FFMpeg分析：第一个函数avformat_open_input

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- 

 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- 

 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,

};

open()函数 函数：open() 1：作用：打开一个文件 2：语法： open(file[, mode[, buffering[, encoding[, errors[, newline[, closefd=True]]]]]]) 3：参数说明： file： 要打开的文件名，需加路径(除非是在当前目录)。
jerry.yin 毕业于上海大学通信与信息工程学院，从事流媒体和视频编解码的研究与开发工作； 研究领域包括视频编解码标准、视频处理和流媒体技术、移动互联网技术等。