add: i/o streaming iface

interfaces/13-io/main.go

@@ -0,0 +1,105 @@
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+// For more tutorials  : https://learngoprogramming.com
+// In-person training  : https://www.linkedin.com/in/inancgumus/
+// Follow me on twitter: https://twitter.com/inancgumus
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+)
+
+func main() {
+	// Stdin, Stdout, and Stderr are *os.File.
+
+	// #1: reads from the standard input
+	// ------------------------------------------------
+	// if err := read(os.Stdin); err != nil {
+	// 	log.Fatalln(err)
+	// }
+
+	// #2: writes to the standard output
+	// ------------------------------------------------
+	// buf := []byte("hi!\n")
+	// if err := write(os.Stdout, buf); err != nil {
+	// 	log.Fatalln(err)
+	// }
+
+	// #2b: reads the entire file content into memory.
+	// ------------------------------------------------
+	// buf, err := ioutil.ReadFile("alice.txt")
+	// if err != nil {
+	// 	log.Fatalln(err)
+	// }
+	// if err := write(os.Stdout, buf); err != nil {
+	// 	log.Fatalln(err)
+	// }
+
+	// #3: reads and writes with 32KB of memory
+	//     no matter how large the source data is.
+	// ------------------------------------------------
+	if err := ioCopy(os.Stdout, os.Stdin); err != nil {
+		log.Fatalln(err)
+	}
+}
+
+func ioCopy(dst, src *os.File) error {
+	buf := make([]byte, 32768)
+	for {
+		nr, er := src.Read(buf)
+		if nr > 0 {
+			_, ew := dst.Write(buf[:nr])
+			if ew != nil {
+				return ew
+			}
+		}
+		if er == io.EOF {
+			return nil
+		}
+		if er != nil {
+			return er
+		}
+	}
+	return nil
+}
+
+func write(dst *os.File, buf []byte) error {
+	nw, ew := dst.Write(buf)
+
+	fmt.Printf("wrote    : %d bytes\n", nw)
+	fmt.Printf("write err: %v\n", ew)
+
+	return ew
+}
+
+func read(src *os.File) error {
+	// Use a fixed-length buffer to efficiently read from src stream in chunks.
+	buf := make([]byte, 32768)
+
+	// #1b: read over and over again to read all the data.
+	for {
+		// #1a: read can read only up to the buffer length.
+		nr, er := src.Read(buf)
+
+		fmt.Printf("\n")
+		// fmt.Printf("buf      : %q\n", buf[0:nr])
+		fmt.Printf("read     : %d bytes\n", nr)
+		fmt.Printf("read err : %v\n", er)
+
+		// #1c: io.EOF = there is nothing left to read—close the loop.
+		if er == io.EOF {
+			return nil
+		}
+		// Only return an error if the reading really fails.
+		if er != nil {
+			return er
+		}
+	}
+	return nil
+}