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FFmpeg源码：avio_seek函数分析

news 来源：原创 2024/10/1 6:27:09

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AVIOContext结构体和其相关的函数分析：

FFmpeg源码：avio_r8、avio_rl16、avio_rl24、avio_rl32、avio_rl64函数分析

FFmpeg源码：read_packet_wrapper、fill_buffer函数分析

FFmpeg源码：avio_read函数分析

FFmpeg源码：avio_seek函数分析

FFmpeg源码：avio_skip函数分析

FFmpeg源码：avio_tell函数分析

FFmpeg源码：ffurl_seek2、ffurl_seek、avio_size函数分析

FFmpeg源码：avio_feof函数分析

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一、avio_seek函数的声明

avio_seek函数声明在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的头文件libavformat/avio.h中：

/*** fseek() equivalent for AVIOContext.* @return new position or AVERROR.*/
int64_t avio_seek(AVIOContext *s, int64_t offset, int whence);

该函数可以看做是AVIOContext版本的fseek函数，根据传入形参whence的值的不同，avio_seek函数的作用也不同。

情况一：whence的值为SEEK_SET时，执行avio_seek函数后，如果执行成功，AVIOContext的文件位置指针将指向以文件开头为基准，偏移offset（指针偏移量）个字节的位置。

情况二：whence的值为SEEK_CUR时，执行avio_seek函数后，如果执行成功，AVIOContext的文件位置指针将指向以当前位置为基准，偏移offset（指针偏移量）个字节的位置。

情况三：whence的值为SEEK_END时，执行avio_seek函数后，如果执行成功，AVIOContext的文件位置指针将指向以文件结尾为基准，偏移offset（指针偏移量）个字节的位置。

形参s：既是输入型参数也是输出型参数。指向一个AVIOContext（字节流上下文结构体）变量。关于AVIOContext结构体可以参考：《FFmpeg源码：avio_r8、avio_rl16、avio_rl24、avio_rl32、avio_rl64函数分析》。

形参offset：输入型参数，表示偏移量。正数表示正向偏移，负数表示负向偏移。

形参whence：输入型参数，设定从文件的哪里开始偏移，可能取值为：

/* The possibilities for the third argument to `fseek'.These values should not be changed.  */
#define SEEK_SET	0	/* Seek from beginning of file.  */
#define SEEK_CUR	1	/* Seek from current position.  */
#define SEEK_END	2	/* Seek from end of file.  */

SEEK_SET：文件开头
SEEK_CUR：当前位置
SEEK_END：文件结尾

返回值：失败返回一个负数，成功返回偏移值。

二、avio_seek函数的使用例子

avio_seek(s,100,SEEK_SET)：把AVIOContext的文件位置指针移动到离文件开头100字节处；

avio_seek(s,100,SEEK_CUR)：把AVIOContext的文件位置指针移动到离文件当前位置100字节处；

avio_seek(s,-100,SEEK_END)：把AVIOContext的文件位置指针退回到离文件结尾100字节处。

三、avio_seek函数的定义

avio_seek函数定义在源文件libavformat/aviobuf.c中：

int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
{FFIOContext *const ctx = ffiocontext(s);int64_t offset1;int64_t pos;int force = whence & AVSEEK_FORCE;int buffer_size;int short_seek;whence &= ~AVSEEK_FORCE;if(!s)return AVERROR(EINVAL);if ((whence & AVSEEK_SIZE))return s->seek ? s->seek(s->opaque, offset, AVSEEK_SIZE) : AVERROR(ENOSYS);buffer_size = s->buf_end - s->buffer;// pos is the absolute position that the beginning of s->buffer corresponds to in the filepos = s->pos - (s->write_flag ? 0 : buffer_size);if (whence != SEEK_CUR && whence != SEEK_SET)return AVERROR(EINVAL);if (whence == SEEK_CUR) {offset1 = pos + (s->buf_ptr - s->buffer);if (offset == 0)return offset1;if (offset > INT64_MAX - offset1)return AVERROR(EINVAL);offset += offset1;}if (offset < 0)return AVERROR(EINVAL);short_seek = ctx->short_seek_threshold;if (ctx->short_seek_get) {int tmp = ctx->short_seek_get(s->opaque);short_seek = FFMAX(tmp, short_seek);}offset1 = offset - pos; // "offset1" is the relative offset from the beginning of s->buffers->buf_ptr_max = FFMAX(s->buf_ptr_max, s->buf_ptr);if ((!s->direct || !s->seek) &&offset1 >= 0 && offset1 <= (s->write_flag ? s->buf_ptr_max - s->buffer : buffer_size)) {/* can do the seek inside the buffer */s->buf_ptr = s->buffer + offset1;} else if ((!(s->seekable & AVIO_SEEKABLE_NORMAL) ||offset1 <= buffer_size + short_seek) &&!s->write_flag && offset1 >= 0 &&(!s->direct || !s->seek) &&(whence != SEEK_END || force)) {while(s->pos < offset && !s->eof_reached)fill_buffer(s);if (s->eof_reached)return AVERROR_EOF;s->buf_ptr = s->buf_end - (s->pos - offset);} else if(!s->write_flag && offset1 < 0 && -offset1 < buffer_size>>1 && s->seek && offset > 0) {int64_t res;pos -= FFMIN(buffer_size>>1, pos);if ((res = s->seek(s->opaque, pos, SEEK_SET)) < 0)return res;s->buf_end =s->buf_ptr = s->buffer;s->pos = pos;s->eof_reached = 0;fill_buffer(s);return avio_seek(s, offset, SEEK_SET | force);} else {int64_t res;if (s->write_flag) {flush_buffer(s);}if (!s->seek)return AVERROR(EPIPE);if ((res = s->seek(s->opaque, offset, SEEK_SET)) < 0)return res;ctx->seek_count++;if (!s->write_flag)s->buf_end = s->buffer;s->buf_ptr = s->buf_ptr_max = s->buffer;s->pos = offset;}s->eof_reached = 0;return offset;
}

四、avio_seek函数的内部实现原理

下面分情况讨论：

（一）whence的值为SEEK_SET

whence的值为SEEK_SET时，avio_seek函数可以化简为：

int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
{FFIOContext *const ctx = ffiocontext(s);int64_t offset1;int64_t pos;int buffer_size;if(!s)return AVERROR(EINVAL);buffer_size = s->buf_end - s->buffer;pos = s->pos - (s->write_flag ? 0 : buffer_size);if (offset < 0)return AVERROR(EINVAL);offset1 = offset - pos;s->buf_ptr_max = FFMAX(s->buf_ptr_max, s->buf_ptr);if ((!s->direct || !s->seek) && offset1 >= 0 && offset1 <= (s->write_flag ? s->buf_ptr_max - s->buffer : buffer_size)) {/* can do the seek inside the buffer */s->buf_ptr = s->buffer + offset1;}s->eof_reached = 0;return offset;
}

大部分情况下，pos的值为0：

    pos = s->pos - (s->write_flag ? 0 : buffer_size);

这时offset1 = offset - pos

= offset - 0

= offset：

    offset1 = offset - pos;

所以这时s->buf_ptr = s->buffer + offset1

= s->buffer + offset：

        s->buf_ptr = s->buffer + offset1;

所以大部分情况下，执行语句avio_seek(s->pb, offset, SEEK_SET)后，s->pb->buf_ptr的值会变为s->pb->buffer + offset。从而实现让AVIOContext的文件位置指针（s->pb->buf_ptr）指向以文件开头（s->pb->buffer）为基准，偏移offset（指针偏移量）个字节的位置。即把文件位置指针移动到离文件开头offset字节处。

（二）whence的值为SEEK_CUR

whence的值为SEEK_CUR时，avio_seek函数可以化简为：

int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
{FFIOContext *const ctx = ffiocontext(s);int64_t offset1;int64_t pos;int buffer_size;if(!s)return AVERROR(EINVAL);buffer_size = s->buf_end - s->buffer;pos = s->pos - (s->write_flag ? 0 : buffer_size);if (whence == SEEK_CUR) {offset1 = pos + (s->buf_ptr - s->buffer);if (offset == 0)return offset1;if (offset > INT64_MAX - offset1)return AVERROR(EINVAL);offset += offset1;}if (offset < 0)return AVERROR(EINVAL);offset1 = offset - pos;s->buf_ptr_max = FFMAX(s->buf_ptr_max, s->buf_ptr);if ((!s->direct || !s->seek) &&offset1 >= 0 && offset1 <= (s->write_flag ? s->buf_ptr_max - s->buffer : buffer_size)) {s->buf_ptr = s->buffer + offset1;} s->eof_reached = 0;return offset;
}

大部分情况下，pos的值为0：

    pos = s->pos - (s->write_flag ? 0 : buffer_size);

这时offset1 = s->buf_ptr - s->buffer。s->buf_ptr - s->buffer为AVIOContext输入缓冲区中当前读取到的位置距离输入缓冲区的开头的偏移。即文件位置指针当前位置（s->buf_ptr）相对于文件首（s->buffer）的偏移字节数：

        offset1 = pos + (s->buf_ptr - s->buffer);

所以这时offset = offset + offset1

= offset + (s->buf_ptr - s->buffer)：

offset += offset1;

这时让offset1 = offset - pos

= offset - 0

= offset

= 最开始的形参offset的值 + (s->buf_ptr - s->buffer) ：

offset1 = offset - pos;

所以这时s->buf_ptr = s->buffer + offset1

= s->buffer + 最开始的形参offset的值 + (s->buf_ptr - s->buffer)

= s->buf_ptr + 最开始的形参offset的值：

s->buf_ptr = s->buffer + offset1;

所以大部分情况下，执行语句avio_seek(s->pb, offset, SEEK_CUR)后，s->pb->buf_ptr的值会变为s->pb->buf_ptr+ offset。从而实现让AVIOContext的文件位置指针（s->pb->buf_ptr）指向以当前位置（s->pb->buf_ptr）为基准，偏移offset（指针偏移量）个字节的位置。即把文件位置指针移动到离文件当前位置offset字节处。