主目录为D:\NMT16\Web\content，background-image: url("../../img/course.jpg"); background-image: url("./images/newCourse.png"); background-image: url("file:///D:/NMT16/css/images/exp.png"); 分析以上代码的意义， 比较三行代码中路径的差异， 并写出"course. jpg" 、 "newCourse.png"、"exp.png"所在文件夹位置。

以上代码中，三行代码中的路径分别为相对路径、相对路径和绝对路径。

第一行代码中的路径 "../../img/course.jpg" 表示该图片文件位于当前CSS文件的上两级目录下的 img 文件夹中。

第二行代码中的路径 "./images/newCourse.png" 表示该图片文件位于当前CSS文件所在的同级目录下的 images 文件夹中。

第三行代码中的路径 "file:///D:/NMT16/css/images/exp.png" 表示该图片文件位于绝对路径 D:\NMT16\css\images\exp.png 中。

因此，"course.jpg" 位于 D:\NMT16\Web\img 文件夹中，"newCourse.png" 位于 D:\NMT16\Web\content\images 文件夹中，"exp.png" 位于 D:\NMT16\css\images 文件夹中。

相关问题

叙述通过"rtsp:\\192.168.0.7\mov\nmt.asp "访问资源"nmt.asp"的错误原因。

"rtsp:\\192.168.0.7\mov\nmt.asp"是一个RTSP协议的地址，而"nmt.asp"是一个ASP页面的资源，两者并不匹配，因此无法通过该地址访问到"nmt.asp"资源。正确的方式应该是使用HTTP协议访问ASP资源，例如"http://192.168.0.7/mov/nmt.asp"。

CANopennode的CANopen.h文件

以下是CANopennode中CANopen.h文件的代码，仅供参考：

/**
 * @file CANopen.h
 * @author CANopenNode, LLC
 * @version 4.0.0
 * @date 25 Feb 2021
 * @brief Header file for CANopenNode stack.
 *
 * This file contains all definitions and declarations for the CANopenNode
 * stack that are relevant for the application. The data types and function
 * prototypes defined here must be used in the user application code.
 *
 * @copyright Copyright (c) CANopenNode, LLC
 * @defgroup CO_CANopen CANopen
 * @{
 */

#ifndef CO_CANOPEN_H
#define CO_CANOPEN_H

#ifdef __cplusplus
extern "C" {
#endif

/* Includes ------------------------------------------------------------------*/
#include <stdint.h>
#include <stdbool.h>
#include <stddef.h>

/* Exported defines ----------------------------------------------------------*/

/**
 * Macros for standard data types.
 *
 * Macros for data types defined in stdint.h and stdbool.h. These macros should
 * be used in the application code instead of direct use of the types.
 *
 * @ingroup CO_CANopen_301
 * @{
 */
#ifndef NULL
#define NULL ((void*) 0)
#endif

#define int8_t    int8_t
#define uint8_t   uint8_t
#define int16_t   int16_t
#define uint16_t  uint16_t
#define int32_t   int32_t
#define uint32_t  uint32_t
#define bool_t    bool
#define true      true
#define false     false
/** @} */

/* Exported types ------------------------------------------------------------*/

/** CAN message structure as in CAN hardware. */
typedef struct {
    uint16_t ident; /**< 11-bit identifier, bits 10..0 are used. */
    uint8_t DLC;    /**< Data length code: 0..8. */
    uint8_t data[8];/**< Data field. Bytes 0..7 are used. */
} CO_CANrxMsg_t;

/** CAN transmit message structure. */
typedef struct {
    uint16_t ident; /**< 11-bit identifier, bits 10..0 are used. */
    bool_t rtr;     /**< RTR (Remote transmission request). */
    bool_t ext;     /**< EXT (Extended identifier, 29-bit). */
    uint8_t DLC;    /**< Data length code: 0..8. */
    uint8_t data[8];/**< Data field. Bytes 0..7 are used. */
} CO_CANtx_t;

/**
 * CAN message reception callback function.
 *
 * Function is called, when new message is received and passed to the CANopenNode.
 *
 * @param[in] msg Received message with necessary informations.
 */
typedef void (*CO_CANrx_callback_t)(const CO_CANrxMsg_t *msg);

/**
 * CANopen receive message structure.
 *
 * Object is storage for received CAN message and additional informations.
 */
typedef struct {
    const uint16_t ident; /**< Standard CAN Identifier or Extended CAN Identifier. */
    const uint16_t mask;  /**< Mask, to determine which bits of the identifier are significant. */
    CO_CANrx_callback_t pCANrx_callback; /**< Pointer to function, which will be called, when CAN message with specified identifier will be received. */
} CO_CANrx_t;

/** CANopen object with static configuration. */
typedef const struct {
    const uint16_t            index;   /**< Index of object in Object Dictionary. */
    const uint8_t             subIndex;/**< Subindex of object in Object Dictionary. */
    const uint8_t             attribute;/**< Attribute of Object Dictionary entry. */
    const uint32_t            length;  /**< Data length in bytes. */
    void* const               pData;   /**< Pointer to data. */
    const CO_CANrx_callback_t pFunct;  /**< Pointer to function, which will be called on RPDO reception. */
} CO_ObjDict_t;

/* Exported variables --------------------------------------------------------*/

/* Exported functions ------------------------------------------------------- */

/**
 * Initialize CANopenNode stack.
 *
 * Function must be called in the communication reset section.
 *
 * Function configures:
 *  - data storage for CANopen objects
 *  - LSS slave
 *  - LSS master
 *  - SDO server
 *  - SDO client
 *  - Emergency object
 *  - Heartbeat consumer
 *  - NMT object
 *  - Time object
 *  - SYNC objects
 *  - RPDO objects and corresponding CAN reception filters
 *  - TPDO objects and corresponding CAN transmission functions
 *  - SDO server objects and corresponding CAN reception filters and CAN transmission functions
 *
 * Function must be called before any other CANopenNode function.
 *
 * @param[out] ppData Pointer to pointer to data memory for CANopen objects.
 *                   If *ppData==NULL, memory for CANopen objects will be
 *                   allocated. If *ppData!=NULL, memory must be
 *                   statically allocated by application and available
 *                   during whole lifetime of the CANopenNode.
 *                   Pointer ppData is pointing to the first free byte
 *                   after the allocated memory.
 * @param[in] nodeId CANopen Node ID.
 * @param[in] bitRate CAN bit-rate.
 * @param[in] CANrx_callback Pointer to function, which will be called, when CAN
 *                            message with appropriate identifier is received.
 *                            It can be NULL.
 *
 * @return 0: Operation completed successfully.
 * @return -1: Error in function parameters.
 * @return -2: Error: CO_LSSslave_init() failed.
 * @return -3: Error: CO_SDOserver_init() failed.
 * @return -4: Error: CO_EM_init() failed.
 * @return -5: Error: CO_NMT_init() failed.
 * @return -6: Error: CO_HBconsumer_init() failed.
 * @return -7: Error: CO_TIME_init() failed.
 * @return -8: Error: CO_SYNC_init() failed.
 * @return -9: Error: CO_RPDO_init() failed.
 * @return -10: Error: CO_TPDO_init() failed.
 */
int16_t CO_init(
        uint8_t** const ppData,
        const uint8_t nodeId,
        const uint16_t bitRate,
        CO_CANrx_callback_t CANrx_callback);

/**
 * CAN receive function.
 *
 * Function must be called, when CAN message is received.
 * For every received message, function will try to find appropriate
 * CO_CANrx_t object (with CO_CANrx_t.ident equal to CAN identifier (msg->ident & CO_CANrx_t.mask).
 *
 * For more information see file CO_CAN.c
 *
 * @param[in] CANrxMsg Pointer to received message.
 * @return 0: Operation completed successfully.
 * @return -1: Error: Received message is NULL.
 * @return -2: Error: No message received.
 * @return -3: Error: Message received, but not processed.
 */
int16_t CO_CANrxBufferProcess(CO_CANrxMsg_t* const CANrxMsg);

/**
 * CAN send function.
 *
 * For more information see file CO_CAN.c
 *
 * @param[in] COB_ID CAN identifier.
 * @param[in] len Length of CAN message in bytes (0 to 8).
 * @param[in] data Pointer to CAN message data bytes.
 * @param[in] rtr Request for transmission. If true, then this is only request for transmission,
 *            no data are sent (length and data pointer are ignored).
 * @return 0: Operation completed successfully.
 * @return -1: Error: Wrong arguments.
 * @return -2: Error: Previous message is still waiting for buffer.
 * @return -3: Error: Timeout in transmission of CAN message.
 */
int16_t CO_CANsend(
        const uint16_t COB_ID,
        const uint8_t len,
        const uint8_t* const data,
        const bool_t rtr);

/**
 * Calculate CAN bit-timing values from register values.
 *
 * Function calculates values for CAN bit timing register and optionally for
 * CAN controller bit rate prescaler.
 *
 * For more information see file CO_driver.c
 *
 * @param[in] brp CAN controller bit rate prescaler.
 * @param[in] tseg1 Time segment 1 (0 to 15).
 * @param[in] tseg2 Time segment 2 (0 to 7).
 * @param[in] sjw Resynchronization jump width (0 to 3).
 * @param[out] pSyncTimeMicroseconds Synchronization time in micro seconds.
 * @param[out] pBitRatePrescaler Bit rate prescaler.
 * @return 0: Operation completed successfully.
 * @return -1: Error: Wrong arguments.
 */
int16_t CO_CANbitRateCalc(
        const uint16_t brp,
        const uint8_t tseg1,
        const uint8_t tseg2,
        const uint8_t sjw,
        uint32_t * const pSyncTimeMicroseconds,
        uint16_t * const pBitRatePrescaler);

#ifdef __cplusplus
}
#endif /* __cplusplus */

#endif /* CO_CANOPEN_H */

/**
 * @}
 */
 
/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */
/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */
/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */

/**
 * @defgroup CO_CANopen_301 CANopen 3.0.1 stack implementation
 *
 * CANopen 3.0.1 stack implementation. File CO_ODinterface.h is not included
 * here and must be included separately.
 *
 * @todo CO_CANrxMsg_t is not used anymore. Remove it in future.
 *
 * @defgroup CO_CANopen_301_Macros Macros
 * @ingroup CO_CANopen_301
 * @{
 *  CANopen error codes
 *  -------------------
 *  0x00000000 - no error
 *  0x05030000 - toggle bit not alternated
 *  0x05040000 - SDO protocol timed out
 *  0x05040001 - SDO protocol bad initial value
 *  0x05040002 - SDO protocol bad object dictionary address
 *  0x05040003 - SDO protocol bad data type
 *  0x05040004 - SDO protocol bad data size
 *  0x05040005 - SDO protocol bad data value
 *  0x06010000 - Object dictionary not found
 *  0x06010001 - Object cannot be mapped to the PDO
 *  0x06010002 - PDO length exceeded
 *  0x06020000 - Object does not exist in the object dictionary
 *  0x06040041 - Object cannot be mapped to the PDO
 *  0x06040042 - the number and length of the objects to be mapped would exceed PDO length
 *  0x06060000 - Access failed due to an hardware error
 *  0x06070010 - Data type does not match, length of service parameter does not match
 *  0x06070012 - Data type does not match, length of service parameter too high
 *  0x06070013 - Data type does not match, length of service parameter too low
 *  0x06090011 - Subindex does not exist
 *  0x06090030 - Value range of parameter exceeded (only for write access)
 *  0x06090031 - Value of parameter written too high
 *  0x06090032 - Value of parameter written too low
 *  0x06090036 - Maximum value is less than minimum value
 *  0x08000000 - General error
 *  0x08000020 - Data cannot be transferred or stored to the application
 *  0x08000021 - Data cannot be transferred or stored to the application because of local control
 *  0x08000022 - Data cannot be transferred or stored to the application because of the present device state
 *  0x08000023 - Object dictionary dynamic generation fails or no object dictionary is present (e.g. object dictionary is generated from file and generation fails because of an file error)
 *  0x08000024 - No data available
 *  0x08000025 - General error in the device
 *  0x08000026 - Data cannot be transferred or stored to the application because of some dependency
 *  0x08000027 - General error reason in the device
 *  0x08000028 - Service is not available
 *  0x08000029 - Attribute not supported
 *  0x0800002A - The value of the parameter written exceeds the range of values that can be written
 *  0x0800002B - Data cannot be transferred or stored to the application because of local control, specific application needed
 *  0x0800002C - Data cannot be transferred or stored to the application because of the present device state, specific application needed
 *  0x0800002D - Object dictionary not present, specific application needed
 *  0x0800002E - No data available, specific application needed
 *  0x0800002F - Data cannot be transferred or stored to the application because of some dependency, specific application needed
 *  0x08000030 - Service is not available, specific application needed
 *  0x08000031 - Attribute not supported, specific application needed
 *  0x0F004000 - Unexpected SYNC data length
 *  0x0F004001 - SYNC timeout
 *  0x0F004002 - Unexpected SYNC data
 *  0x0F004003 - Heartbeat consumer timeout
 *  0x0F004004 - PDO not processed due to length error
 *  0x0F004005 - PDO length exceeded
 *  0x0F004006 - DAM MPDO not processed, destination object not available
 *  0x0F004007 - Unexpected emergency object
 *  0x0F004008 - Error in error register
 *  0x0F004009 - Additional functions not available
 *  0x0F00400A - Parameter incompatibility (configuration or mode)
 *  0x0F00400B - CANopen service not supported
 *  0x0F00400C - CANopen invalid state transition
 *  0x0F00400D - CAN frame received is wrong
 *  0x0F00400E - PDO not processed, communication object not available
 *  0x0F00400F - PDO length exceeded
 *  0x0F004010 - Data type of service parameter does not match
 *  0x0F004011 - Data type of service parameter is not implemented