FFmpeg源代码简单分析:avio_open2()
来源:程序员人生 发布时间:2015-03-13 07:59:22 阅读次数:10949次
本文简单分析FFmpeg中1个经常使用的函数avio_open2()。该函数用于打开FFmpeg的输入输出文件。avio_open2()的声明位于libavformatavio.h文件中,以下所示。
/**
* Create and initialize a AVIOContext for
accessing the
* resource indicated by url.
* @note When the resource indicated by url has been opened in
* read+write mode, the AVIOContext can be used only for writing.
*
* @param s Used to return the pointer to the created AVIOContext.
* In case of failure the pointed to value is set to NULL.
* @param url resource to
access
* @param flags flags which control how the resource indicated by url
* is to be opened
* @param int_cb an interrupt callback to be used at the protocols level
* @param options A dictionary filled with protocol-private options. On return
* this parameter will be destroyed and replaced with a dict containing options
* that were not found. May be NULL.
* @return >= 0 in case of success, a negative value corresponding to an
* AVERROR code in case of failure
*/
int avio_open2(AVIOContext **s, const char *url, int flags,
const AVIOInterruptCB *int_cb, AVDictionary **options);
avio_open2()函数参数的含义以下:
s:函数调用成功以后创建的AVIOContext结构体。
url:输入输出协议的地址(文件也是1种“广义”的协议,对文件来讲就是文件的路径)。
flags:打开地址的方式。可以选择只读,只写,或读写。取值以下。
AVIO_FLAG_READ:只读。
AVIO_FLAG_WRITE:只写。
AVIO_FLAG_READ_WRITE:读写。
int_cb:目前还没有用过。
options:目前还没有用过。
该函数最典型的例子可以参考:最简单的基于FFMPEG的视频编码器(YUV编码为H.264)
函数调用结构图
首先贴出来终究分析得出的函数调用结构图,以下所示。
单击查看更清晰的图片
avio_open()
有1个和avio_open2()“长得很像”的函数avio_open(),应当是avio_open2()的初期版本。avio_open()比avio_open2()少了最后2个参数。而它前面几个参数的含义和avio_open2()是1样的。从源代码中可以看出,avio_open()内部调用了avio_open2(),并且把avio_open2()的后2个参数设置成了NULL,因此它的功能实际上和avio_open2()是1样的。avio_open()源代码以下所示。
int avio_open(AVIOContext **s, const char *filename, int flags)
{
return avio_open2(s, filename, flags, NULL, NULL);
}
avio_open2()
下面看1下avio_open2()的源代码,位于libavformataviobuf.c文件中。
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;
}
从avio_open2()的源代码可以看出,它主要调用了2个函数:ffurl_open()和ffio_fdopen()。其中ffurl_open()用于初始化URLContext,ffio_fdopen()用于根据URLContext初始化AVIOContext。URLContext中包括的URLProtocol完成了具体的协议读写等工作。AVIOContext则是在URLContext的读写函数外面加上了1层“包装”(通过retry_transfer_wrapper()函数)。
URLProtocol和URLContext
在查看ffurl_open()和ffio_fdopen()函数之前,首先查看1下URLContext和URLProtocol的定义。这两个结构体在FFmpeg的初期版本的SDK中是定义在头文件中可以直接使用的。但是近期的FFmpeg的SDK中已找不到这两个结构体的定义了。FFmpeg把这两个结构体移动到了源代码的内部,变成了内部结构体。
URLProtocol的定义位于libavformaturl.h,以下所示。
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;
从URLProtocol的定义可以看出,其中包括了用于协议读写的函数指针。例如:
url_open():打开协议。
url_read():读数据。
url_write():写数据。
url_close():关闭协议。
每种具体的协议都包括了1个URLProtocol结构体,例如:
FILE协议(“文件”在FFmpeg中也被当作1种协议)的结构体ff_file_protocol的定义以下所示(位于libavformatfile.c)。
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,
};
在使用FILE协议进行读写的时候,调用url_open()实际上就是调用了file_open()函数,这里限于篇幅不再对file_open()的源代码进行分析。file_open()函数实际上调用了系统的打开文件函数open()。同理,调用url_read()实际上就是调用了file_read()函数;file_read()函数实际上调用了系统的读取文件函数read()。url_write(),url_seek()等函数的道理都是1样的。
LibRTMP协议的结构体ff_librtmp_protocol的定义以下所示(位于libavformatlibrtmp.c)。
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,
};
UDP协议的结构体ff_udp_protocol的定义以下所示(位于libavformatudp.c)。
URLProtocol ff_udp_protocol = {
.name = "udp",
.url_open = udp_open,
.url_read = udp_read,
.url_write = udp_write,
.url_close = udp_close,
.url_get_file_handle = udp_get_file_handle,
.priv_data_size = sizeof(UDPContext),
.priv_data_class = &udp_context_class,
.flags = URL_PROTOCOL_FLAG_NETWORK,
};
上文中简单介绍了URLProtocol结构体。下面看1下URLContext结构体。URLContext的定义也位于libavformaturl.h,以下所示。
typedef struct URLContext {
const AVClass *av_class; /**< information for av_log(). Set by url_open(). */
struct URLProtocol *prot;
void *priv_data;
char *filename; /**< specified URL */
int flags;
int max_packet_size; /**< if non zero, the stream is packetized with this max packet size */
int is_streamed; /**< true if streamed (no seek possible), default = false */
int is_connected;
AVIOInterruptCB interrupt_callback;
int64_t rw_timeout; /**< maximum time to wait for (network) read/write operation completion, in mcs */
} URLContext;
从代码中可以看出,URLProtocol结构体是URLContext结构体的1个成员。由于还没有对URLContext结构体进行详细研究,有关该结构体的代码不再做过量分析。
ffurl_open()
前文提到AVIOContext中主要调用了2个函数:ffurl_open()和ffio_fdopen()。其中ffurl_open()用于初始化URLContext,ffio_fdopen()用于根据URLContext初始化AVIOContext。下面首先看1下初始化URLContext的函数ffurl_open()。
ffurl_open()的函数定义位于libavformatavio.c中,以下所示。
int ffurl_open(URLContext **puc, const char *filename, int flags,
const AVIOInterruptCB *int_cb, AVDictionary **options)
{
int ret = ffurl_alloc(puc, filename, flags, int_cb);
if (ret < 0)
return ret;
if (options && (*puc)->prot->priv_data_class &&
(ret = av_opt_set_dict((*puc)->priv_data, options)) < 0)
goto fail;
if ((ret = av_opt_set_dict(*puc, options)) < 0)
goto fail;
ret = ffurl_connect(*puc, options);
if (!ret)
return 0;
fail:
ffurl_close(*puc);
*puc = NULL;
return ret;
}
从代码中可以看出,ffurl_open()主要调用了2个函数:ffurl_alloc()和ffurl_connect()。ffurl_alloc()用于查找适合的URLProtocol,并创建1个URLContext;ffurl_connect()用于打开取得的URLProtocol。
ffurl_alloc()
ffurl_alloc()的定义位于libavformatavio.c中,以下所示。
int ffurl_alloc(URLContext **puc, const char *filename, int flags,
const AVIOInterruptCB *int_cb)
{
URLProtocol *p = NULL;
if (!first_protocol) {
av_log(NULL, AV_LOG_WARNING, "No URL Protocols are registered. "
"Missing call to av_register_all()?
");
}
p = url_find_protocol(filename);
if (p)
return url_alloc_for_protocol(puc, p, filename, flags, int_cb);
*puc = NULL;
if (av_strstart(filename, "https:", NULL))
av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile with openssl or gnutls enabled.
");
return AVERROR_PROTOCOL_NOT_FOUND;
}
从代码中可以看出,ffurl_alloc()主要调用了2个函数:url_find_protocol()根据文件路径查找适合的URLProtocol,url_alloc_for_protocol()为查找到的URLProtocol创建URLContext。
url_find_protocol()
先来看1下url_find_protocol()函数,定义以下所示。
#define URL_SCHEME_CHARS
"abcdefghijklmnopqrstuvwxyz"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"0123456789+-."
static struct URLProtocol *url_find_protocol(const char *filename)
{
URLProtocol *up = NULL;
char proto_str[128], proto_nested[128], *ptr;
size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
if (filename[proto_len] != ':' &&
(filename[proto_len] != ',' || !strchr(filename + proto_len + 1, ':')) ||
is_dos_path(filename))
strcpy(proto_str, "file");
else
av_strlcpy(proto_str, filename,
FFMIN(proto_len + 1, sizeof(proto_str)));
if ((ptr = strchr(proto_str, ',')))
*ptr = ' ';
av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));
if ((ptr = strchr(proto_nested, '+')))
*ptr = ' ';
while (up = ffurl_protocol_next(up)) {
if (!strcmp(proto_str, up->name))
break;
if (up->flags & URL_PROTOCOL_FLAG_NESTED_SCHEME &&
!strcmp(proto_nested, up->name))
break;
}
return up;
}
url_find_protocol()函数表明了FFmpeg根据文件路径猜想协议的方法。该函数首先根据strspn()函数查找字符串中第1个“非字母或数字”的字符的位置,并保存在proto_len中。1般情况下,协议URL中都是包括“:”的,比如说RTMP的URL格式是“rtmp://xxx…”,UDP的URL格式是“udp://…”,HTTP的URL格式是“http://...”。因此,1般情况下proto_len的数值就是“:”的下标(代表了“:”前面的协议名称的字符的个数,例如rtmp://的proto_len为4)。
接下来函数将filename的前proto_len个字节拷贝至proto_str字符串中。
PS:
这个地方比较纠结,源代码中av_strlcpy()函数的第3个参数size写的字符串的长度是(proto_len+1),但是查了1下av_strlcpy()的定义,发现该函数最多拷贝(size⑴)个字符。这么1涨1消,终究还是拷贝了proto_len个字节。例如RTMP协议就拷贝了“rtmp”,UDP协议就拷贝了“udp”。
av_strlcpy()是FFMpeg的1个工具函数,声明位于libavutilavstring.h,以下所示。
/**
* Copy the string src to dst, but no more than size - 1 bytes, and
* null-terminate dst.
*
* This function is the same as BSD strlcpy().
*
* @param dst destination buffer
* @param src source string
* @param size size of destination buffer
* @return the length of src
*
* @warning since the return value is the length of src, src absolutely
* _must_ be a properly 0-terminated string, otherwise this will read beyond
* the end of the buffer and possibly crash.
*/
size_t av_strlcpy(char *dst, const char *src, size_t size);
这里有1种例外,那就是文件路径。“文件”在FFmpeg中也是1种“协议”,并且前缀是“file”。也就是标准的文件路径应当是“file://...”格式的。但是这太不符合我们1般人的使用习惯,我们1般是不会在文件路径前面加上“file”协议名称的。所以该函数采取的方法是:1旦检测出来输入的URL是文件路径而不是网络协议,就自动向proto_str中拷贝“file”。
其中判断文件路径那里有1个很复杂的if()语句。根据我的理解,“||”前面的语句用于判断是不是是相对文件路径,“||”后面的语句用于判断是不是是绝对路径。判断绝对路径的时候用到了1个函数is_dos_path(),定义位于libavformatos_support.h,以下所示。
static inline int is_dos_path(const char *path)
{
#if HAVE_DOS_PATHS
if (path[0] && path[1] == ':')
return 1;
#endif
return 0;
}
注意“&&”优先级低于“==”。如果文件路径第1个字符不为空(1般情况下是盘符)而且第2个字符为“:”,就认为它是绝对文件路径。
另外url_find_protocol()函数中还触及到1个函数ffurl_protocol_next()。该函数用于取得下1个URLProtocol(所有的URLProtocol在FFmpeg初始化注册的时候构成1个链表结构)。ffurl_protocol_next()代码极为简单,以下所示。
URLProtocol *ffurl_protocol_next(const URLProtocol *prev)
{
return prev ? prev->next : first_protocol;
}
url_alloc_for_protocol()
url_alloc_for_protocol()的定义位于libavformatavio.c中,以下所示。
static int url_alloc_for_protocol(URLContext **puc, struct URLProtocol *up,
const char *filename, int flags,
const AVIOInterruptCB *int_cb)
{
URLContext *uc;
int err;
#if CONFIG_NETWORK
if (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())
return AVERROR(EIO);
#endif
if ((flags & AVIO_FLAG_READ) && !up->url_read) {
av_log(NULL, AV_LOG_ERROR,
"Impossible to open the '%s' protocol for reading
", up->name);
return AVERROR(EIO);
}
if ((flags & AVIO_FLAG_WRITE) && !up->url_write) {
av_log(NULL, AV_LOG_ERROR,
"Impossible to open the '%s' protocol for writing
", up->name);
return AVERROR(EIO);
}
uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);
if (!uc) {
err = AVERROR(ENOMEM);
goto fail;
}
uc->av_class = &ffurl_context_class;
uc->filename = (char *)&uc[1];
strcpy(uc->filename, filename);
uc->prot = up;
uc->flags = flags;
uc->is_streamed = 0; /* default = not streamed */
uc->max_packet_size = 0; /* default: stream file */
if (up->priv_data_size) {
uc->priv_data = av_mallocz(up->priv_data_size);
if (!uc->priv_data) {
err = AVERROR(ENOMEM);
goto fail;
}
if (up->priv_data_class) {
int proto_len= strlen(up->name);
char *start = strchr(uc->filename, ',');
*(const AVClass **)uc->priv_data = up->priv_data_class;
av_opt_set_defaults(uc->priv_data);
if(!strncmp(up->name, uc->filename, proto_len) && uc->filename + proto_len == start){
int ret= 0;
char *p= start;
char sep= *++p;
char *key, *val;
p++;
while(ret >= 0 && (key= strchr(p, sep)) && p<key && (val = strchr(key+1, sep))){
*val= *key= 0;
ret= av_opt_set(uc->priv_data, p, key+1, 0);
if (ret == AVERROR_OPTION_NOT_FOUND)
av_log(uc, AV_LOG_ERROR, "Key '%s' not found.
", p);
*val= *key= sep;
p= val+1;
}
if(ret<0 || p!=key){
av_log(uc, AV_LOG_ERROR, "Error parsing options string %s
", start);
av_freep(&uc->priv_data);
av_freep(&uc);
err = AVERROR(EINVAL);
goto fail;
}
memmove(start, key+1, strlen(key));
}
}
}
if (int_cb)
uc->interrupt_callback = *int_cb;
*puc = uc;
return 0;
fail:
*puc = NULL;
if (uc)
av_freep(&uc->priv_data);
av_freep(&uc);
#if CONFIG_NETWORK
if (up->flags & URL_PROTOCOL_FLAG_NETWORK)
ff_network_close();
#endif
return err;
}
url_alloc_for_protocol()完成了以下步骤:首先,检查输入的URLProtocol是不是支持指定的flag。比如flag中如果指定了AVIO_FLAG_READ,则URLProtocol中必须包括url_read();如果指定了AVIO_FLAG_WRITE,则URLProtocol中必须包括url_write()。在检查无误以后,接着就能够调用av_mallocz()为行将创建的URLContext分配内存了。接下来基本上就是各种赋值工作,在这里不再详细记录。
ffurl_connect()
ffurl_connect()用于打开取得的URLProtocol。该函数的定义位于libavformatavio.c中,以下所示。
int ffurl_connect(URLContext *uc, AVDictionary **options)
{
int err =
uc->prot->url_open2 ? uc->prot->url_open2(uc,
uc->filename,
uc->flags,
options) :
uc->prot->url_open(uc, uc->filename, uc->flags);
if (err)
return err;
uc->is_connected = 1;
/* We must be careful here as ffurl_seek() could be slow,
* for example for http */
if ((uc->flags & AVIO_FLAG_WRITE) || !strcmp(uc->prot->name, "file"))
if (!uc->is_streamed && ffurl_seek(uc, 0, SEEK_SET) < 0)
uc->is_streamed = 1;
return 0;
}
该函数最重要的函数就是它的第1句:URLProtocol中是不是包括url_open2()?如果包括的话,就调用url_open2(),否则就调用url_open()。
url_open()本身是URLProtocol的1个函数指针,这个地方根据不同的协议调用的url_open()具体实现函数也是不1样的,例如file协议的url_open()对应的是file_open(),而file_open()终究调用了_wsopen(),_sopen()(Windows下)或open()(Linux下,类似于fopen())这样的系统中打开文件的API函数;而libRTMP的url_open()对应的是rtmp_open(),而rtmp_open()终究调用了libRTMP的API函数RTMP_Init(),RTMP_SetupURL(),RTMP_Connect() 和RTMP_ConnectStream()。
ffio_fdopen()
ffio_fdopen()使用已取得的URLContext初始化AVIOContext。它的函数定义位于libavformataviobuf.c中,以下所示。
#define IO_BUFFER_SIZE 32768
int ffio_fdopen(AVIOContext **s, URLContext *h)
{
uint8_t *buffer;
int buffer_size, max_packet_size;
max_packet_size = h->max_packet_size;
if (max_packet_size) {
buffer_size = max_packet_size; /* no need to bufferize more than one packet */
} else {
buffer_size = IO_BUFFER_SIZE;
}
buffer = av_malloc(buffer_size);
if (!buffer)
return AVERROR(ENOMEM);
*s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,
(void*)ffurl_read, (void*)ffurl_write, (void*)ffurl_seek);
if (!*s) {
av_free(buffer);
return AVERROR(ENOMEM);
}
(*s)->direct = h->flags & AVIO_FLAG_DIRECT;
(*s)->seekable = h->is_streamed ? 0 : AVIO_SEEKABLE_NORMAL;
(*s)->max_packet_size = max_packet_size;
if(h->prot) {
(*s)->read_pause = (int (*)(void *, int))h->prot->url_read_pause;
(*s)->read_seek = (int64_t (*)(void *, int, int64_t, int))h->prot->url_read_seek;
}
(*s)->av_class = &ffio_url_class;
return 0;
}
ffio_fdopen()函数首先初始化AVIOContext中的Buffer。如果URLContext中设置了max_packet_size,则将Buffer的大小设置为max_packet_size。如果没有设置的话(仿佛大部份URLContext都没有设置该值),则会分配IO_BUFFER_SIZE个字节给Buffer。IO_BUFFER_SIZE取值为32768。
avio_alloc_context()
ffio_fdopen()接下来会调用avio_alloc_context()初始化1个AVIOContext。avio_alloc_context()本身是1个FFmpeg的API函数。它的声明位于libavformatavio.h中,以下所示。
/**
* Allocate and initialize an AVIOContext for buffered I/O. It must be later
* freed with av_free().
*
* @param buffer Memory block for input/output operations via AVIOContext.
* The buffer must be allocated with av_malloc() and friends.
* @param buffer_size The buffer size is very important for performance.
* For protocols with fixed blocksize it should be set to this blocksize.
* For others a typical size is a cache page, e.g. 4kb.
* @param write_flag Set to 1 if the buffer should be writable, 0 otherwise.
* @param opaque An opaque pointer to user-specific data.
* @param read_packet A function for refilling the buffer, may be NULL.
* @param write_packet A function for writing the buffer contents, may be NULL.
* The function may not change the input buffers content.
* @param seek A function for seeking to specified byte position, may be NULL.
*
* @return Allocated AVIOContext or NULL on failure.
*/
AVIOContext *avio_alloc_context(
unsigned char *buffer,
int buffer_size,
int write_flag,
void *opaque,
int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
int64_t (*seek)(void *opaque, int64_t offset, int whence));
avio_alloc_context()看上去参数很多,但实际上其实不复杂。先简单解释1下它各个参数的含义:
buffer:AVIOContext中的Buffer。
buffer_size:AVIOContext中的Buffer的大小。
write_flag:设置为1则Buffer可写;否则Buffer只可读。
opaque:用户自定义数据。
read_packet():读取外部数据,填充Buffer的函数。
write_packet():向Buffer中写入数据的函数。
seek():用于Seek的函数。
该函数成功履行的话则会返回1个创建好的AVIOContext。
下面看1下avio_alloc_context()的定义,位于libavformataviobuf.c,以下所示。
AVIOContext *avio_alloc_context(
unsigned char *buffer,
int buffer_size,
int write_flag,
void *opaque,
int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
int64_t (*seek)(void *opaque, int64_t offset, int whence))
{
AVIOContext *s = av_mallocz(sizeof(AVIOContext));
if (!s)
return NULL;
ffio_init_context(s, buffer, buffer_size, write_flag, opaque,
read_packet, write_packet, seek);
return s;
}
该函数代码很简单:首先调用av_mallocz()为AVIOContext分配1块内存空间,然后基本上将所有输入参数传递给ffio_init_context()。
ffio_init_context()
ffio_init_context()的定义以下。
int ffio_init_context(AVIOContext *s,
unsigned char *buffer,
int buffer_size,
int write_flag,
void *opaque,
int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
int64_t (*seek)(void *opaque, int64_t offset, int whence))
{
s->buffer = buffer;
s->orig_buffer_size =
s->buffer_size = buffer_size;
s->buf_ptr = buffer;
s->opaque = opaque;
s->direct = 0;
url_resetbuf(s, write_flag ? AVIO_FLAG_WRITE : AVIO_FLAG_READ);
s->write_packet = write_packet;
s->read_packet = read_packet;
s->seek = seek;
s->pos = 0;
s->must_flush = 0;
s->eof_reached = 0;
s->error = 0;
s->seekable = seek ? AVIO_SEEKABLE_NORMAL : 0;
s->max_packet_size = 0;
s->update_checksum = NULL;
if (!read_packet && !write_flag) {
s->pos = buffer_size;
s->buf_end = s->buffer + buffer_size;
}
s->read_pause = NULL;
s->read_seek = NULL;
return 0;
}
可以看出,这个函数的工作就是各种赋值,不算很有“技术含量”,不再详述。
ffurl_read(),ffurl_write(),ffurl_seek()
现在我们再回到ffio_fdopen(),会发现它初始化AVIOContext的结构体的时候,首先将自己分配的Buffer设置为该AVIOContext的Buffer;然后将URLContext作为用户自定义数据(对应AVIOContext的opaque变量)提供给该AVIOContext;最后分别将3个函数作为该AVIOContext的读,写,跳转函数:ffurl_read(),ffurl_write(),ffurl_seek()。下面我们选择1个ffurl_read()看看它的定义。
ffurl_read()的定义位于libavformatavio.c,以下所示。
int ffurl_read(URLContext *h, unsigned char *buf, int size)
{
if (!(h->flags & AVIO_FLAG_READ))
return AVERROR(EIO);
return retry_transfer_wrapper(h, buf, size, 1, h->prot->url_read);
}
该函数先判断了1下输入的URLContext是不是支持“读”操作,接着调用了1个函数:retry_transfer_wrapper()。
如果我们看ffurl_write()的代码,以下所示。
int ffurl_write(URLContext *h, const unsigned char *buf, int size)
{
if (!(h->flags & AVIO_FLAG_WRITE))
return AVERROR(EIO);
/* avoid sending too big packets */
if (h->max_packet_size && size > h->max_packet_size)
return AVERROR(EIO);
return retry_transfer_wrapper(h, (unsigned char *)buf, size, size, (void*)h->prot->url_write);
}
会发现他也调用了一样的1个函数retry_transfer_wrapper()。唯1的不同在于ffurl_read()调用retry_transfer_wrapper()的时候,最后1个参数是URLProtocol的url_read(),而ffurl_write()调用retry_transfer_wrapper()的时候,最后1个参数是URLProtocol的url_write()。
下面我们看1下retry_transfer_wrapper()的定义,位于libavformatavio.c,以下所示。
static inline int retry_transfer_wrapper(URLContext *h, uint8_t *buf,
int size, int size_min,
int (*transfer_func)(URLContext *h,
uint8_t *buf,
int size))
{
int ret, len;
int fast_retries = 5;
int64_t wait_since = 0;
len = 0;
while (len < size_min) {
if (ff_check_interrupt(&h->interrupt_callback))
return AVERROR_EXIT;
ret = transfer_func(h, buf + len, size - len);
if (ret == AVERROR(EINTR))
continue;
if (h->flags & AVIO_FLAG_NONBLOCK)
return ret;
if (ret == AVERROR(EAGAIN)) {
ret = 0;
if (fast_retries) {
fast_retries--;
} else {
if (h->rw_timeout) {
if (!wait_since)
wait_since = av_gettime_relative();
else if (av_gettime_relative() > wait_since + h->rw_timeout)
return AVERROR(EIO);
}
av_usleep(1000);
}
} else if (ret < 1)
return (ret < 0 && ret != AVERROR_EOF) ? ret : len;
if (ret)
fast_retries = FFMAX(fast_retries, 2);
len += ret;
}
return len;
}
从代码中可以看出,它的核心实际上是调用了1个名称为transfer_func()的函数。而该函数就是retry_transfer_wrapper()的第4个参数。该函数实际上是对URLProtocol的读写操作中的毛病进行了1些“容错”处理,可让数据的读写更加的稳定。
avio_alloc_context()履行终了后,ffio_fdopen()函数的工作就基本完成了,avio_open2()的工作也就做完了。
雷霄骅 (Lei Xiaohua)
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020
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