1.avio接口

const char *avio_find_protocol_name(const char *url);
int avio_check(const char *url, int flags);
int avpriv_io_move(const char *url_src, const char *url_dst);
int avpriv_io_delete(const char *url);
int avio_open_dir(AVIODirContext **s, const char *url, AVDictionary **options);
int avio_read_dir(AVIODirContext *s, AVIODirEntry **next);
int avio_close_dir(AVIODirContext **s);
void avio_free_directory_entry(AVIODirEntry **entry);
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));
void avio_context_free(AVIOContext **s);

void avio_w8(AVIOContext *s, int b);
void avio_write(AVIOContext *s, const unsigned char *buf, int size);
void avio_wl64(AVIOContext *s, uint64_t val);
void avio_wb64(AVIOContext *s, uint64_t val);
void avio_wl32(AVIOContext *s, unsigned int val);
void avio_wb32(AVIOContext *s, unsigned int val);
void avio_wl24(AVIOContext *s, unsigned int val);
void avio_wb24(AVIOContext *s, unsigned int val);
void avio_wl16(AVIOContext *s, unsigned int val);
void avio_wb16(AVIOContext *s, unsigned int val);
int avio_put_str(AVIOContext *s, const char *str);
int avio_put_str16le(AVIOContext *s, const char *str);
int avio_put_str16be(AVIOContext *s, const char *str);
void avio_write_marker(AVIOContext *s, int64_t time, enum AVIODataMarkerType type);
int64_t avio_seek(AVIOContext *s, int64_t offset, int whence);
int64_t avio_skip(AVIOContext *s, int64_t offset);
static av_always_inline int64_t avio_tell(AVIOContext *s);
int64_t avio_size(AVIOContext *s);
int avio_feof(AVIOContext *s);
void avio_flush(AVIOContext *s);

int avio_read(AVIOContext *s, unsigned char *buf, int size);
int avio_read_partial(AVIOContext *s, unsigned char *buf, int size);
int          avio_r8  (AVIOContext *s);
unsigned int avio_rl16(AVIOContext *s);
unsigned int avio_rl24(AVIOContext *s);
unsigned int avio_rl32(AVIOContext *s);
uint64_t     avio_rl64(AVIOContext *s);
unsigned int avio_rb16(AVIOContext *s);
unsigned int avio_rb24(AVIOContext *s);
unsigned int avio_rb32(AVIOContext *s);
uint64_t     avio_rb64(AVIOContext *s);
int avio_get_str(AVIOContext *pb, int maxlen, char *buf, int buflen);
int avio_get_str16le(AVIOContext *pb, int maxlen, char *buf, int buflen);
int avio_get_str16be(AVIOContext *pb, int maxlen, char *buf, int buflen);

int avio_open(AVIOContext **s, const char *url, int flags);
int avio_open2(AVIOContext **s, const char *url, int flags,
               const AVIOInterruptCB *int_cb, AVDictionary **options);
int avio_close(AVIOContext *s);
int avio_closep(AVIOContext **s);
int avio_open_dyn_buf(AVIOContext **s);
int avio_get_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
const char *avio_enum_protocols(void **opaque, int output);
int     avio_pause(AVIOContext *h, int pause);
int64_t  avio_seek_time(AVIOContext *h, int stream_index,
                       int64_t timestamp, int flags);
int avio_read_to_bprint(AVIOContext *h, struct AVBPrint *pb, size_t max_size);
int avio_accept(AVIOContext *s, AVIOContext **c);
int avio_handshake(AVIOContext *c);

2.avio结构

typedef struct AVIODirEntry {
    char *name;                           /**< Filename */
    int type;                             /**< Type of the entry */
    int utf8;                             /**< Set to 1 when name is encoded with UTF-8, 0 otherwise.
                                               Name can be encoded with UTF-8 even though 0 is set. */
    int64_t size;                         /**< File size in bytes, -1 if unknown. */
    int64_t modification_timestamp;       /**< Time of last modification in microseconds since unix
                                               epoch, -1 if unknown. */
    int64_t access_timestamp;             /**< Time of last access in microseconds since unix epoch,
                                               -1 if unknown. */
    int64_t status_change_timestamp;      /**< Time of last status change in microseconds since unix
                                               epoch, -1 if unknown. */
    int64_t user_id;                      /**< User ID of owner, -1 if unknown. */
    int64_t group_id;                     /**< Group ID of owner, -1 if unknown. */
    int64_t filemode;                     /**< Unix file mode, -1 if unknown. */
} AVIODirEntry;

typedef struct AVIODirContext {
    struct URLContext *url_context;
} AVIODirContext;


typedef struct AVIOContext {
    /**
     * 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.
     */
    const AVClass *av_class;

    /*
     * The following shows the relationship between buffer, buf_ptr,
     * buf_ptr_max, buf_end, buf_size, and pos, when reading and when writing
     * (since AVIOContext is used for both):
     *
     **********************************************************************************
     *                                   READING
     **********************************************************************************
     *
     *                            |              buffer_size              |
     *                            |---------------------------------------|
     *                            |                                       |
     *
     *                         buffer          buf_ptr       buf_end
     *                            +---------------+-----------------------+
     *                            |/ / / / / / / /|/ / / / / / /|         |
     *  read buffer:              |/ / consumed / | to be read /|         |
     *                            |/ / / / / / / /|/ / / / / / /|         |
     *                            +---------------+-----------------------+
     *
     *                                                         pos
     *              +-------------------------------------------+-----------------+
     *  input file: |                                           |                 |
     *              +-------------------------------------------+-----------------+
     *
     *
     **********************************************************************************
     *                                   WRITING
     **********************************************************************************
     *
     *                             |          buffer_size                 |
     *                             |--------------------------------------|
     *                             |                                      |
     *
     *                                                buf_ptr_max
     *                          buffer                 (buf_ptr)       buf_end
     *                             +-----------------------+--------------+
     *                             |/ / / / / / / / / / / /|              |
     *  write buffer:              | / / to be flushed / / |              |
     *                             |/ / / / / / / / / / / /|              |
     *                             +-----------------------+--------------+
     *                               buf_ptr can be in this
     *                               due to a backward seek
     *
     *                            pos
     *               +-------------+----------------------------------------------+
     *  output file: |             |                                              |
     *               +-------------+----------------------------------------------+
     *
     */
    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 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;

    /**
     * avio_read and avio_write should if possible be satisfied directly
     * instead of going through a buffer, and avio_seek will always
     * call the underlying seek function directly.
     */
    int direct;

    /**
     * Bytes read statistic
     * This field is internal to libavformat and access from outside is not allowed.
     */
    int64_t bytes_read;

    /**
     * seek statistic
     * This field is internal to libavformat and access from outside is not allowed.
     */
    int seek_count;

    /**
     * writeout statistic
     * This field is internal to libavformat and access from outside is not allowed.
     */
    int writeout_count;

    /**
     * Original buffer size
     * used internally after probing and ensure seekback to reset the buffer size
     * This field is internal to libavformat and access from outside is not allowed.
     */
    int orig_buffer_size;

    /**
     * Threshold to favor readahead over seek.
     * This is current internal only, do not use from outside.
     */
    int short_seek_threshold;

    /**
     * ',' separated list of allowed protocols.
     */
    const char *protocol_whitelist;

    /**
     * ',' separated list of disallowed protocols.
     */
    const char *protocol_blacklist;

    /**
     * A callback that is used instead of write_packet.
     */
    int (*write_data_type)(void *opaque, uint8_t *buf, int buf_size,
                           enum AVIODataMarkerType type, int64_t time);
    /**
     * If set, don't call write_data_type separately for AVIO_DATA_MARKER_BOUNDARY_POINT,
     * but ignore them and treat them as AVIO_DATA_MARKER_UNKNOWN (to avoid needlessly
     * small chunks of data returned from the callback).
     */
    int ignore_boundary_point;

    /**
     * Internal, not meant to be used from outside of AVIOContext.
     */
    enum AVIODataMarkerType current_type;
    int64_t last_time;

    /**
     * A callback that is used instead of short_seek_threshold.
     * This is current internal only, do not use from outside.
     */
    int (*short_seek_get)(void *opaque);

    int64_t written;

    /**
     * Maximum reached position before a backward seek in the write buffer,
     * used keeping track of already written data for a later flush.
     */
    unsigned char *buf_ptr_max;

    /**
     * Try to buffer at least this amount of data before flushing it
     */
    int min_packet_size;
} AVIOContext;

3.测试demo

#include "libavformat/avio.h"
//遍历支持的protocol
int show_protocols()
{
    void *opaque = NULL;
    const char *name;

    printf("Supported file protocols:\n"
        "Input:\n");
    while ((name = avio_enum_protocols(&opaque, 0)))
        printf("  %s\n", name);
    printf("Output:\n");
    while ((name = avio_enum_protocols(&opaque, 1)))
        printf("  %s\n", name);
    return 0;
}

int main()
{
    char* srcName = "src1.mp4";
    char* dstName = "test.mp4";
    AVIOContext* pReadCtx = NULL;
    AVIOContext* pWriteCtx = NULL;
    unsigned char buf[1024];
    int nRet = 0;

    show_protocols();
    
    char* protocolName = avio_find_protocol_name(srcName);
    printf("protocol name: %s\n", protocolName);

    nRet = avio_open(&pReadCtx, srcName, AVIO_FLAG_READ);
    nRet = avio_open(&pWriteCtx, dstName, AVIO_FLAG_WRITE);

    int64_t srcSize = avio_size(pReadCtx);
    printf("srcSize: %lld\n", srcSize);

    nRet = avio_read(pReadCtx, buf, 1024);

    avio_write(pWriteCtx, buf, 1024);

    nRet = avio_close(pReadCtx);
    nRet = avio_close(pWriteCtx);
    
    AVIOContext* pWriteOnlyBufCtx = NULL;
    uint8_t* pWriteBuf = NULL;
    uint8_t* pWriteBuf1 = NULL;

    nRet = avio_open_dyn_buf(&pWriteOnlyBufCtx);
    avio_write(pWriteOnlyBufCtx, buf, 1024);
    //int nSize = avio_get_dyn_buf(pWriteOnlyBufCtx, &pWriteBuf);
    nRet = avio_close_dyn_buf(pWriteOnlyBufCtx, &pWriteBuf1);
    

    av_free(pWriteBuf1);

    printf("end\n");
    return 0;
}

4.找文件protocol类型

//libavformat/protocols.c
const URLProtocol **ffurl_get_protocols(const char *whitelist,
                                        const char *blacklist)
{
    const URLProtocol **ret;
    int i, ret_idx = 0;
    //url_protocols数组中保存了支持的Protocol类型
    ret = av_mallocz_array(FF_ARRAY_ELEMS(url_protocols), sizeof(*ret));
    if (!ret)
        return NULL;

    for (i = 0; url_protocols[i]; i++) {
        const URLProtocol *up = url_protocols[i];

        if (whitelist && *whitelist && !av_match_name(up->name, whitelist))
            continue;
        if (blacklist && *blacklist && av_match_name(up->name, blacklist))
            continue;

        ret[ret_idx++] = up;
    }

    return ret;
}

static const struct URLProtocol *url_find_protocol(const char *filename)
{
    const URLProtocol **protocols;
    char proto_str[128], proto_nested[128], *ptr;
    size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
    int i;

    if (filename[proto_len] != ':' &&
        (strncmp(filename, "subfile,", 8) || !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)));

    av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));
    if ((ptr = strchr(proto_nested, '+')))
        *ptr = '\0';

    protocols = ffurl_get_protocols(NULL, NULL);
    if (!protocols)
        return NULL;
    for (i = 0; protocols[i]; i++) {
            const URLProtocol *up = protocols[i];
        if (!strcmp(proto_str, up->name)) {
            av_freep(&protocols);
            return up;
        }
        if (up->flags & URL_PROTOCOL_FLAG_NESTED_SCHEME &&
            !strcmp(proto_nested, up->name)) {
            av_freep(&protocols);
            return up;
        }
    }
    av_freep(&protocols);

    return NULL;
}

url_protocols数组定义在libavformat/protocol_list.c中

static const URLProtocol *url_protocols[] = {
    &ff_async_protocol,
    &ff_bluray_protocol,
    &ff_cache_protocol,
    &ff_concat_protocol,
    &ff_crypto_protocol,
    &ff_data_protocol,
    &ff_ffrtmpcrypt_protocol,
    &ff_ffrtmphttp_protocol,
    &ff_file_protocol,
    &ff_ftp_protocol,
    &ff_gopher_protocol,
    &ff_hls_protocol,
    &ff_http_protocol,
    &ff_httpproxy_protocol,
    &ff_https_protocol,
    &ff_icecast_protocol,
    &ff_mmsh_protocol,
    &ff_mmst_protocol,
    &ff_md5_protocol,
    &ff_pipe_protocol,
    &ff_prompeg_protocol,
    &ff_rtmp_protocol,
    &ff_rtmpe_protocol,
    &ff_rtmps_protocol,
    &ff_rtmpt_protocol,
    &ff_rtmpte_protocol,
    &ff_rtmpts_protocol,
    &ff_rtp_protocol,
    &ff_srtp_protocol,
    &ff_subfile_protocol,
    &ff_tee_protocol,
    &ff_tcp_protocol,
    &ff_tls_protocol,
    &ff_udp_protocol,
    &ff_udplite_protocol,
    &ff_libssh_protocol,
    NULL };