MAC上查询设备索引号

例如查询avfoundation支持的设备列表

ffmpeg -f avfoundation -list_devices true -i ""

执行结果：

[AVFoundation input device @ 0x0] AVFoundation video devices:
[AVFoundation input device @ 0x0] [0] FaceTime HD Camera
[AVFoundation input device @ 0x0] [1] Capture screen 0

[AVFoundation input device @ 0x0] AVFoundation audio devices:
[AVFoundation input device @ 0x0] [0] Built-in Microphone
: Input/output error