实现音频播放后的自动退出功能

本文讨论了在 Windows 上使用 bass.dll 和 Go syscall 编写命令行音频播放器时实现自动退出的问题。原始代码需要添加 time.sleep 代码才能播放音频，否则会快速退出。文章提供了使用 bass_channelgetlength 和 bass_channelgetposition 函数的解决方案，这些函数可以获取音频的长度和当前播放位置。通过循环比较当前位置和音频长度，可以确定音频何时播放完毕，然后触发退出。

我正在写一个小工具，它可以像sox一样播放command/terminal中的音频文件。我在 windows 上使用 bass.dll 和 golang syscall。

这是我的代码，可以从评论中下载文件，只能在windows x64上运行。

github 上的 bass.go 要点

package main

import (
    "fmt"
    "syscall"
    "time"
    "unsafe"
)

/*
基于 [bass.dll](http://us2.un4seen.com/files/bass24.zip)
和 [Golang syscall](https://github.com/golang/go/wiki/WindowsDLLs) 
实现的命令行版播放器。
*/

type BassLib struct {
    libBass syscall.Handle
    init   uintptr
    free   uintptr
    streamCreateFile uintptr
    channelPlay uintptr
    channelPause uintptr
    channelStop uintptr
}

func (bass *BassLib) LoadBass(bassDllPath string) bool {
    bass.libBass, _ = syscall.LoadLibrary(bassDllPath)
    if bass.libBass == 0 {
        fmt.Println("load library result")
        fmt.Println(bass.libBass)
        return false
    }
    bass.init, _ = syscall.GetProcAddress(bass.libBass, "BASS_Init")
    // BASS_init(device, freq, flags, win, clsid)
    // see http://www.un4seen.com/doc/#bass/BASS_Init.html
    device := 1
    syscall.Syscall6(bass.init, 5, uintptr(device), uintptr(44100), uintptr(0), uintptr(0), uintptr(0), 0)
    return true
}


func StrPtr(s string) uintptr {
    // return uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(s)))
    p, _ := syscall.UTF16PtrFromString(s)
    return uintptr(unsafe.Pointer(p))
}

func (bass *BassLib) PlayFile(filePath string) {
    bass.streamCreateFile, _ = syscall.GetProcAddress(bass.libBass, "BASS_StreamCreateFile")
    // hstream = BASS_StreamCreateFile(mem=0, &file, offset=0, length=0, flags=(A_IsUnicode ? 0x80000000 : 0x40000))
    // see http://www.un4seen.com/doc/#bass/BASS_StreamCreateFile.html
    var bassUnicode uint32 = 0x80000000
    hstream, _, _ := syscall.Syscall6(bass.streamCreateFile, 5,  uintptr(0), StrPtr(filePath), uintptr(0), uintptr(0), uintptr(bassUnicode), 0)
    // bassErrorGetCode, _ := syscall.GetProcAddress(bass.libBass, "BASS_ErrorGetCode")
    // errCode, _, _ := syscall.Syscall(uintptr(bassErrorGetCode), 0, 0, 0, 0)
    // fmt.Println(errCode)
    fmt.Println("hstream")
    fmt.Println(hstream)
    bass.channelPlay, _ = syscall.GetProcAddress(bass.libBass, "BASS_ChannelPlay")
    // BASS_ChannelPlay(hstream)
    // see http://www.un4seen.com/doc/#bass/BASS_ChannelPlay.html
    ret, _, _ := syscall.Syscall(bass.channelPlay, 2, hstream, uintptr(0), 0)
    bassErrorGetCode, _ := syscall.GetProcAddress(bass.libBass, "BASS_ErrorGetCode")
    errCode, _, _ := syscall.Syscall(bassErrorGetCode, 0, 0, 0, 0)
    fmt.Println(errCode)
    fmt.Println(ret)
    // sleep to wait playing mp3 file
    time.Sleep(time.Second * 10)
    // bass.channelPause, _ = syscall.GetProcAddress(bass.libBass, "BASS_ChannelPause")
    // bass.channelStop, _ = syscall.GetProcAddress(bass.libBass, "BASS_ChannelStop")
    // return true
}

func (bass *BassLib) UnLoad() {
    if bass.libBass != 0 {
        bass.free, _ = syscall.GetProcAddress(bass.libBass, "BASS_Free")
        syscall.Syscall(bass.free, 0, 0, 0, 0)
        // BASS_Free()
        // see http://www.un4seen.com/doc/#bass/BASS_Free.html
        syscall.FreeLibrary(bass.libBass)
    }
}

func main() {
    bass := &BassLib{}
    bass.LoadBass("C:\\workspace\\play\\bass.dll")
    bass.PlayFile("C:\\workspace\\play\\sample.mp3")
    bass.UnLoad()
}

有一个大问题：

• 如果没有添加time.sleep代码（bass.go第68行），则没有声音播放，快速退出。
• 当我添加 time.sleep(time.second * 10) 代码时，音频持续时间可能超过 10 秒。

有没有可能让程序在音频播放完毕后自动退出？

解决方案

谢谢大家。可以用bass_channelgetlength和bass_channelgetposition函数解决问题。

这是代码：

// +build windows

package main

import (
    "fmt"
    "syscall"
    "time"
    "unsafe"
)

/*
基于 [bass.dll](http://us2.un4seen.com/files/bass24.zip)
和 [golang syscall](https://github.com/golang/go/wiki/windowsdlls)
实现的命令行版播放器。
*/

type (
    basserrorgetcode int32
)

const (
    bassunicode uint32 = 0x80000000 // bass_unicode
    basssamplefloat uint32 = 256 // bass_sample_float
    bassposbyte uint64 = 0  // bass_pos_byte
)

type basslib struct {
    libbass syscall.handle
    init   uintptr
    free   uintptr
    streamcreatefile uintptr
    channelplay uintptr
    channelpause uintptr
    channelstop uintptr
    channelgetlength uintptr
    channelgetposition uintptr
    channelbytes2seconds uintptr
}

func (bass *basslib) loadbass(bassdllpath string) bool {
    bass.libbass, _ = syscall.loadlibrary(bassdllpath)
    if bass.libbass == 0 {
        fmt.println("load `bass.dll` library failed!")
        errcode := bass.getbasserrorgetcode()
        fmt.println("bass_init failed!")
        fmt.println(errcode)
        return false
    }
    bass.init, _ = syscall.getprocaddress(bass.libbass, "bass_init")
    // bass_init(device, freq, flags, win, clsid)
    // see http://www.un4seen.com/doc/#bass/bass_init.html
    device := 1
    r, _, _ := syscall.syscall6(bass.init, 5, uintptr(device), uintptr(44100), uintptr(0), uintptr(0), uintptr(0), 0)
    // var ret = *(* int)(unsafe.pointer(&r))
    if r == 0 {
        errcode := bass.getbasserrorgetcode()
        fmt.println("bass_init failed!")
        fmt.println(errcode)
        return false
    }
    return true
}


func strptr(s string) uintptr {
    p, _ := syscall.utf16ptrfromstring(s)
    return uintptr(unsafe.pointer(p))
}

func (bass *basslib) playfile(filepath string) {
    bass.streamcreatefile, _ = syscall.getprocaddress(bass.libbass, "bass_streamcreatefile")
    // hstream = bass_streamcreatefile(mem=0, &file, offset=0, length=0, flags=(a_isunicode ? 0x80000000 : 0x40000))
    // see http://www.un4seen.com/doc/#bass/bass_streamcreatefile.html
    hstream, _, _ := syscall.syscall6(bass.streamcreatefile, 5,  uintptr(0), strptr(filepath), uintptr(0), uintptr(0), uintptr(bassunicode|basssamplefloat), 0)
    bass.channelplay, _ = syscall.getprocaddress(bass.libbass, "bass_channelplay")
    // bass_channelplay(hstream)
    // see http://www.un4seen.com/doc/#bass/bass_channelplay.html
    r, _, _ := syscall.syscall(bass.channelplay, 2, hstream, uintptr(0), 0)
    if r == 1 {
        totalduration := bass.getaudiobytelength(hstream)
        // currentpos := bass.getaudiocurrentbyteposition(hstream)
        fmt.println(totalduration)
        // fmt.println(currentpos)
        time.sleep(time.second*1)
        for {
            currentpos := bass.getaudiocurrentbyteposition(hstream)
            if currentpos >= totalduration {
                break
            }
        }
    } else {
        errcode := bass.getbasserrorgetcode()
        fmt.println("bass_channelplay failed!")
        fmt.println(errcode)
    }
}

func (bass *basslib) getbasserrorgetcode() basserrorgetcode {
    basserrorgetcode, _ := syscall.getprocaddress(bass.libbass, "bass_errorgetcode")
    // bass_errorgetcode()
    // bass_ok              basserrorgetcode = 0    // all is ok
    // bass_error_mem       basserrorgetcode = 1    // memory error
    // ...
    // see http://www.un4seen.com/doc/#bass/bass_errorgetcode.html
    errcode, _, _ := syscall.syscall(basserrorgetcode, 0, 0, 0, 0)
    var ierrcode = *(*basserrorgetcode)(unsafe.pointer(&errcode))
    return ierrcode
}

func (bass *basslib) getaudiobytelength(handle uintptr) uintptr {
    // (qword) bass_channelgetlength(handle=hstream, mode)
    // see http://www.un4seen.com/doc/#bass/bass_channelgetlength.html
    bass.channelgetlength, _ = syscall.getprocaddress(bass.libbass, "bass_channelgetlength")
    len, _, _ := syscall.syscall(bass.channelgetlength, 2, handle,  uintptr(bassposbyte), 0)
    return len
}


func (bass *basslib) getaudiocurrentbyteposition(handle uintptr) uintptr {
    // bass_channelgetposition(handle=hstream, mode)
    // see http://www.un4seen.com/doc/#bass/bass_channelgetposition.html
    bass.channelgetposition, _ = syscall.getprocaddress(bass.libbass, "bass_channelgetposition")
    pos, _, _ := syscall.syscall(bass.channelgetposition, 2, handle,  uintptr(bassposbyte), 0)
    return pos
}

func (bass *basslib) getchannelbytes2seconds(handle uintptr, pos uintptr) uintptr {
    // bass_channelbytes2seconds(handle=hstream, pos)
    // see http://www.un4seen.com/doc/#bass/bass_channelbytes2seconds.html
    // bass.channelbytes2seconds, _ = syscall.getprocaddress(bass.libbass, "bass_channelbytes2seconds")
    len, _, _ := syscall.syscall(bass.channelbytes2seconds, 2, handle, pos, 0)
    return len
}




func (bass *basslib) unload() {
    if bass.libbass != 0 {
        bass.free, _ = syscall.getprocaddress(bass.libbass, "bass_free")
        syscall.syscall(bass.free, 0, 0, 0, 0)
        // bass_free()
        // see http://www.un4seen.com/doc/#bass/bass_free.html
        syscall.freelibrary(bass.libbass)
    }
}

func main() {
    bass := &basslib{}
    bass.loadbass("c:\\workspace\\play\\bass.dll")
    bass.playfile("c:\\workspace\\play\\sample.mp3")
    bass.unload()
}

你也可以拨打 https://gist.github.com/ycrao/e7d1df181f870091b4a6d298d6ea2770#file-bass_play-go-L81-L91 。

我认为你可以使用 golang 的 defer 关键字在 play 函数完成时触发退出。

您可以参考这里：A Tour of Go | Defer 或者这里：Golang Blog | Defer

package main

import "fmt"

func main() {
    defer fmt.Println("world")

    fmt.Println("hello")
}

==========
$ go run main.go
hello
world

到这里，我们也就讲完了《实现音频播放后的自动退出功能》的内容了。个人认为，基础知识的学习和巩固，是为了更好的将其运用到项目中，欢迎关注golang学习网公众号，带你了解更多关于的知识点！