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* leixiaohua1020@126.com**//*** 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 thanbuffer+buffer_size if the read function returnedless data than requested, e.g. for streams whereno 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");elseav_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_NETWORKif (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())return AVERROR(EIO);
#endifif ((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)) && 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.\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_NETWORKif (up->flags & URL_PROTOCOL_FLAG_NETWORK)ff_network_close();
#endifreturn 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*leixiaohua1020@126.com*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_PROTOCOLstatic 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_BINARYaccess |= O_BINARY;
#endiffd = 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

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