func OpenFile(name string, flag int, perm FileMode) (*File, error) 

// Flags to OpenFile wrapping those of the underlying system. Not all
// flags may be implemented on a given system.
const (
    // Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
    // 只读模式
    O_RDONLY int = syscall.O_RDONLY // open the file read-only.
    // 只写模式
    O_WRONLY int = syscall.O_WRONLY // open the file write-only.
    // 可读可写
    O_RDWR   int = syscall.O_RDWR   // open the file read-write.
    // The remaining values may be or‘ed in to control behavior.
    // 追加内容
    O_APPEND int = syscall.O_APPEND // append data to the file when writing.
    // 创建文件，如果文件不存在
    O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
    // 与创建文件一同使用，文件必须存在
    O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
    // 打开一个同步的文件流
    O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
    // 如果可能，打开时缩短文件
    O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.
)

// The defined file mode bits are the most significant bits of the FileMode.
// The nine least-significant bits are the standard Unix rwxrwxrwx permissions.
// The values of these bits should be considered part of the public API and
// may be used in wire protocols or disk representations: they must not be
// changed, although new bits might be added.
const (
    // The single letters are the abbreviations
    // used by the String method‘s formatting.
    // 文件夹模式
    ModeDir        FileMode = 1 << (32 - 1 - iota) // d: is a directory
    // 追加模式
    ModeAppend                                     // a: append-only
    // 单独使用
    ModeExclusive                                  // l: exclusive use
    // 临时文件
    ModeTemporary                                  // T: temporary file; Plan 9 only
    // 象征性的关联
    ModeSymlink                                    // L: symbolic link
    // 设备文件
    ModeDevice                                     // D: device file
    // 命名管道
    ModeNamedPipe                                  // p: named pipe (FIFO)
    // Unix 主机 socket
    ModeSocket                                     // S: Unix domain socket
    // 设置uid
    ModeSetuid                                     // u: setuid
    // 设置gid
    ModeSetgid                                     // g: setgid
    // UNIX 字符串设备，当设备模式是设置unix
    ModeCharDevice                                 // c: Unix character device, when ModeDevice is set
    // 粘性的
    ModeSticky                                     // t: sticky
    // 非常规文件；对该文件一无所知
    ModeIrregular                                  // ?: non-regular file; nothing else is known about this file

    // bit位遮盖，不变的文件设置为none
    // Mask for the type bits. For regular files, none will be set.
    ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice | ModeCharDevice | ModeIrregular
    // 权限位
    ModePerm FileMode = 0777 // Unix permission bits
)

package main

import (
    "os"
)
func main() {
    fname := "/tmp/t.txt"
    f, err := os.OpenFile(fname, os.O_CREATE|os.O_RDWR|os.O_APPEND, os.ModeAppend|os.ModePerm)
    if err != nil {
        fmt.Println(err)
    }
    f.WriteString("test")
    f.Close()
}

os.MkdirAll(dname,os.ModeDir|os.ModePerm)