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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。 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)) && 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_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。 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： 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：https://blog.csdn.net/leixiaohua1020/article/details/14215369 原文链接：https://blog.csdn.net/guxinkobe/article/details/100514689 |
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