标签：ret context standard caller 读取 有一个 support writing lang
在上文FFmpeg 结构体学习(六)： AVCodecContext 分析我们学习了AVCodec结构体的相关内容。本文，我们将讲述一下AVIOContext。
AVIOContext是FFMPEG管理输入输出数据的结构体。下面我们来分析一下该结构体里重要变量的含义和作用。
首先我们先看一下结构体AVIOContext的定义的结构体源码(位于libavformat/avio.h)：
/** * 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;
AVIOContext中有以下几个变量比较重要：
unsigned char *buffer：缓存开始位置 int buffer_size：缓存大小（默认32768） unsigned char *buf_ptr：当前指针读取到的位置 unsigned char *buf_end：缓存结束的位置 void *opaque：URLContext结构体
在解码的情况下，buffer用于存储ffmpeg读入的数据。例如打开一个视频文件的时候，先把数据从硬盘读入buffer，然后在送给解码器用于解码。
其中opaque指向了URLContext。注意，这个结构体并不在FFMPEG提供的头文件中，而是在FFMPEG的源代码中。从FFMPEG源代码中翻出的定义如下所示：
typedef struct URLContext { const AVClass *av_class; ///< information for av_log(). Set by url_open(). struct URLProtocol *prot; int flags; int is_streamed; /**< true if streamed (no seek possible), default = false */ int max_packet_size; /**< if non zero, the stream is packetized with this max packet size */ void *priv_data; char *filename; /**< specified URL */ int is_connected; AVIOInterruptCB interrupt_callback; } URLContext;
URLContext结构体中还有一个结构体URLProtocol。注：每种协议（rtp，rtmp，file等）对应一个URLProtocol。这个结构体也不在FFMPEG提供的头文件中。从FFMPEG源代码中翻出其的定义：
typedef struct URLProtocol { const char *name; int (*url_open)(URLContext *h, const char *url, int flags); 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 priv_data_size; const AVClass *priv_data_class; int flags; int (*url_check)(URLContext *h, int mask); } URLProtocol;
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, };
libRTMP中代码如下（libRTMP.c）：
URLProtocol ff_rtmp_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(RTMP), .flags = URL_PROTOCOL_FLAG_NETWORK, };
udp协议代码如下（udp.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), .flags = URL_PROTOCOL_FLAG_NETWORK, };
等号右边的函数是完成具体读写功能的函数。可以看一下file协议的几个函数（其实就是读文件，写文件这样的操作）（file.c）：
/* 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); }
FFmpeg 结构体学习(七)： AVIOContext 分析
标签：ret context standard caller 读取 有一个 support writing lang
原文地址：https://www.cnblogs.com/renhui/p/9494887.html
