Simple knowledge of embedded system development

**Simple Knowledge Points for Embedded System Development** 1. **The Difference Between Include and Require, and the Efficiency of Require and Require_once?** In PHP, `include` is used to include and evaluate a specified file during the execution of the script. If a page has 10 `include` statements, PHP will interpret each one separately. On the other hand, `require` also includes a file, but it only does so once, even if the file is referenced multiple times. This makes `require` more efficient than `include`. Additionally, `require_once` checks whether the file has already been included, which adds an extra layer of checking and slightly reduces performance compared to `require`. When using `require`, if the file is not found or contains syntax errors, the script will stop executing and throw a fatal error. In contrast, `include` will issue a warning and allow the script to continue running. Also, `include` can be used conditionally, while `require` is always executed. 2. **Difference Between Cookie and Session, Can Session Work Without Cookies? What Are the Disadvantages of Using Sessions? Where Is the Session Stored on the Server? Is It Shared or Private?** Cookies are stored on the client side, meaning they can be modified by users, making them less secure. They have a size limit of around 3KB. Sessions, on the other hand, are stored on the server side, which makes them more secure and allows for larger data storage. However, sessions rely on cookies to work properly. If cookies are disabled, session IDs cannot be passed between the client and server, and the session will not function correctly. Disadvantages of using sessions include increased server resource usage because every session requires server memory. By default, sessions are stored in files on the server, with the location defined by the `session.save_path` configuration. These session files are typically stored in a public directory, which means they can be accessed if the server is not properly secured. 3. **How to Prevent SQL Injection?** To prevent SQL injection, you should: - Filter out common SQL keywords such as `SELECT`, `INSERT`, `UPDATE`, `DELETE`, `AND`, `*`, etc. - Use functions like `addslashes()` to escape special characters. - Disable `register_globals` in the PHP configuration to avoid direct access to user input variables. - Always use quotes when writing SQL queries to prevent malformed statements. - Rename database fields to make them harder to guess. - Encapsulate SQL logic into reusable functions to avoid exposing raw queries. - Enable `magic_quotes_gpc` to automatically escape incoming data. - Avoid displaying detailed error messages to users; log them instead. - Use prepared statements with `mysqli` or `PDO` for safer query execution. 4. **What Are the Types of Database Indexes, and When Should You Use Them?** Common types of database indexes include: - Normal index: Improves query speed for non-unique values. - Primary key index: Uniquely identifies each row in a table. - Unique index: Ensures that all values in a column are unique. You should create an index on a column if it's frequently used in WHERE clauses, JOINs, or ORDER BY statements, as this can significantly improve query performance. 5. **What Is the Difference Between Passing by Value and Passing by Reference? When Should You Use Each?** Passing by value means that a copy of the variable is made within the function, so changes inside the function do not affect the original variable. Passing by reference means that the function operates directly on the original variable, so any changes made inside the function are reflected outside. Passing by reference is more efficient for large data structures, as it avoids copying. Use pass-by-value when you don't want to modify the original data, and use pass-by-reference when you need to update the original variable. 6. **List Some Magic Methods and Explain Their Roles** - `__call()`: Invoked when a method is called that doesn't exist in the class. - `__autoload()`: Automatically loads a class file when an undefined class is instantiated. - `__set()`: Called when trying to set a value to an undefined property. - `__get()`: Called when trying to get the value of an undefined property. - `__construct()`: The constructor method called when an object is created. - `__destruct()`: The destructor method called when an object is destroyed. - `__unset()`: Called when `unset()` is used on an undefined variable. - `__isset()`: Called when `isset()` is used on an undefined variable. - `__clone()`: Called when an object is cloned. - `__toString()`: Called when an object is converted to a string. 7. **What Do $_REQUEST, $_POST, $_GET, $_COOKIE, $_SESSION, and $_FILES Mean?** These are built-in superglobal arrays in PHP: - `$_REQUEST`: Contains data from both GET and POST requests. - `$_POST`: Holds data sent via HTTP POST. - `$_GET`: Holds data sent via HTTP GET. - `$_COOKIE`: Stores cookie values. - `$_SESSION`: Stores session data. - `$_FILES`: Contains information about uploaded files. 8. **What Type of Array Subscript Is Best, and Why?** Numeric subscripts are generally faster to process in arrays, as they are handled more efficiently by the interpreter. String keys are useful for readability but may be slower for large datasets. 9. **Which Is More Efficient: ++i or i++? Why?** `++i` is more efficient than `i++` because it increments the value before returning it, whereas `i++` returns the original value before incrementing, which involves an extra step. 10. **What Do magic_quotes_gpc() and magic_quotes_runtime() Mean?** `magic_quotes_gpc()` is a PHP setting that automatically escapes quotes in GET, POST, and COOKIE data. `magic_quotes_runtime()` automatically escapes quotes from database results, but it's deprecated in newer versions of PHP. 11. **What Is the Difference Between Echo(), Print(), and Print_r()?** - `echo()` is a language construct that outputs one or more strings. - `print()` is a function that outputs a single string. - `print_r()` is a function that prints the contents of arrays and objects in a human-readable format. 12. **What Is Your Understanding of MVC?** MVC stands for Model-View-Controller, a design pattern that separates an application into three components: - **Model**: Manages the data and business logic. - **View**: Handles the user interface. - **Controller**: Acts as an intermediary between the model and view, handling user input and updating the model or view accordingly. Advantages of MVC include better organization, reusability, and maintainability. However, it can be overkill for small projects and may require more initial setup. 13. **What Are the Advantages of Single Entry and Multiple Entries in a Framework?** - **Single entry**: All requests are directed to a single file, simplifying security and access control. - **Multiple entries**: Each request is handled by different files, offering flexibility but potentially complicating security. Single entry is better for security, but may result in less SEO-friendly URLs. 14. **When Concatenating Strings, Is Using '.' More Efficient Than Using ','?** In PHP, using `.` (dot) to concatenate strings is more efficient than using `,` (comma), especially when dealing with multiple variables. The comma operator is mainly used in `echo` statements for outputting multiple values. 15. **What Do the Status Codes 200, 404, and 502 Mean?** - 200: The request was successful. - 404: The requested resource was not found. - 502: A bad gateway error, usually caused by a server misconfiguration. 16. **Write a Custom Function to Extract the File Extension from a URL** Example: `"www/hello/test.php.html?a=3&b=4"` Function: ```php function geturltype($url) { $info = parse_url($url); return end(explode('.', $info['path'])); } ``` 17. **What Are the Advantages of Using Memcache?** Memcache is a distributed memory caching system that stores data in memory to reduce database load and increase response time. Benefits include fast access, scalability, and reduced server stress under high traffic. It’s simple to configure, supports multi-server setups, and is widely used in high-performance web applications. In summary, learning embedded systems and related technologies is a continuous process. It requires consistent practice, reading technical documentation, and working on real-world projects. Combining theory with hands-on experience is the most effective way to master these skills.

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