**Course Title:** Mastering Go: From Basics to Advanced Development **Section Title:** Working with the Standard Library: File I/O and Networking **Topic:** Create a Go application that handles file I/O and networking ====================================================== **Objective:** By the end of this lab topic, you will have a deep understanding of how to create a Go application that handles file I/O and networking. You will be able to apply your knowledge to build real-world applications that interact with files and networks. **Prerequisites:** Before starting this lab topic, make sure you have completed the previous topics and have a good understanding of the following concepts: * File I/O in Go (reading and writing files) * Networking in Go (TCP and HTTP) * Error handling in Go **Lab Topic: Create a Go Application that Handles File I/O and Networking** ------------------------------------------------------------------------ In this lab topic, we will create a simple Go application that handles file I/O and networking. Our application will be a simple file server that allows clients to upload and download files. **Step 1: Set up the project structure** Create a new directory for your project and navigate to it in your terminal or command prompt. Run the following command to create a new Go module: ```go go mod init file-server ``` This will create a new file called `go.mod` with the following content: ```go module file-server go 1.19 ``` **Step 2: Create the file server** Create a new file called `main.go` and add the following code: ```go package main import ( "fmt" "io" "log" "net/http" ) const ( port = 8080 ) func main() { http.HandleFunc("/upload", uploadFile) http.HandleFunc("/download", downloadFile) fmt.Printf("Server is listening on port %d...\n", port) log.Fatal(http.ListenAndServe(fmt.Sprintf(":%d", port), nil)) } func uploadFile(w http.ResponseWriter, r *http.Request) { // Handle file upload } func downloadFile(w http.ResponseWriter, r *http.Request) { // Handle file download } ``` This code sets up an HTTP server that listens on port 8080 and defines two handlers for the `/upload` and `/download` routes. However, the handlers are not implemented yet. **Step 3: Implement file upload** In the `uploadFile` function, we need to read the file from the HTTP request and save it to disk. We can use the `io.Copy` function to copy the file from the request body to a file on disk. Here's the implementation: ```go func uploadFile(w http.ResponseWriter, r *http.Request) { file, err := os.Create("uploads/" + r.URL.Path) if err != nil { http.Error(w, "Error creating file", http.StatusInternalServerError) return } defer file.Close() _, err = io.Copy(file, r.Body) if err != nil { http.Error(w, "Error uploading file", http.StatusInternalServerError) return } fmt.Fprintf(w, "File uploaded successfully!") } ``` This code creates a new file on disk with the same name as the URL path, and then copies the file from the request body to the new file using `io.Copy`. **Step 4: Implement file download** In the `downloadFile` function, we need to read the file from disk and send it back to the client in the HTTP response. We can use the `io.Copy` function again to copy the file from disk to the response writer. Here's the implementation: ```go func downloadFile(w http.ResponseWriter, r *http.Request) { file, err := os.Open("uploads/" + r.URL.Path) if err != nil { http.Error(w, "Error opening file", http.StatusInternalServerError) return } defer file.Close() _, err = io.Copy(w, file) if err != nil { http.Error(w, "Error downloading file", http.StatusInternalServerError) return } } ``` This code opens the file on disk and copies it to the response writer using `io.Copy`. **Step 5: Run the application** Run the following command to start the server: ``` go run main.go ``` This will start the server and you can access it by navigating to `http://localhost:8080` in your web browser. **Conclusion:** In this lab topic, we created a simple Go application that handles file I/O and networking. We implemented file upload and download using the `io.Copy` function and the `net/http` package. This application can be used as a starting point for more complex applications that require file I/O and networking. **What's next?** In the next topic, we will explore the `net/http` package in more depth and learn how to build web applications using Go. We will cover topics such as routing, middleware, and template rendering. **External Links:** * Go documentation: [https://golang.org/pkg/io/](https://golang.org/pkg/io/) * Go documentation: [https://golang.org/pkg/net/http/](https://golang.org/pkg/net/http/) **Leave a comment or ask for help:** If you have any questions or need help with this lab topic, feel free to leave a comment below.