在上篇文章中的demo中，main函数的流程里调用的第一个函数就是avformat_open_input()。直观看来，其最明显的功能就是制定了要播放的文件名了。但是除了问价名之外还有几个结构体作为了函数的参数。那么这个函数的功能是什么？又是怎么完成的？一起慢慢研究。
先贴代码：
int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options)
{
AVFormatContext *s = *ps;
int ret = 0;
AVDictionary *tmp = NULL;
ID3v2ExtraMeta *id3v2_extra_meta = NULL;
if (!s && !(s = avformat_alloc_context()))
return AVERROR(ENOMEM);
if (!s->av_class)
{
av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");
return AVERROR(EINVAL);
}
if (fmt)
s->iformat = fmt;
if (options)
av_dict_copy(&tmp, *options, 0);
if ((ret = av_opt_set_dict(s, &tmp)) < 0)
goto fail;
if ((ret = init_input(s, filename, &tmp)) < 0)
goto fail;
s->probe_score = ret;
avio_skip(s->pb, s->skip_initial_bytes);
/* check filename in case an image number is expected */
if (s->iformat->flags & AVFMT_NEEDNUMBER)
{
if (!av_filename_number_test(filename))
{
ret = AVERROR(EINVAL);
goto fail;
}
}
s->duration = s->start_time = AV_NOPTS_VALUE;
av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
/* allocate private data */
if (s->iformat->priv_data_size > 0)
{
if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size)))
{
ret = AVERROR(ENOMEM);
goto fail;
}
if (s->iformat->priv_class)
{
*(const AVClass**)s->priv_data = s->iformat->priv_class;
av_opt_set_defaults(s->priv_data);
if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
goto fail;
}
}
/* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
if (s->pb)
ff_id3v2_read(s, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);
if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
if ((ret = s->iformat->read_header(s)) < 0)
goto fail;
if (id3v2_extra_meta)
{
if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
!strcmp(s->iformat->name, "tta")) {
if((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
goto fail;
} else
av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skipping\n");
}
ff_id3v2_free_extra_meta(&id3v2_extra_meta);
if ((ret = avformat_queue_attached_pictures(s)) < 0)
goto fail;
if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->data_offset)
s->data_offset = avio_tell(s->pb);
s->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;
if (options) {
av_dict_free(options);
*options = tmp;
}
*ps = s;
return 0;
fail:
ff_id3v2_free_extra_meta(&id3v2_extra_meta);
av_dict_free(&tmp);
if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
avio_close(s->pb);
avformat_free_context(s);
*ps = NULL;
return ret;
}函数的各个参数：1、AVFormatContext **ps：指向用户提供的结构体，一般可以将这个参数定义指向空然后传递到函数中，这样avformat_open_input函数将会分配这个结构体的内存空间并初始化。
2、const char *filename：打开视频文件的文件名。
3、AVInputFormat *fmt：如果这个参数不为空，则指定固定的输入格式，否则自动检测输入格式；一般设为空即可。
4、AVDictionary **options：由AVFormatContext和demuxer-private options组成的字典结构，可设为空。
该函数中调用了init_input()函数实现打开目标文件和检测文件格式等操作，代码如下：
static int init_input(AVFormatContext *s, const char *filename, AVDictionary **options)
{
int ret;
AVProbeData pd = {filename, NULL, 0};
int score = AVPROBE_SCORE_RETRY;
if (s->pb)
{
s->flags |= AVFMT_FLAG_CUSTOM_IO;
if (!s->iformat)
return av_probe_input_buffer2(s->pb, &s->iformat, filename, s, 0, s->probesize);
else if (s->iformat->flags & AVFMT_NOFILE)
av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
"will be ignored with AVFMT_NOFILE format.\n");
return 0;
}
if ( (s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
(!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
return score;
if ((ret = avio_open2(&s->pb, filename, AVIO_FLAG_READ | s->avio_flags,
&s->interrupt_callback, options)) < 0)
return ret;
if (s->iformat)
return 0;
return av_probe_input_buffer2(s->pb, &s->iformat, filename, s, 0, s->probesize);
}根据条件的不同，该函数内部调用了av_probe_input_buffer2()或avio_open2()两个函数。av_probe_input_buffer2()函数通过分析bitstream来确定输入的格式，代码如下：
int av_probe_input_buffer2(AVIOContext *pb, AVInputFormat **fmt, const char *filename, void *logctx, unsigned int offset, unsigned int max_probe_size)
{
AVProbeData pd = { filename ? filename : "", NULL, -offset };
unsigned char *buf = NULL;
uint8_t *mime_type;
int ret = 0, probe_size, buf_offset = 0;
int score = 0;
if (!max_probe_size) {
max_probe_size = PROBE_BUF_MAX;
} else if (max_probe_size > PROBE_BUF_MAX) {
max_probe_size = PROBE_BUF_MAX;
} else if (max_probe_size < PROBE_BUF_MIN) {
av_log(logctx, AV_LOG_ERROR,
"Specified probe size value %u cannot be < %u\n", max_probe_size, PROBE_BUF_MIN);
return AVERROR(EINVAL);
}
if (offset >= max_probe_size) {
return AVERROR(EINVAL);
}
if (!*fmt && pb->av_class && av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type) >= 0 && mime_type) {
if (!av_strcasecmp(mime_type, "audio/aacp")) {
*fmt = av_find_input_format("aac");
}
av_freep(&mime_type);
}
for(probe_size= PROBE_BUF_MIN; probe_size<=max_probe_size && !*fmt;
probe_size = FFMIN(probe_size<<1, FFMAX(max_probe_size, probe_size+1))) {
if (probe_size < offset) {
continue;
}
score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;
/* read probe data */
if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
return ret;
if ((ret = avio_read(pb, buf + buf_offset, probe_size - buf_offset)) < 0) {
/* fail if error was not end of file, otherwise, lower score */
if (ret != AVERROR_EOF) {
av_free(buf);
return ret;
}
score = 0;
ret = 0; /* error was end of file, nothing read */
}
pd.buf_size = buf_offset += ret;
pd.buf = &buf[offset];
memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);
/* guess file format */
*fmt = av_probe_input_format2(&pd, 1, &score);
if(*fmt){
if(score <= AVPROBE_SCORE_RETRY){ //this can only be true in the last iteration
av_log(logctx, AV_LOG_WARNING, "Format %s detected only with low score of %d, misdetection possible!\n", (*fmt)->name, score);
}else
av_log(logctx, AV_LOG_DEBUG, "Format %s probed with size=%d and score=%d\n", (*fmt)->name, probe_size, score);
}
}
if (!*fmt) {
av_free(buf);
return AVERROR_INVALIDDATA;
}
/* rewind. reuse probe buffer to avoid seeking */
ret = ffio_rewind_with_probe_data(pb, &buf, pd.buf_size);
return ret < 0 ? ret : score;
}其中调用了av_probe_input_format2()函数确定输入格式，其具体实现在函数av_probe_input_format3()中。
avio_open2()的代码如下：
int avio_open2(AVIOContext **s, const char *filename, int flags, const AVIOInterruptCB *int_cb, AVDictionary **options)
{
URLContext *h;
int err;
err = ffurl_open(&h, filename, flags, int_cb, options);
if (err < 0)
return err;
err = ffio_fdopen(s, h);
if (err < 0) {
ffurl_close(h);
return err;
}
return 0;
}该函数用于建立和初始化一个AVIOContext结构体，用于打开指定的url指向的资源。各个参数的含义：
1、AVIOContext **s：用于指向返回的AVIOContext结构体的指针，调用时应设为NULL；
2、const char *url：指定资源的地址；考虑本地播放时，就是文件名的字符串的地址；
3、int flags：标志位，控制文件打开的方式；
4、const AVIOInterruptCB *int_cb：在协议层使用的中断回调指针；
5、AVDictionary **options：protocol-private options组成的字典元素，设为空即可。
avio_open2()调用了ffurl_open()函数，创建一个URLContext结构体用于获取并打开url指向的资源（分别调用ffurl_alloc()和ffurl_connect()创建和链接URLContext结构体）；调用ffio_fdopen()函数创建AVIOContext()结构体并获取URLContext结构体引用的资源（调用avio_alloc_context()实现）；
整个函数的大致框图如下：
在实际运行的过程中，url_read、url_write等函数都由一个URLProtocal实现，代码如下：
typedef struct URLProtocol
{
const char *name;
int (*url_open)( URLContext *h, const char *url, int flags);
/**
* This callback is to be used by protocols which open further nested
* protocols. options are then to be passed to ffurl_open()/ffurl_connect()
* for those nested protocols.
*/
int (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);
/**
* Read data from the protocol.
* If data is immediately available (even less than size), EOF is
* reached or an error occurs (including EINTR), return immediately.
* Otherwise:
* In non-blocking mode, return AVERROR(EAGAIN) immediately.
* In blocking mode, wait for data/EOF/error with a short timeout (0.1s),
* and return AVERROR(EAGAIN) on timeout.
* Checking interrupt_callback, looping on EINTR and EAGAIN and until
* enough data has been read is left to the calling function; see
* retry_transfer_wrapper in avio.c.
*/
int (*url_read)( URLContext *h, unsigned char *buf, int size);
int (*url_write)(URLContext *h, const unsigned char *buf, int size);
int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);
int (*url_close)(URLContext *h);
struct URLProtocol *next;
int (*url_read_pause)(URLContext *h, int pause);
int64_t (*url_read_seek)(URLContext *h, int stream_index,
int64_t timestamp, int flags);
int (*url_get_file_handle)(URLContext *h);
int (*url_get_multi_file_handle)(URLContext *h, int **handles,
int *numhandles);
int (*url_shutdown)(URLContext *h, int flags);
int priv_data_size;
const AVClass *priv_data_class;
int flags;
int (*url_check)(URLContext *h, int mask);
} URLProtocol;针对不同的协议，分别定义了不同的URLPrococol对象来进行具体的操作，如：
文件协议：
URLProtocol ff_pipe_protocol = {
.name = "pipe",
.url_open = pipe_open,
.url_read = file_read,
.url_write = file_write,
.url_get_file_handle = file_get_handle,
.url_check = file_check,
.priv_data_size = sizeof(FileContext),
.priv_data_class = &pipe_class,
};ftp协议：
URLProtocol ff_ftp_protocol = {
.name = "ftp",
.url_open = ftp_open,
.url_read = ftp_read,
.url_write = ftp_write,
.url_seek = ftp_seek,
.url_close = ftp_close,
.url_get_file_handle = ftp_get_file_handle,
.url_shutdown = ftp_shutdown,
.priv_data_size = sizeof(FTPContext),
.priv_data_class = &ftp_context_class,
.flags = URL_PROTOCOL_FLAG_NETWORK,
};http协议：
URLProtocol ff_http_protocol = {
.name = "http",
.url_open2 = http_open,
.url_read = http_read,
.url_write = http_write,
.url_seek = http_seek,
.url_close = http_close,
.url_get_file_handle = http_get_file_handle,
.url_shutdown = http_shutdown,
.priv_data_size = sizeof(HTTPContext),
.priv_data_class = &http_context_class,
.flags = URL_PROTOCOL_FLAG_NETWORK,
};rtmp协议：
URLProtocol ff_librtmp_protocol =
{
.name = "rtmp",
.url_open = rtmp_open,
.url_read = rtmp_read,
.url_write = rtmp_write,
.url_close = rtmp_close,
.url_read_pause = rtmp_read_pause,
.url_read_seek = rtmp_read_seek,
.url_get_file_handle = rtmp_get_file_handle,
.priv_data_size = sizeof(LibRTMPContext),
.priv_data_class = &librtmp_class,
.flags = URL_PROTOCOL_FLAG_NETWORK,
};