FFMPEG源码分析（一）
FFMPEG源码分析（一）
ffmpeg之前公司项目中就使用过，但是多停留于应用层面，实现某个功能时，需要哪些结构体以及调用哪些函数。最近想系统的学习一下ffmpeg，于是开始看雷霄骅https://blog.csdn.net/leixiaohua1020的博客（在此缅怀下大神）。看到ffmpeg中重要的结构体方面，于是想要到源码层看看各个结构体初始化等工作，以便对各个结构体有一个更深入的了解。废话不多说，结构体的介绍请大家去雷神的博客了解，下面进入第一个结构体AVIOContext：
AVIOContext
/* 雷霄骅
* 中国传媒大学/数字电视技术
* [email protected]
*
*/
/**
* Bytestream IO Context.
* New fields can be added to the end with minor version bumps.
* Removal, reordering and changes to existing fields require a major
* version bump.
* sizeof(AVIOContext) must not be used outside libav*.
*
* @note None of the function pointers in AVIOContext should be called
* directly, they should only be set by the client application
* when implementing custom I/O. Normally these are set to the
* function pointers specified in avio_alloc_context()
*/
typedef struct {
/**
* A class for private options.
*
* If this AVIOContext is created by avio_open2(), av_class is set and
* passes the options down to protocols.
*
* If this AVIOContext is manually allocated, then av_class may be set by
* the caller.
*
* warning -- this field can be NULL, be sure to not pass this AVIOContext
* to any av_opt_* functions in that case.
*/
AVClass *av_class;
unsigned char *buffer; /**< Start of the buffer. */
int buffer_size; /**< Maximum buffer size */
unsigned char *buf_ptr; /**< Current position in the buffer */
unsigned char *buf_end; /**< End of the data, may be less than
buffer+buffer_size if the read function returned
less data than requested, e.g. for streams where
no more data has been received yet. */
void *opaque; /**< A private pointer, passed to the read/write/seek/...
functions. */
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);
int64_t pos; /**< position in the file of the current buffer */
int must_flush; /**< true if the next seek should flush */
int eof_reached; /**< true if eof reached */
int write_flag; /**< true if open for writing */
int max_packet_size;
unsigned long checksum;
unsigned char *checksum_ptr;
unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size);
int error; /**< contains the error code or 0 if no error happened */
/**
* Pause or resume playback for network streaming protocols - e.g. MMS.
*/
int (*read_pause)(void *opaque, int pause);
/**
* Seek to a given timestamp in stream with the specified stream_index.
* Needed for some network streaming protocols which don't support seeking
* to byte position.
*/
int64_t (*read_seek)(void *opaque, int stream_index,
int64_t timestamp, int flags);
/**
* A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable.
*/
int seekable;
/**
* max filesize, used to limit allocations
* This field is internal to libavformat and access from outside is not allowed.
*/
int64_t maxsize;
} AVIOContext;
该结构体可以通过avio_open2函数来分配初始化，也可以手动分配初始化。下面看看avio_open2源码中在做什么。
在此之前，为了看源码时理解更顺畅一些，首先明确几个结构体间的关系，
AVIOContext中包含有一个URLContext 结构体，而在URLContext 中包含有一个URLProtocol结构体。
源码分析
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;
}
可以看到，该函数比较简单，其中两个主要的函数ffurl_open和ffio_fdopen，而需要生成的结构体AVIOContext **s，主要就是初始化了一个URLContext结构体给它。
一 ffurl_open函数
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); //--------------------(1)
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); //------------------(2)
if (!ret)
return 0;
fail:
ffurl_close(*puc);
*puc = NULL;
return ret;
}
- 先看第(1)个函数：
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()?\n");
}
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 FFmpeg with "
"openssl, gnutls,\n"
"or securetransport enabled.\n");
return AVERROR_PROTOCOL_NOT_FOUND;
}
这里出现了URLProtocol 这个结构体，用于解析路径的协议，比如file、udp、rtmp等等。
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 = '\0';
av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));
if ((ptr = strchr(proto_nested, '+')))
*ptr = '\0';
/* 这里proto_str已经储存为protocol的名字，比如"file"、“rtmp”、“http”等等，然后会循环获取当前注册了的协议结构体URLProtocol，
* 比较其中的名字up->name是否 一样，如果一样就匹配成功，返回该URLProtocol
*/
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;
}
这个函数内部主要是对路径的字符串做一些处理，从而获取到该路径对应的URLProtocol。如果成功获取，那么会 return url_alloc_for_protocol(puc, p, filename, flags, int_cb)
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\n", 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\n", 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)) && ppriv_data, p, key+1, 0);
if (ret == AVERROR_OPTION_NOT_FOUND)
av_log(uc, AV_LOG_ERROR, "Key '%s' not found.\n", p);
*val= *key= sep;
p= val+1;
}
if(ret<0 || p!=key){
av_log(uc, AV_LOG_ERROR, "Error parsing options string %s\n", 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;
}
这个函数中主要就是malloc分配内存，并初始化URLContext 中一些变量的值。
至此，ffurl_alloc函数已经初始化一个URLContext结构体，并分配好空间。
- 第(2)个函数
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;
}
在该函数中会查看结构体URLContext 中是否已经赋值了url_open2，如果有则通过url_open2 打开文件，否则使用url_open函数来打开文件。这个函数根据不同的协议会传入不同的回调，比如file类型其实就是调用open函数等。
二 ffio_fdopen函数
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);
//分配AVIOContext 结构体内存空间，赋值一些参数和回调函数
*s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,
(int (*)(void *, uint8_t *, int)) ffurl_read,
(int (*)(void *, uint8_t *, int)) ffurl_write,
(int64_t (*)(void *, int64_t, int)) 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 = &ff_avio_class;
return 0;
}
该函数首先初始化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（似乎大部分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。后面就是一些属性的赋值操作。
其中有几个函数注意一下，ffurl_read、ffurl_write和ffurl_seek。
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);
}
该函数首先判断一下URLContext 结构体是否支持读操作，如果支持就会调用retry_transfer_wrapper函数。
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;
}
可以看到核心就是 ret = transfer_func(h, buf + len, size - len)，而该函数是传入的回调函数，在read中就是h->prot->url_read，这个函数会根据不同的协议而不同，也就是在进行读文件的实际操作，比如file协议中会调用read函数去实际读取数据。读完以后会进行一些容错处理。
下面给大家看一下file协议的函数就明白上面的意思了。文件协议中代码如下（file.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,
};
其中的所有file_xxx函数如下：
/*
*雷霄骅
*[email protected]
*中国传媒大学/数字电视技术
*/
/* standard file protocol */
static int file_read(URLContext *h, unsigned char *buf, int size)
{
int fd = (intptr_t) h->priv_data;
int r = read(fd, buf, size);
return (-1 == r)?AVERROR(errno):r;
}
static int file_write(URLContext *h, const unsigned char *buf, int size)
{
int fd = (intptr_t) h->priv_data;
int r = write(fd, buf, size);
return (-1 == r)?AVERROR(errno):r;
}
static int file_get_handle(URLContext *h)
{
return (intptr_t) h->priv_data;
}
static int file_check(URLContext *h, int mask)
{
struct stat st;
int ret = stat(h->filename, &st);
if (ret < 0)
return AVERROR(errno);
ret |= st.st_mode&S_IRUSR ? mask&AVIO_FLAG_READ : 0;
ret |= st.st_mode&S_IWUSR ? mask&AVIO_FLAG_WRITE : 0;
return ret;
}
#if CONFIG_FILE_PROTOCOL
static int file_open(URLContext *h, const char *filename, int flags)
{
int access;
int fd;
av_strstart(filename, "file:", &filename);
if (flags & AVIO_FLAG_WRITE && flags & AVIO_FLAG_READ) {
access = O_CREAT | O_TRUNC | O_RDWR;
} else if (flags & AVIO_FLAG_WRITE) {
access = O_CREAT | O_TRUNC | O_WRONLY;
} else {
access = O_RDONLY;
}
#ifdef O_BINARY
access |= O_BINARY;
#endif
fd = open(filename, access, 0666);
if (fd == -1)
return AVERROR(errno);
h->priv_data = (void *) (intptr_t) fd;
return 0;
}
/* XXX: use llseek */
static int64_t file_seek(URLContext *h, int64_t pos, int whence)
{
int fd = (intptr_t) h->priv_data;
if (whence == AVSEEK_SIZE) {
struct stat st;
int ret = fstat(fd, &st);
return ret < 0 ? AVERROR(errno) : st.st_size;
}
return lseek(fd, pos, whence);
}
static int file_close(URLContext *h)
{
int fd = (intptr_t) h->priv_data;
return close(fd);
}
到此，结构体AVIOContext就被初始化完成。
参考资料
[1]: https://blog.csdn.net/leixiaohua1020/article/details/14215369