标签：ffmpeg 源代码 关闭 avformatcontext 函数
本文简单分析FFmpeg的avformat_close_input()函数。该函数用于关闭一个AVFormatContext，一般情况下是和avformat_open_input()成对使用的。/** * Close an opened input AVFormatContext. Free it and all its contents * and set *s to NULL. */ void avformat_close_input(AVFormatContext **s);
该函数最典型的例子可以参考：
最简单的基于FFMPEG+SDL的视频播放器 ver2 （采用SDL2.0）
函数的调用关系如下图所示。
void avformat_close_input(AVFormatContext **ps)
{
AVFormatContext *s;
AVIOContext *pb;
if (!ps || !*ps)
return;
s = *ps;
pb = s->pb;
if ((s->iformat && strcmp(s->iformat->name, "image2") && s->iformat->flags & AVFMT_NOFILE) ||
(s->flags & AVFMT_FLAG_CUSTOM_IO))
pb = NULL;
flush_packet_queue(s);
if (s->iformat)
if (s->iformat->read_close)
s->iformat->read_close(s);
avformat_free_context(s);
*ps = NULL;
avio_close(pb);
}
（1）调用AVInputFormat的read_close()方法关闭输入流下面我们分别来看上述几个步骤。
（2）调用avformat_free_context()释放AVFormatContext
（3）调用avio_close()关闭并且释放AVIOContext
AVInputFormat ff_flv_demuxer = {
.name = "flv",
.long_name = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
.priv_data_size = sizeof(FLVContext),
.read_probe = flv_probe,
.read_header = flv_read_header,
.read_packet = flv_read_packet,
.read_seek = flv_read_seek,
.read_close = flv_read_close,
.extensions = "flv",
.priv_class = &flv_class,
};static int flv_read_close(AVFormatContext *s)
{
int i;
FLVContext *flv = s->priv_data;
for (i=0; i<FLV_STREAM_TYPE_NB; i++)
av_freep(&flv->new_extradata[i]);
return 0;
}有关avformat_free_context()可以参考文章：
avio_close()是一个FFmpeg的API函数，用于关闭和释放AVIOContext。它的声明位于libavformat\avio.h，如下所示。
/** * Close the resource accessed by the AVIOContext s and free it. * This function can only be used if s was opened by avio_open(). * * The internal buffer is automatically flushed before closing the * resource. * * @return 0 on success, an AVERROR < 0 on error. * @see avio_closep */ int avio_close(AVIOContext *s);
int avio_close(AVIOContext *s)
{
URLContext *h;
if (!s)
return 0;
avio_flush(s);
h = s->opaque;
av_freep(&s->buffer);
if (s->write_flag)
av_log(s, AV_LOG_DEBUG, "Statistics: %d seeks, %d writeouts\n", s->seek_count, s->writeout_count);
else
av_log(s, AV_LOG_DEBUG, "Statistics: %"PRId64" bytes read, %d seeks\n", s->bytes_read, s->seek_count);
av_free(s);
return ffurl_close(h);
}
（1）调用avio_flush()强制清除缓存中的数据
（2）调用av_freep()释放掉AVIOContext种的buffer
（3）调用av_free()释放掉AVIOContext结构体
（4）调用ffurl_close()关闭并且释放掉URLContext
下面按照顺序分别看看avio_flush()和ffurl_close()这两个函数。
/** * Force flushing of buffered data. * * For write streams, force the buffered data to be immediately written to the output, * without to wait to fill the internal buffer. * * For read streams, discard all currently buffered data, and advance the * reported file position to that of the underlying stream. This does not * read new data, and does not perform any seeks. */ void avio_flush(AVIOContext *s);
void avio_flush(AVIOContext *s)
{
flush_buffer(s);
s->must_flush = 0;
}static void flush_buffer(AVIOContext *s)
{
if (s->write_flag && s->buf_ptr > s->buffer) {
writeout(s, s->buffer, s->buf_ptr - s->buffer);
if (s->update_checksum) {
s->checksum = s->update_checksum(s->checksum, s->checksum_ptr,
s->buf_ptr - s->checksum_ptr);
s->checksum_ptr = s->buffer;
}
}
s->buf_ptr = s->buffer;
if (!s->write_flag)
s->buf_end = s->buffer;
}static void writeout(AVIOContext *s, const uint8_t *data, int len)
{
if (s->write_packet && !s->error) {
int ret = s->write_packet(s->opaque, (uint8_t *)data, len);
if (ret < 0) {
s->error = ret;
}
}
s->writeout_count ++;
s->pos += len;
}从定义可以看出，writeout()调用了AVIOContext的write_packet()方法。根据此前文章《FFmpeg源代码简单分析：avio_open2()》中的分析我们可以了解到，AVIOContext的write_packet()实际指向了ffurl_write()函数，而ffurl_write()经过retry_transfer_wrapper()函数最终调用了URLProtocol的url_write()函数。url_write()是一个函数指针，不同的URLProtocol的url_write()指向不同的函数。
例如，file（文件）对应的URLProtocol的定义位于libavformat\file.c，如下所示。
URLProtocol ff_file_protocol = {
.name = "file",
.url_open = file_open,
.url_read = file_read,
.url_write = file_write,
.url_seek = file_seek,
.url_close = file_close,
.url_get_file_handle = file_get_handle,
.url_check = file_check,
.priv_data_size = sizeof(FileContext),
.priv_data_class = &file_class,
};static int file_write(URLContext *h, const unsigned char *buf, int size)
{
FileContext *c = h->priv_data;
int r;
size = FFMIN(size, c->blocksize);
r = write(c->fd, buf, size);
return (-1 == r)?AVERROR(errno):r;
}/** * Close the resource accessed by the URLContext h, and free the * memory used by it. Also set the URLContext pointer to NULL. * * @return a negative value if an error condition occurred, 0 * otherwise */ int ffurl_closep(URLContext **h); int ffurl_close(URLContext *h);
int ffurl_close(URLContext *h)
{
return ffurl_closep(&h);
}int ffurl_closep(URLContext **hh)
{
URLContext *h= *hh;
int ret = 0;
if (!h)
return 0; /* can happen when ffurl_open fails */
if (h->is_connected && h->prot->url_close)
ret = h->prot->url_close(h);
#if CONFIG_NETWORK
if (h->prot->flags & URL_PROTOCOL_FLAG_NETWORK)
ff_network_close();
#endif
if (h->prot->priv_data_size) {
if (h->prot->priv_data_class)
av_opt_free(h->priv_data);
av_freep(&h->priv_data);
}
av_freep(hh);
return ret;
}
（1）调用URLProtocol的url_close()其中URLProtocol的url_close()是一个函数指针，其指向的函数与具体的URLProtocol有关，这里我们还是看一下file（文件）对应的URLProtocol，如下所示。
（2）调用av_freep()释放URLContext结构体
URLProtocol ff_file_protocol = {
.name = "file",
.url_open = file_open,
.url_read = file_read,
.url_write = file_write,
.url_seek = file_seek,
.url_close = file_close,
.url_get_file_handle = file_get_handle,
.url_check = file_check,
.priv_data_size = sizeof(FileContext),
.priv_data_class = &file_class,
};static int file_close(URLContext *h)
{
FileContext *c = h->priv_data;
return close(c->fd);
}至此avio_close()函数分析完毕。
雷霄骅
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020
FFmpeg源代码简单分析：avformat_close_input()
标签：ffmpeg 源代码 关闭 avformatcontext 函数
原文地址：http://blog.csdn.net/leixiaohua1020/article/details/44110683
