Go语言文件操作深度解析：O_APPEND模式与文件偏移量的行为

package main

import (
    "io"
    "log"
    "os"
    "strings"
)

func main() {
    filePath := "example_append.txt"
    contentToAppend := "This is new appended content.\n"
    initialContent := "Original content line 1.\nOriginal content line 2.\n"

    // 1. 准备文件，写入初始内容
    err := os.WriteFile(filePath, []byte(initialContent), 0666)
    if err != nil {
        log.Fatalf("Failed to write initial content: %v", err)
    }
    log.Printf("Initial file content written to %s", filePath)

    // 2. 以O_APPEND模式打开文件，并尝试Seek
    file, err := os.OpenFile(filePath, os.O_RDWR|os.O_APPEND, 0666)
    if err != nil {
        log.Fatalf("Failed to open file with O_APPEND: %v", err)
    }
    defer file.Close()

    // 尝试Seek到文件开头，或者某个中间位置
    // 注意：这里的Seek操作会被O_APPEND的底层行为覆盖
    seekPos, err := file.Seek(0, io.SeekStart) // 尝试Seek到文件开头
    if err != nil {
        log.Fatalf("Failed to seek: %v", err)
    }
    log.Printf("Attempted to seek to position: %d", seekPos)

    // 写入数据
    reader := strings.NewReader(contentToAppend)
    n, err := io.CopyN(file, reader, int64(len(contentToAppend)))
    if err != nil && err != io.EOF { // io.EOF在io.CopyN读取完所有数据时可能返回，通常不是错误
        log.Fatalf("Failed to copy data: %v", err)
    }
    log.Printf("Copied %d bytes with O_APPEND mode.", n)

    // 验证文件内容
    finalContent, err := os.ReadFile(filePath)
    if err != nil {
        log.Fatalf("Failed to read final content: %v", err)
    }
    log.Printf("File content after O_APPEND write:\n%s", string(finalContent))
    // 预期输出：Initial content...后面直接跟着 This is new appended content.
    // 即：Original content line 1.
    //     Original content line 2.
    //     This is new appended content.
}

package main

import (
    "io"
    "log"
    "os"
    "strings"
)

func main() {
    filePath := "example_overwrite.txt"
    contentToOverwrite := "OVERWRITTEN" // 长度11
    initialContent := "0123456789ABCDEF" // 长度16

    // 1. 准备文件，写入初始内容
    err := os.WriteFile(filePath, []byte(initialContent), 0666)
    if err != nil {
        log.Fatalf("Failed to write initial content: %v", err)
    }
    log.Printf("Initial file content written to %s", filePath)

    // 2. 以O_RDWR模式打开文件 (不带O_APPEND)
    file, err := os.OpenFile(filePath, os.O_RDWR, 0666)
    if err != nil {
        log.Fatalf("Failed to open file without O_APPEND: %v", err)
    }
    defer file.Close()

    // 尝试Seek到特定位置 (例如，从索引2开始)
    seekPos := int64(2)
    _, err = file.Seek(seekPos, io.SeekStart)
    if err != nil {
        log.Fatalf("Failed to seek: %v", err)
    }
    log.Printf("Successfully sought to position: %d", seekPos)

    // 写入数据
    reader := strings.NewReader(contentToOverwrite)
    n, err := io.CopyN(file, reader, int64(len(contentToOverwrite)))
    if err != nil && err != io.EOF {
        log.Fatalf("Failed to copy data: %v", err)
    }
    log.Printf("Copied %d bytes without O_APPEND mode.", n)

    // 验证文件内容
    finalContent, err := os.ReadFile(filePath)
    if err != nil {
        log.Fatalf("Failed to read final content: %v", err)
    }
    log.Printf("File content after overwrite write:\n%s", string(finalContent))
    // 预期输出：01OVERWRITTENCDEF
}