Free function for global transport context
       */
      httpd_free_ctx_fn_t global_transport_ctx_free_fn;
  
      /**
       * Custom session opening callback.
       *
       * Called on a new session socket just after accept(), but before reading any data.
       *
       * This is an opportunity to set up e.g. SSL encryption using global_transport_ctx
       * and the send/recv/pending session overrides.
       *
       * If a context needs to be maintained between these functions, store it in the session using
       * httpd_sess_set_transport_ctx() and retrieve it later with httpd_sess_get_transport_ctx()
       *
       * Returning a value other than ESP_OK will immediately close the new socket.
       */
      httpd_open_func_t open_fn;
  
      /**
       * Custom session closing callback.
       *
       * Called when a session is deleted, before freeing user and transport contexts and before
       * closing the socket. This is a place for custom de-init code common to all sockets.
       *
       * Set the user or transport context to NULL if it was freed here, so the server does not
       * try to free it again.
       *
       * This function is run for all terminated sessions, including sessions where the socket
       * was closed by the network stack - that is, the file descriptor may not be valid anymore.
       */
      httpd_close_func_t close_fn;
  
      /**
       * URI matcher function.
       *
       * Called when searching for a matching URI:
       *     1) whose request handler is to be executed right
       *        after an HTTP request is successfully parsed
       *     2) in order to prevent duplication while registering
       *        a new URI handler using `httpd_register_uri_handler()`
       *
       * Available options are:
       *     1) NULL : Internally do basic matching using `strncmp()`
       *     2) `httpd_uri_match_wildcard()` : URI wildcard matcher
       *
       * Users can implement their own matching functions (See description
       * of the `httpd_uri_match_func_t` function prototype)
       */
      httpd_uri_match_func_t uri_match_fn;
  } httpd_config_t;
  
  /**
   * @brief Starts the web server
   *
   * Create an instance of HTTP server and allocate memory/resources for it
   * depending upon the specified configuration.
   *
   * Example usage:
   * @code{c}
   *
   * //Function for starting the webserver
   * httpd_handle_t start_webserver(void)
   * {
   *      // Generate default configuration
   *      httpd_config_t config = HTTPD_DEFAULT_CONFIG();
   *
   *      // Empty handle to http_server
   *      httpd_handle_t server = NULL;
   *
   *      // Start the httpd server
   *      if (httpd_start(&server, &config) == ESP_OK) {
   *          // Register URI handlers
   *          httpd_register_uri_handler(server, &uri_get);
   *          httpd_register_uri_handler(server, &uri_post);
   *      }
   *      // If server failed to start, handle will be NULL
   *      return server;
   * }
   *
   * @endcode
   *
   * @param[in]  config   Configuration for new instance of the server
   * @param[out] handle   Handle to newly created instance of the server. NULL on error
   * @return
   *  - ESP_OK    : Instance created successfully
   *  - ESP_ERR_INVALID_ARG      : Null argument(s)
   *  - ESP_ERR_HTTPD_ALLOC_MEM  : Failed to allocate memory for instance
   *  - ESP_ERR_HTTPD_TASK       : Failed to launch server task
   */
  esp_err_t  httpd_start(httpd_handle_t *handle, const httpd_config_t *config);
  
  /**
   * @brief Stops the web server
   *
   * Deallocates memory/resources used by an HTTP server instance and
   * deletes it. Once deleted the handle can no longer be used for accessing
   * the instance.
   *
   * Example usage:
   * @code{c}
   *
   * // Function for stopping the webserver
   * void stop_webserver(httpd_handle_t server)
   * {
   *      // Ensure handle is non NULL
   *      if (server != NULL) {
   *          // Stop the httpd server
   *          httpd_stop(server);
   *      }
   * }
   *
   * @endcode
   *
   * @param[in] handle Handle to server returned by httpd_start
   * @return
   *  - ESP_OK : Server stopped successfully
   *  - ESP_ERR_INVALID_ARG : Handle argument is Null
   */
  esp_err_t httpd_stop(httpd_handle_t handle);
  
  /** End of Group Initialization
   * @}
   */
  
  /* ************** Group: URI Handlers ************** */
  /** @name URI Handlers
   * APIs related to the URI handlers
   * @{
   */
  
  /* Max supported HTTP request header length */
  #define HTTPD_MAX_REQ_HDR_LEN CONFIG_HTTPD_MAX_REQ_HDR_LEN
  
  /* Max supported HTTP request URI length */
  #define HTTPD_MAX_URI_LEN CONFIG_HTTPD_MAX_URI_LEN
  
  /**
   * @brief HTTP Request Data Structure
   */
  typedef struct httpd_req {
      httpd_handle_t  handle;                     /*!< Handle to server instance */
      int             method;                     /*!< The type of HTTP request, -1 if unsupported method */
      const char      uri[HTTPD_MAX_URI_LEN + 1]; /*!< The URI of this request (1 byte extra for null termination) */
      size_t          content_len;                /*!< Length of the request body */
      void           *aux;                        /*!< Internally used members */
  
      /**
       * User context pointer passed during URI registration.
       */
      void *user_ctx;
  
      /**
       * Session Context Pointer
       *
       * A session context. Contexts are maintained across 'sessions' for a
       * given open TCP connection. One session could have multiple request
       * responses. The web server will ensure that the context persists
       * across all these request and responses.
       *
       * By default, this is NULL. URI Handlers can set this to any meaningful
       * value.
       *
       * If the underlying socket gets closed, and this pointer is non-NULL,
       * the web server will free up the context by calling free(), unless
       * free_ctx function is set.
       */
      void *sess_ctx;
  
      /**
       * Pointer to free context hook
       *
       * Function to free session context
       *
       * If the web server's socket closes, it frees up the session context by
       * calling free() on the sess_ctx member. If you wish to use a custom
       * function for freeing the session context, please specify that here.
       */
      httpd_free_ctx_fn_t free_ctx;
  
      /**
       * Flag indicating if Session Context changes should be ignored
       *
       * By default, if you change the sess_ctx in some URI handler, the http server
       * will internally free the earlier context (if non NULL), after the URI handler
       * returns. If you want to manage the allocation/reallocation/freeing of
       * sess_ctx yourself, set this flag to true, so that the server will not
       * perform any checks on it. The context will be cleared by the server
       * (by calling free_ctx or free()) only if the socket gets closed.
       */
      bool ignore_sess_ctx_changes;
  } httpd_req_t;
  
  /**
   * @brief Structure for URI handler
   */
  typedef struct httpd_uri {
      const char       *uri;    /*!< The URI to handle */
      httpd_method_t    method; /*!< Method supported by the URI */
  
      /**
       * Handler to call for supported request method. This must
       * return ESP_OK, or else the underlying socket will be closed.
       */
      esp_err_t (*handler)(httpd_req_t *r);
  
      /**
       * Pointer to user context data which will be available to handler
       */
      void *user_ctx;
  } httpd_uri_t;
  
  /**
   * @brief   Registers a URI handler
   *
   * @note    URI handlers can be registered in real time as long as the
   *          server handle is valid.
   *
   * Example usage:
   * @code{c}
   *
   * esp_err_t my_uri_handler(httpd_req_t* req)
   * {
   *     // Recv , Process and Send
   *     ....
   *     ....
   *     ....
   *
   *     // Fail condition
   *     if (....) {
   *         // Return fail to close session //
   *         return ESP_FAIL;
   *     }
   *
   *     // On success
   *     return ESP_OK;
   * }
   *
   * // URI handler structure
   * httpd_uri_t my_uri {
   *     .uri      = "/my_uri/path/xyz",
   *     .method   = HTTPD_GET,
   *     .handler  = my_uri_handler,
   *     .user_ctx = NULL
   * };
   *
   * // Register handler
   * if (httpd_register_uri_handler(server_handle, &my_uri) != ESP_OK) {
   *    // If failed to register handler
   *    ....
   * }
   *
   * @endcode
   *
   * @param[in] handle      handle to HTTPD server instance
   * @param[in] uri_handler pointer to handler that needs to be registered
   *
   * @return
   *  - ESP_OK : On successfully registering the handler
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_HANDLERS_FULL  : If no slots left for new handler
   *  - ESP_ERR_HTTPD_HANDLER_EXISTS : If handler with same URI and
   *                                   method is already registered
   */
  esp_err_t httpd_register_uri_handler(httpd_handle_t handle,
                                       const httpd_uri_t *uri_handler);
  
  /**
   * @brief   Unregister a URI handler
   *
   * @param[in] handle    handle to HTTPD server instance
   * @param[in] uri       URI string
   * @param[in] method    HTTP method
   *
   * @return
   *  - ESP_OK : On successfully deregistering the handler
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_NOT_FOUND   : Handler with specified URI and method not found
   */
  esp_err_t httpd_unregister_uri_handler(httpd_handle_t handle,
                                         const char *uri, httpd_method_t method);
  
  /**
   * @brief   Unregister all URI handlers with the specified uri string
   *
   * @param[in] handle   handle to HTTPD server instance
   * @param[in] uri      uri string specifying all handlers that need
   *                     to be deregisterd
   *
   * @return
   *  - ESP_OK : On successfully deregistering all such handlers
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_NOT_FOUND   : No handler registered with specified uri string
   */
  esp_err_t httpd_unregister_uri(httpd_handle_t handle, const char* uri);
  
  /** End of URI Handlers
   * @}
   */
  
  /* ************** Group: HTTP Error ************** */
  /** @name HTTP Error
   * Prototype for HTTP errors and error handling functions
   * @{
   */
  
  /**
   * @brief Error codes sent as HTTP response in case of errors
   *        encountered during processing of an HTTP request
   */
  typedef enum {
      /* For any unexpected errors during parsing, like unexpected
       * state transitions, or unhandled errors.
       */
      HTTPD_500_INTERNAL_SERVER_ERROR = 0,
  
      /* For methods not supported by http_parser. Presently
       * http_parser halts parsing when such methods are
       * encountered and so the server responds with 400 Bad
       * Request error instead.
       */
      HTTPD_501_METHOD_NOT_IMPLEMENTED,
  
      /* When HTTP version is not 1.1 */
      HTTPD_505_VERSION_NOT_SUPPORTED,
  
      /* Returned when http_parser halts parsing due to incorrect
       * syntax of request, unsupported method in request URI or
       * due to chunked encoding / upgrade field present in headers
       */
      HTTPD_400_BAD_REQUEST,
  
      /* When requested URI is not found */
      HTTPD_404_NOT_FOUND,
  
      /* When URI found, but method has no handler registered */
      HTTPD_405_METHOD_NOT_ALLOWED,
  
      /* Intended for recv timeout. Presently it's being sent
       * for other recv errors as well. Client should expect the
       * server to immediately close the connection after
       * responding with this.
       */
      HTTPD_408_REQ_TIMEOUT,
  
      /* Intended for responding to chunked encoding, which is
       * not supported currently. Though unhandled http_parser
       * callback for chunked request returns "400 Bad Request"
       */
      HTTPD_411_LENGTH_REQUIRED,
  
      /* URI length greater than CONFIG_HTTPD_MAX_URI_LEN */
      HTTPD_414_URI_TOO_LONG,
  
      /* Headers section larger than CONFIG_HTTPD_MAX_REQ_HDR_LEN */
      HTTPD_431_REQ_HDR_FIELDS_TOO_LARGE,
  
      /* Used internally for retrieving the total count of errors */
      HTTPD_ERR_CODE_MAX
  } httpd_err_code_t;
  
  /**
   * @brief  Function prototype for HTTP error handling.
   *
   * This function is executed upon HTTP errors generated during
   * internal processing of an HTTP request. This is used to override
   * the default behavior on error, which is to send HTTP error response
   * and close the underlying socket.
   *
   * @note
   *  - If implemented, the server will not automatically send out HTTP
   *    error response codes, therefore, httpd_resp_send_err() must be
   *    invoked inside this function if user wishes to generate HTTP
   *    error responses.
   *  - When invoked, the validity of `uri`, `method`, `content_len`
   *    and `user_ctx` fields of the httpd_req_t parameter is not
   *    guaranteed as the HTTP request may be partially received/parsed.
   *  - The function must return ESP_OK if underlying socket needs to
   *    be kept open. Any other value will ensure that the socket is
   *    closed. The return value is ignored when error is of type
   *    `HTTPD_500_INTERNAL_SERVER_ERROR` and the socket closed anyway.
   *
   * @param[in] req    HTTP request for which the error needs to be handled
   * @param[in] error  Error type
   *
   * @return
   *  - ESP_OK   : error handled successful
   *  - ESP_FAIL : failure indicates that the underlying socket needs to be closed
   */
  typedef esp_err_t (*httpd_err_handler_func_t)(httpd_req_t *req,
                                                httpd_err_code_t error);
  
  /**
   * @brief  Function for registering HTTP error handlers
   *
   * This function maps a handler function to any supported error code
   * given by `httpd_err_code_t`. See prototype `httpd_err_handler_func_t`
   * above for details.
   *
   * @param[in] handle     HTTP server handle
   * @param[in] error      Error type
   * @param[in] handler_fn User implemented handler function
   *                       (Pass NULL to unset any previously set handler)
   *
   * @return
   *  - ESP_OK : handler registered successfully
   *  - ESP_ERR_INVALID_ARG : invalid error code or server handle
   */
  esp_err_t httpd_register_err_handler(httpd_handle_t handle,
                                       httpd_err_code_t error,
                                       httpd_err_handler_func_t handler_fn);
  
  /** End of HTTP Error
   * @}
   */
  
  /* ************** Group: TX/RX ************** */
  /** @name TX / RX
   * Prototype for HTTPDs low-level send/recv functions
   * @{
   */
  
  #define HTTPD_SOCK_ERR_FAIL      -1
  #define HTTPD_SOCK_ERR_INVALID   -2
  #define HTTPD_SOCK_ERR_TIMEOUT   -3
  
  /**
   * @brief  Prototype for HTTPDs low-level send function
   *
   * @note   User specified send function must handle errors internally,
   *         depending upon the set value of errno, and return specific
   *         HTTPD_SOCK_ERR_ codes, which will eventually be conveyed as
   *         return value of httpd_send() function
   *
   * @param[in] hd        server instance
   * @param[in] sockfd    session socket file descriptor
   * @param[in] buf       buffer with bytes to send
   * @param[in] buf_len   data size
   * @param[in] flags     flags for the send() function
   * @return
   *  - Bytes : The number of bytes sent successfully
   *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
   *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket send()
   *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket send()
   */
  typedef int (*httpd_send_func_t)(httpd_handle_t hd, int sockfd, const char *buf, size_t buf_len, int flags);
  
  /**
   * @brief  Prototype for HTTPDs low-level recv function
   *
   * @note   User specified recv function must handle errors internally,
   *         depending upon the set value of errno, and return specific
   *         HTTPD_SOCK_ERR_ codes, which will eventually be conveyed as
   *         return value of httpd_req_recv() function
   *
   * @param[in] hd        server instance
   * @param[in] sockfd    session socket file descriptor
   * @param[in] buf       buffer with bytes to send
   * @param[in] buf_len   data size
   * @param[in] flags     flags for the send() function
   * @return
   *  - Bytes : The number of bytes received successfully
   *  - 0     : Buffer length parameter is zero / connection closed by peer
   *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
   *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket recv()
   *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket recv()
   */
  typedef int (*httpd_recv_func_t)(httpd_handle_t hd, int sockfd, char *buf, size_t buf_len, int flags);
  
  /**
   * @brief  Prototype for HTTPDs low-level "get pending bytes" function
   *
   * @note   User specified pending function must handle errors internally,
   *         depending upon the set value of errno, and return specific
   *         HTTPD_SOCK_ERR_ codes, which will be handled accordingly in
   *         the server task.
   *
   * @param[in] hd       server instance
   * @param[in] sockfd   session socket file descriptor
   * @return
   *  - Bytes : The number of bytes waiting to be received
   *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
   *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket pending()
   *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket pending()
   */
  typedef int (*httpd_pending_func_t)(httpd_handle_t hd, int sockfd);
  
  /** End of TX / RX
   * @}
   */
  
  /* ************** Group: Request/Response ************** */
  /** @name Request / Response
   * APIs related to the data send/receive by URI handlers.
   * These APIs are supposed to be called only from the context of
   * a URI handler where httpd_req_t* request pointer is valid.
   * @{
   */
  
  /**
   * @brief   Override web server's receive function (by session FD)
   *
   * This function overrides the web server's receive function. This same function is
   * used to read HTTP request packets.
   *
   * @note    This API is supposed to be called either from the context of
   *          - an http session APIs where sockfd is a valid parameter
   *          - a URI handler where sockfd is obtained using httpd_req_to_sockfd()
   *
   * @param[in] hd        HTTPD instance handle
   * @param[in] sockfd    Session socket FD
   * @param[in] recv_func The receive function to be set for this session
   *
   * @return
   *  - ESP_OK : On successfully registering override
   *  - ESP_ERR_INVALID_ARG : Null arguments
   */
  esp_err_t httpd_sess_set_recv_override(httpd_handle_t hd, int sockfd, httpd_recv_func_t recv_func);
  
  /**
   * @brief   Override web server's send function (by session FD)
   *
   * This function overrides the web server's send function. This same function is
   * used to send out any response to any HTTP request.
   *
   * @note    This API is supposed to be called either from the context of
   *          - an http session APIs where sockfd is a valid parameter
   *          - a URI handler where sockfd is obtained using httpd_req_to_sockfd()
   *
   * @param[in] hd        HTTPD instance handle
   * @param[in] sockfd    Session socket FD
   * @param[in] send_func The send function to be set for this session
   *
   * @return
   *  - ESP_OK : On successfully registering override
   *  - ESP_ERR_INVALID_ARG : Null arguments
   */
  esp_err_t httpd_sess_set_send_override(httpd_handle_t hd, int sockfd, httpd_send_func_t send_func);
  
  /**
   * @brief   Override web server's pending function (by session FD)
   *
   * This function overrides the web server's pending function. This function is
   * used to test for pending bytes in a socket.
   *
   * @note    This API is supposed to be called either from the context of
   *          - an http session APIs where sockfd is a valid parameter
   *          - a URI handler where sockfd is obtained using httpd_req_to_sockfd()
   *
   * @param[in] hd           HTTPD instance handle
   * @param[in] sockfd       Session socket FD
   * @param[in] pending_func The receive function to be set for this session
   *
   * @return
   *  - ESP_OK : On successfully registering override
   *  - ESP_ERR_INVALID_ARG : Null arguments
   */
  esp_err_t httpd_sess_set_pending_override(httpd_handle_t hd, int sockfd, httpd_pending_func_t pending_func);
  
  /**
   * @brief   Get the Socket Descriptor from the HTTP request
   *
   * This API will return the socket descriptor of the session for
   * which URI handler was executed on reception of HTTP request.
   * This is useful when user wants to call functions that require
   * session socket fd, from within a URI handler, ie. :
   *      httpd_sess_get_ctx(),
   *      httpd_sess_trigger_close(),
   *      httpd_sess_update_lru_counter().
   *
   * @note    This API is supposed to be called only from the context of
   *          a URI handler where httpd_req_t* request pointer is valid.
   *
   * @param[in] r The request whose socket descriptor should be found
   *
   * @return
   *  - Socket descriptor : The socket descriptor for this request
   *  - -1 : Invalid/NULL request pointer
   */
  int httpd_req_to_sockfd(httpd_req_t *r);
  
  /**
   * @brief   API to read content data from the HTTP request
   *
   * This API will read HTTP content data from the HTTP request into
   * provided buffer. Use content_len provided in httpd_req_t structure
   * to know the length of data to be fetched. If content_len is too
   * large for the buffer then user may have to make multiple calls to
   * this function, each time fetching 'buf_len' number of bytes,
   * while the pointer to content data is incremented internally by
   * the same number.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - If an error is returned, the URI handler must further return an error.
   *    This will ensure that the erroneous socket is closed and cleaned up by
   *    the web server.
   *  - Presently Chunked Encoding is not supported
   *
   * @param[in] r         The request being responded to
   * @param[in] buf       Pointer to a buffer that the data will be read into
   * @param[in] buf_len   Length of the buffer
   *
   * @return
   *  - Bytes : Number of bytes read into the buffer successfully
   *  - 0     : Buffer length parameter is zero / connection closed by peer
   *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
   *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket recv()
   *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket recv()
   */
  int httpd_req_recv(httpd_req_t *r, char *buf, size_t buf_len);
  
  /**
   * @brief   Search for a field in request headers and
   *          return the string length of it's value
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once httpd_resp_send() API is called all request headers
   *    are purged, so request headers need be copied into separate
   *    buffers if they are required later.
   *
   * @param[in]  r        The request being responded to
   * @param[in]  field    The header field to be searched in the request
   *
   * @return
   *  - Length    : If field is found in the request URL
   *  - Zero      : Field not found / Invalid request / Null arguments
   */
  size_t httpd_req_get_hdr_value_len(httpd_req_t *r, const char *field);
  
  /**
   * @brief   Get the value string of a field from the request headers
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once httpd_resp_send() API is called all request headers
   *    are purged, so request headers need be copied into separate
   *    buffers if they are required later.
   *  - If output size is greater than input, then the value is truncated,
   *    accompanied by truncation error as return value.
   *  - Use httpd_req_get_hdr_value_len() to know the right buffer length
   *
   * @param[in]  r        The request being responded to
   * @param[in]  field    The field to be searched in the header
   * @param[out] val      Pointer to the buffer into which the value will be copied if the field is found
   * @param[in]  val_size Size of the user buffer "val"
   *
   * @return
   *  - ESP_OK : Field found in the request header and value string copied
   *  - ESP_ERR_NOT_FOUND          : Key not found
   *  - ESP_ERR_INVALID_ARG        : Null arguments
   *  - ESP_ERR_HTTPD_INVALID_REQ  : Invalid HTTP request pointer
   *  - ESP_ERR_HTTPD_RESULT_TRUNC : Value string truncated
   */
  esp_err_t httpd_req_get_hdr_value_str(httpd_req_t *r, const char *field, char *val, size_t val_size);
  
  /**
   * @brief   Get Query string length from the request URL
   *
   * @note    This API is supposed to be called only from the context of
   *          a URI handler where httpd_req_t* request pointer is valid
   *
   * @param[in]  r    The request being responded to
   *
   * @return
   *  - Length    : Query is found in the request URL
   *  - Zero      : Query not found / Null arguments / Invalid request
   */
  size_t httpd_req_get_url_query_len(httpd_req_t *r);
  
  /**
   * @brief   Get Query string from the request URL
   *
   * @note
   *  - Presently, the user can fetch the full URL query string, but decoding
   *    will have to be performed by the user. Request headers can be read using
   *    httpd_req_get_hdr_value_str() to know the 'Content-Type' (eg. Content-Type:
   *    application/x-www-form-urlencoded) and then the appropriate decoding
   *    algorithm needs to be applied.
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid
   *  - If output size is greater than input, then the value is truncated,
   *    accompanied by truncation error as return value
   *  - Prior to calling this function, one can use httpd_req_get_url_query_len()
   *    to know the query string length beforehand and hence allocate the buffer
   *    of right size (usually query string length + 1 for null termination)
   *    for storing the query string
   *
   * @param[in]  r         The request being responded to
   * @param[out] buf       Pointer to the buffer into which the query string will be copied (if found)
   * @param[in]  buf_len   Length of output buffer
   *
   * @return
   *  - ESP_OK : Query is found in the request URL and copied to buffer
   *  - ESP_ERR_NOT_FOUND          : Query not found
   *  - ESP_ERR_INVALID_ARG        : Null arguments
   *  - ESP_ERR_HTTPD_INVALID_REQ  : Invalid HTTP request pointer
   *  - ESP_ERR_HTTPD_RESULT_TRUNC : Query string truncated
   */
  esp_err_t httpd_req_get_url_query_str(httpd_req_t *r, char *buf, size_t buf_len);
  
  /**
   * @brief   Helper function to get a URL query tag from a query
   *          string of the type param1=val1&param2=val2
   *
   * @note
   *  - The components of URL query string (keys and values) are not URLdecoded.
   *    The user must check for 'Content-Type' field in the request headers and
   *    then depending upon the specified encoding (URLencoded or otherwise) apply
   *    the appropriate decoding algorithm.
   *  - If actual value size is greater than val_size, then the value is truncated,
   *    accompanied by truncation error as return value.
   *
   * @param[in]  qry       Pointer to query string
   * @param[in]  key       The key to be searched in the query string
   * @param[out] val       Pointer to the buffer into which the value will be copied if the key is found
   * @param[in]  val_size  Size of the user buffer "val"
   *
   * @return
   *  - ESP_OK : Key is found in the URL query string and copied to buffer
   *  - ESP_ERR_NOT_FOUND          : Key not found
   *  - ESP_ERR_INVALID_ARG        : Null arguments
   *  - ESP_ERR_HTTPD_RESULT_TRUNC : Value string truncated
   */
  esp_err_t httpd_query_key_value(const char *qry, const char *key, char *val, size_t val_size);
  
  /**
   * @brief Test if a URI matches the given wildcard template.
   *
   * Template may end with "?" to make the previous character optional (typically a slash),
   * "*" for a wildcard match, and "?*" to make the previous character optional, and if present,
   * allow anything to follow.
   *
   * Example:
   *   - * matches everything
   *   - /foo/? matches /foo and /foo/
   *   - /foo/\* (sans the backslash) matches /foo/ and /foo/bar, but not /foo or /fo
   *   - /foo/?* or /foo/\*?  (sans the backslash) matches /foo/, /foo/bar, and also /foo, but not /foox or /fo
   *
   * The special characters "?" and "*" anywhere else in the template will be taken literally.
   *
   * @param[in] uri_template   URI template (pattern)
   * @param[in] uri_to_match   URI to be matched
   * @param[in] match_upto     how many characters of the URI buffer to test
   *                          (there may be trailing query string etc.)
   *
   * @return true if a match was found
   */
  bool httpd_uri_match_wildcard(const char *uri_template, const char *uri_to_match, size_t match_upto);
  
  /**
   * @brief   API to send a complete HTTP response.
   *
   * This API will send the data as an HTTP response to the request.
   * This assumes that you have the entire response ready in a single
   * buffer. If you wish to send response in incremental chunks use
   * httpd_resp_send_chunk() instead.
   *
   * If no status code and content-type were set, by default this
   * will send 200 OK status code and content type as text/html.
   * You may call the following functions before this API to configure
   * the response headers :
   *      httpd_resp_set_status() - for setting the HTTP status string,
   *      httpd_resp_set_type()   - for setting the Content Type,
   *      httpd_resp_set_hdr()    - for appending any additional field
   *                                value entries in the response header
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once this API is called, the request has been responded to.
   *  - No additional data can then be sent for the request.
   *  - Once this API is called, all request headers are purged, so
   *    request headers need be copied into separate buffers if
   *    they are required later.
   *
   * @param[in] r         The request being responded to
   * @param[in] buf       Buffer from where the content is to be fetched
   * @param[in] buf_len   Length of the buffer, HTTPD_RESP_USE_STRLEN to use strlen()
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null request pointer
   *  - ESP_ERR_HTTPD_RESP_HDR    : Essential headers are too large for internal buffer
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request
   */
  esp_err_t httpd_resp_send(httpd_req_t *r, const char *buf, ssize_t buf_len);
  
  /**
   * @brief   API to send one HTTP chunk
   *
   * This API will send the data as an HTTP response to the
   * request. This API will use chunked-encoding and send the response
   * in the form of chunks. If you have the entire response contained in
   * a single buffer, please use httpd_resp_send() instead.
   *
   * If no status code and content-type were set, by default this will
   * send 200 OK status code and content type as text/html. You may
   * call the following functions before this API to configure the
   * response headers
   *      httpd_resp_set_status() - for setting the HTTP status string,
   *      httpd_resp_set_type()   - for setting the Content Type,
   *      httpd_resp_set_hdr()    - for appending any additional field
   *                                value entries in the response header
   *
   * @note
   * - This API is supposed to be called only from the context of
   *   a URI handler where httpd_req_t* request pointer is valid.
   * - When you are finished sending all your chunks, you must call
   *   this function with buf_len as 0.
   * - Once this API is called, all request headers are purged, so
   *   request headers need be copied into separate buffers if they
   *   are required later.
   *
   * @param[in] r         The request being responded to
   * @param[in] buf       Pointer to a buffer that stores the data
   * @param[in] buf_len   Length of the buffer, HTTPD_RESP_USE_STRLEN to use strlen()
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet chunk
   *  - ESP_ERR_INVALID_ARG : Null request pointer
   *  - ESP_ERR_HTTPD_RESP_HDR    : Essential headers are too large for internal buffer
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  esp_err_t httpd_resp_send_chunk(httpd_req_t *r, const char *buf, ssize_t buf_len);
  
  /**
   * @brief   API to send a complete string as HTTP response.
   *
   * This API simply calls http_resp_send with buffer length
   * set to string length assuming the buffer contains a null
   * terminated string
   *
   * @param[in] r         The request being responded to
   * @param[in] str       String to be sent as response body
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null request pointer
   *  - ESP_ERR_HTTPD_RESP_HDR    : Essential headers are too large for internal buffer
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request
   */
  static inline esp_err_t httpd_resp_sendstr(httpd_req_t *r, const char *str) {
      return httpd_resp_send(r, str, (str == NULL) ? 0 : strlen(str));
  }
  
  /**
   * @brief   API to send a string as an HTTP response chunk.
   *
   * This API simply calls http_resp_send_chunk with buffer length
   * set to string length assuming the buffer contains a null
   * terminated string
   *
   * @param[in] r    The request being responded to
   * @param[in] str  String to be sent as response body (NULL to finish response packet)
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null request pointer
   *  - ESP_ERR_HTTPD_RESP_HDR    : Essential headers are too large for internal buffer
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request
   */
  static inline esp_err_t httpd_resp_sendstr_chunk(httpd_req_t *r, const char *str) {
      return httpd_resp_send_chunk(r, str, (str == NULL) ? 0 : strlen(str));
  }
  
  /* Some commonly used status codes */
  #define HTTPD_200      "200 OK"                     /*!< HTTP Response 200 */
  #define HTTPD_204      "204 No Content"             /*!< HTTP Response 204 */
  #define HTTPD_207      "207 Multi-Status"           /*!< HTTP Response 207 */
  #define HTTPD_400      "400 Bad Request"            /*!< HTTP Response 400 */
  #define HTTPD_404      "404 Not Found"              /*!< HTTP Response 404 */
  #define HTTPD_408      "408 Request Timeout"        /*!< HTTP Response 408 */
  #define HTTPD_500      "500 Internal Server Error"  /*!< HTTP Response 500 */
  
  /**
   * @brief   API to set the HTTP status code
   *
   * This API sets the status of the HTTP response to the value specified.
   * By default, the '200 OK' response is sent as the response.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - This API only sets the status to this value. The status isn't
   *    sent out until any of the send APIs is executed.
   *  - Make sure that the lifetime of the status string is valid till
   *    send function is called.
   *
   * @param[in] r         The request being responded to
   * @param[in] status    The HTTP status code of this response
   *
   * @return
   *  - ESP_OK : On success
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  esp_err_t httpd_resp_set_status(httpd_req_t *r, const char *status);
  
  /* Some commonly used content types */
  #define HTTPD_TYPE_JSON   "application/json"            /*!< HTTP Content type JSON */
  #define HTTPD_TYPE_TEXT   "text/html"                   /*!< HTTP Content type text/HTML */
  #define HTTPD_TYPE_OCTET  "application/octet-stream"    /*!< HTTP Content type octext-stream */
  
  /**
   * @brief   API to set the HTTP content type
   *
   * This API sets the 'Content Type' field of the response.
   * The default content type is 'text/html'.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - This API only sets the content type to this value. The type
   *    isn't sent out until any of the send APIs is executed.
   *  - Make sure that the lifetime of the type string is valid till
   *    send function is called.
   *
   * @param[in] r     The request being responded to
   * @param[in] type  The Content Type of the response
   *
   * @return
   *  - ESP_OK   : On success
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  esp_err_t httpd_resp_set_type(httpd_req_t *r, const char *type);
  
  /**
   * @brief   API to append any additional headers
   *
   * This API sets any additional header fields that need to be sent in the response.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - The header isn't sent out until any of the send APIs is executed.
   *  - The maximum allowed number of additional headers is limited to
   *    value of max_resp_headers in config structure.
   *  - Make sure that the lifetime of the field value strings are valid till
   *    send function is called.
   *
   * @param[in] r     The request being responded to
   * @param[in] field The field name of the HTTP header
   * @param[in] value The value of this HTTP header
   *
   * @return
   *  - ESP_OK : On successfully appending new header
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_RESP_HDR    : Total additional headers exceed max allowed
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  esp_err_t httpd_resp_set_hdr(httpd_req_t *r, const char *field, const char *value);
  
  /**
   * @brief   For sending out error code in response to HTTP request.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once this API is called, all request headers are purged, so
   *    request headers need be copied into separate buffers if
   *    they are required later.
   *  - If you wish to send additional data in the body of the
   *    response, please use the lower-level functions directly.
   *
   * @param[in] req     Pointer to the HTTP request for which the response needs to be sent
   * @param[in] error   Error type to send
   * @param[in] msg     Error message string (pass NULL for default message)
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  esp_err_t httpd_resp_send_err(httpd_req_t *req, httpd_err_code_t error, const char *msg);
  
  /**
   * @brief   Helper function for HTTP 404
   *
   * Send HTTP 404 message. If you wish to send additional data in the body of the
   * response, please use the lower-level functions directly.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once this API is called, all request headers are purged, so
   *    request headers need be copied into separate buffers if
   *    they are required later.
   *
   * @param[in] r The request being responded to
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  static inline esp_err_t httpd_resp_send_404(httpd_req_t *r) {
      return httpd_resp_send_err(r, HTTPD_404_NOT_FOUND, NULL);
  }
  
  /**
   * @brief   Helper function for HTTP 408
   *
   * Send HTTP 408 message. If you wish to send additional data in the body of the
   * response, please use the lower-level functions directly.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once this API is called, all request headers are purged, so
   *    request headers need be copied into separate buffers if
   *    they are required later.
   *
   * @param[in] r The request being responded to
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  static inline esp_err_t httpd_resp_send_408(httpd_req_t *r) {
      return httpd_resp_send_err(r, HTTPD_408_REQ_TIMEOUT, NULL);
  }
  
  /**
   * @brief   Helper function for HTTP 500
   *
   * Send HTTP 500 message. If you wish to send additional data in the body of the
   * response, please use the lower-level functions directly.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Once this API is called, all request headers are purged, so
   *    request headers need be copied into separate buffers if
   *    they are required later.
   *
   * @param[in] r The request being responded to
   *
   * @return
   *  - ESP_OK : On successfully sending the response packet
   *  - ESP_ERR_INVALID_ARG : Null arguments
   *  - ESP_ERR_HTTPD_RESP_SEND   : Error in raw send
   *  - ESP_ERR_HTTPD_INVALID_REQ : Invalid request pointer
   */
  static inline esp_err_t httpd_resp_send_500(httpd_req_t *r) {
      return httpd_resp_send_err(r, HTTPD_500_INTERNAL_SERVER_ERROR, NULL);
  }
  
  /**
   * @brief   Raw HTTP send
   *
   * Call this API if you wish to construct your custom response packet.
   * When using this, all essential header, eg. HTTP version, Status Code,
   * Content Type and Length, Encoding, etc. will have to be constructed
   * manually, and HTTP delimeters (CRLF) will need to be placed correctly
   * for separating sub-sections of the HTTP response packet.
   *
   * If the send override function is set, this API will end up
   * calling that function eventually to send data out.
   *
   * @note
   *  - This API is supposed to be called only from the context of
   *    a URI handler where httpd_req_t* request pointer is valid.
   *  - Unless the response has the correct HTTP structure (which the
   *    user must now ensure) it is not guaranteed that it will be
   *    recognized by the client. For most cases, you wouldn't have
   *    to call this API, but you would rather use either of :
   *          httpd_resp_send(),
   *          httpd_resp_send_chunk()
   *
   * @param[in] r         The request being responded to
   * @param[in] buf       Buffer from where the fully constructed packet is to be read
   * @param[in] buf_len   Length of the buffer
   *
   * @return
   *  - Bytes : Number of bytes that were sent successfully
   *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
   *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket send()
   *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket send()
   */
  int httpd_send(httpd_req_t *r, const char *buf, size_t buf_len);
  
  /** End of Request / Response
   * @}
   */
  
  /* ************** Group: Session ************** */
  /** @name Session
   * Functions for controlling sessions and accessing context data
   * @{
   */
  
  /**
   * @brief   Get session context from socket descriptor
   *
   * Typically if a session context is created, it is available to URI handlers
   * through the httpd_req_t structure. But, there are cases where the web
   * server's send/receive functions may require the context (for example, for
   * accessing keying information etc). Since the send/receive function only have
   * the socket descriptor at their disposal, this API provides them with a way to
   * retrieve the session context.
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] sockfd    The socket descriptor for which the context should be extracted.
   *
   * @return
   *  - void* : Pointer to the context associated with this session
   *  - NULL  : Empty context / Invalid handle / Invalid socket fd
   */
  void *httpd_sess_get_ctx(httpd_handle_t handle, int sockfd);
  
  /**
   * @brief   Set session context by socket descriptor
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] sockfd    The socket descriptor for which the context should be extracted.
   * @param[in] ctx       Context object to assign to the session
   * @param[in] free_fn   Function that should be called to free the context
   */
  void httpd_sess_set_ctx(httpd_handle_t handle, int sockfd, void *ctx, httpd_free_ctx_fn_t free_fn);
  
  /**
   * @brief   Get session 'transport' context by socket descriptor
   * @see     httpd_sess_get_ctx()
   *
   * This context is used by the send/receive functions, for example to manage SSL context.
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] sockfd    The socket descriptor for which the context should be extracted.
   * @return
   *  - void* : Pointer to the transport context associated with this session
   *  - NULL  : Empty context / Invalid handle / Invalid socket fd
   */
  void *httpd_sess_get_transport_ctx(httpd_handle_t handle, int sockfd);
  
  /**
   * @brief   Set session 'transport' context by socket descriptor
   * @see     httpd_sess_set_ctx()
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] sockfd    The socket descriptor for which the context should be extracted.
   * @param[in] ctx       Transport context object to assign to the session
   * @param[in] free_fn   Function that should be called to free the transport context
   */
  void httpd_sess_set_transport_ctx(httpd_handle_t handle, int sockfd, void *ctx, httpd_free_ctx_fn_t free_fn);
  
  /**
   * @brief   Get HTTPD global user context (it was set in the server config struct)
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @return global user context
   */
  void *httpd_get_global_user_ctx(httpd_handle_t handle);
  
  /**
   * @brief   Get HTTPD global transport context (it was set in the server config struct)
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @return global transport context
   */
  void *httpd_get_global_transport_ctx(httpd_handle_t handle);
  
  /**
   * @brief   Trigger an httpd session close externally
   *
   * @note    Calling this API is only required in special circumstances wherein
   *          some application requires to close an httpd client session asynchronously.
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] sockfd    The socket descriptor of the session to be closed
   *
   * @return
   *  - ESP_OK    : On successfully initiating closure
   *  - ESP_FAIL  : Failure to queue work
   *  - ESP_ERR_NOT_FOUND   : Socket fd not found
   *  - ESP_ERR_INVALID_ARG : Null arguments
   */
  esp_err_t httpd_sess_trigger_close(httpd_handle_t handle, int sockfd);
  
  /**
   * @brief   Update LRU counter for a given socket
   *
   * LRU Counters are internally associated with each session to monitor
   * how recently a session exchanged traffic. When LRU purge is enabled,
   * if a client is requesting for connection but maximum number of
   * sockets/sessions is reached, then the session having the earliest
   * LRU counter is closed automatically.
   *
   * Updating the LRU counter manually prevents the socket from being purged
   * due to the Least Recently Used (LRU) logic, even though it might not
   * have received traffic for some time. This is useful when all open
   * sockets/session are frequently exchanging traffic but the user specifically
   * wants one of the sessions to be kept open, irrespective of when it last
   * exchanged a packet.
   *
   * @note    Calling this API is only necessary if the LRU Purge Enable option
   *          is enabled.
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] sockfd    The socket descriptor of the session for which LRU counter
   *                      is to be updated
   *
   * @return
   *  - ESP_OK : Socket found and LRU counter updated
   *  - ESP_ERR_NOT_FOUND   : Socket not found
   *  - ESP_ERR_INVALID_ARG : Null arguments
   */
  esp_err_t httpd_sess_update_lru_counter(httpd_handle_t handle, int sockfd);
  
  /** End of Session
   * @}
   */
  
  /* ************** Group: Work Queue ************** */
  /** @name Work Queue
   * APIs related to the HTTPD Work Queue
   * @{
   */
  
  /**
   * @brief   Prototype of the HTTPD work function
   *          Please refer to httpd_queue_work() for more details.
   * @param[in] arg   The arguments for this work function
   */
  typedef void (*httpd_work_fn_t)(void *arg);
  
  /**
   * @brief   Queue execution of a function in HTTPD's context
   *
   * This API queues a work function for asynchronous execution
   *
   * @note    Some protocols require that the web server generate some asynchronous data
   *          and send it to the persistently opened connection. This facility is for use
   *          by such protocols.
   *
   * @param[in] handle    Handle to server returned by httpd_start
   * @param[in] work      Pointer to the function to be executed in the HTTPD's context
   * @param[in] arg       Pointer to the arguments that should be passed to this function
   *
   * @return
   *  - ESP_OK   : On successfully queueing the work
   *  - ESP_FAIL : Failure in ctrl socket
   *  - ESP_ERR_INVALID_ARG : Null arguments
   */
  esp_err_t httpd_queue_work(httpd_handle_t handle, httpd_work_fn_t work, void *arg);
  
  /** End of Group Work Queue
   * @}
   */
  
  #ifdef __cplusplus
  }
  #endif
  
  #endif /* ! _ESP_HTTP_SERVER_H_ */
  