diff --git a/mp4/libav.py b/mp4/libav.py
index b1e06bb..03646fc 100644
--- a/mp4/libav.py
+++ b/mp4/libav.py
@@ -18,18 +18,14 @@ else:
 
 AVERROR_EOF = _errtag('E', 'O', 'F', ' ')
 
-AV_ERROR_MAX_STRING_SIZE = 64
+_AV_ERROR_MAX_STRING_SIZE = 64
 
-_avutil.av_strerror.restype = ctypes.c_int
-_avutil.av_strerror.argtypes = [
-    ctypes.c_int, # errno
-    ctypes.c_char_p, # errbuf
-    ctypes.c_size_t] # errbuff_size
-
-def strerror(errno):
-    errbuf = ctypes.create_string_buffer(AV_ERROR_MAX_STRING_SIZE)
-    _avutil.av_strerror(errno, errbuf, AV_ERROR_MAX_STRING_SIZE)
-    return errbuf.value.decode("utf-8")
+AVMEDIA_TYPE_UNKNOWN = -1
+AVMEDIA_TYPE_VIDEO = 0
+AVMEDIA_TYPE_AUDIO = 1
+AVMEDIA_TYPE_DATA = 2
+AVMEDIA_TYPE_SUBTITLE = 3
+AVMEDIA_TYPE_ATTACHMENT = 4
 
 class AVFrame(ctypes.Structure):
     pass
@@ -37,18 +33,6 @@ class AVFrame(ctypes.Structure):
 AVFrame_p = ctypes.POINTER(AVFrame)
 AVFrame_pp = ctypes.POINTER(AVFrame_p)
 
-_avutil.av_frame_alloc.restype = AVFrame_p
-_avutil.av_frame_alloc.argtypes = None
-
-def frame_alloc():
-    return _avutil.av_frame_alloc()
-
-_avutil.av_frame_free.restype = None
-_avutil.av_frame_free.argtypes = [AVFrame_pp]
-
-def frame_free(frame):
-    _avutil.av_frame_free(ctypes.byref(frame))
-
 class AVCodecParameters(ctypes.Structure):
     pass
 
@@ -80,6 +64,57 @@ class AVFormatContext(ctypes.Structure):
 AVFormatContext_p = ctypes.POINTER(AVFormatContext)
 AVFormatContext_pp = ctypes.POINTER(AVFormatContext_p)
 
+class AVPacket(ctypes.Structure):
+    _fields_ = [
+        ("buf", ctypes.c_void_p),
+        ("pts", ctypes.c_int64),
+        ("dts", ctypes.c_int64),
+        ("data", ctypes.c_void_p),
+        ("size", ctypes.c_int),
+        ("stream_index", ctypes.c_int)]
+        # ...
+
+AVPacket_p = ctypes.POINTER(AVPacket)
+AVPacket_pp = ctypes.POINTER(AVPacket_p)
+
+class AVCodec(ctypes.Structure):
+    _fields_ = [
+        ("name", ctypes.c_char_p),
+        ("long_name", ctypes.c_char_p)]
+        # ...
+
+AVCodec_p = ctypes.POINTER(AVCodec)
+AVCodec_pp = ctypes.POINTER(AVCodec_p)
+
+class AVCodecContext(ctypes.Structure):
+    pass
+
+AVCodecContext_p = ctypes.POINTER(AVCodecContext)
+AVCodecContext_pp = ctypes.POINTER(AVCodecContext_p)
+
+_avutil.av_strerror.restype = ctypes.c_int
+_avutil.av_strerror.argtypes = [
+    ctypes.c_int, # errno
+    ctypes.c_char_p, # errbuf
+    ctypes.c_size_t] # errbuff_size
+
+def strerror(errno):
+    errbuf = ctypes.create_string_buffer(_AV_ERROR_MAX_STRING_SIZE)
+    _avutil.av_strerror(errno, errbuf, _AV_ERROR_MAX_STRING_SIZE)
+    return errbuf.value.decode("utf-8")
+
+_avutil.av_frame_alloc.restype = AVFrame_p
+_avutil.av_frame_alloc.argtypes = None
+
+def frame_alloc():
+    return _avutil.av_frame_alloc()
+
+_avutil.av_frame_free.restype = None
+_avutil.av_frame_free.argtypes = [AVFrame_pp]
+
+def frame_free(frame):
+    _avutil.av_frame_free(ctypes.byref(frame))
+
 _avformat.avformat_alloc_context.restype = AVFormatContext_p
 _avformat.avformat_alloc_context.argtypes = None
 
@@ -116,22 +151,6 @@ _avformat.avformat_find_stream_info.argtypes = [
 def format_find_stream_info(context):
     return _avformat.avformat_find_stream_info(context, None)
 
-AVMEDIA_TYPE_UNKNOWN = -1
-AVMEDIA_TYPE_VIDEO = 0
-AVMEDIA_TYPE_AUDIO = 1
-AVMEDIA_TYPE_DATA = 2
-AVMEDIA_TYPE_SUBTITLE = 3
-AVMEDIA_TYPE_ATTACHMENT = 4
-
-class AVCodec(ctypes.Structure):
-    _fields_ = [
-        ("name", ctypes.c_char_p),
-        ("long_name", ctypes.c_char_p)]
-        # ...
-
-AVCodec_p = ctypes.POINTER(AVCodec)
-AVCodec_pp = ctypes.POINTER(AVCodec_p)
-
 _avformat.av_find_best_stream.restype = ctypes.c_int
 _avformat.av_find_best_stream.argtypes = [
     AVFormatContext_p,
@@ -146,19 +165,6 @@ def format_find_best_stream(context, type):
     index = _avformat.av_find_best_stream(context, type, -1, -1, ctypes.byref(codec), 0)
     return index, codec
 
-class AVPacket(ctypes.Structure):
-    _fields_ = [
-        ("buf", ctypes.c_void_p),
-        ("pts", ctypes.c_int64),
-        ("dts", ctypes.c_int64),
-        ("data", ctypes.c_void_p),
-        ("size", ctypes.c_int),
-        ("stream_index", ctypes.c_int)]
-        # ...
-
-AVPacket_p = ctypes.POINTER(AVPacket)
-AVPacket_pp = ctypes.POINTER(AVPacket_p)
-
 _avformat.av_packet_alloc.restype = AVPacket_p
 _avformat.av_packet_alloc.argtypes = None
 
@@ -177,12 +183,6 @@ _avformat.av_read_frame.argtypes = [AVFormatContext_p, AVPacket_p]
 def read_frame(context, packet):
     return _avformat.av_read_frame(context, packet)
 
-class AVCodecContext(ctypes.Structure):
-    pass
-
-AVCodecContext_p = ctypes.POINTER(AVCodecContext)
-AVCodecContext_pp = ctypes.POINTER(AVCodecContext_p)
-
 _avcodec.avcodec_alloc_context3.restype = AVCodecContext_p
 _avcodec.avcodec_alloc_context3.argtypes = [AVCodec_p]