在C中为ffmpeg自定义实时输入

Question

I'm implementing a custom io with avformat_alloc_context and avio_alloc_context to be able to read the output of another function in real-time. A buffer is populated by this function in a boost asio thread, while ffmpeg is reading this buffer from another thread.

I initialise an io buffer where this function writes into, and which ffmpeg reads:

BufferData input_buffer = {0};
input_buffer.size = 65536;
input_buffer.ptr = (uint8_t *) av_malloc(buf_size);
memset(input_buffer.ptr,'0',100); 

fprintf(stdout, "initialisation: buffer pointer %p buffer data pointer: %p\n", &input_buffer, input_buffer.ptr);

Why i do the memset is explained here . Then to test out the pointer address I did:

BufferData * decode_buffer;
decode_buffer->size = 65536;
decode_buffer->ptr = (uint8_t *) av_malloc(decode_buffer->size);

AVIOContext * av_io_ctx = avio_alloc_context(decode_buffer->ptr, decode_buffer->size, 0, &input_buffer, &read_function, NULL, NULL);

AVFormatContext *av_fmt_ctx = avformat_alloc_context();
av_fmt_ctx->pb = av_io_ctx;


BufferData * tmpPtr = (BufferData * ) video_input_file->av_io_ctx->opaque;

fprintf(stdout, "video decoder before: buffer pointer %p, buffer data pointer: %p\n", tmpPtr, tmpPtr->ptr);

open_res = avformat_open_input(&av_fmt_ctx, "anyname", in_fmt, options ? &options : NULL);

fprintf(stdout, "video decoder after: buffer pointer %p, buffer data pointer: %p\n", tmpPtr, tmpPtr->ptr);

For reference

typedef struct {
    uint8_t *ptr;
    size_t size;
} BufferData;

The read function

static int read_function(void* opaque, uint8_t* buf, int buf_size) {
    BufferData *bd = (BufferData *) opaque;
    buf_size = FFMIN(buf_size, bd->size);
    memcpy(buf, bd->ptr, buf_size);
    bd->ptr  += buf_size; //This seemed to cause the problem
    bd->size -= buf_size;
    return buf_size;
}

And the result will be:

initialisation: buffer pointer 0x7f2c4a613620, buffer data pointer: 0x7f2c48c56040

video decoder before: buffer pointer 0x7f2c4a613620, buffer data pointer: 0x7f2c48c56040

video decoder after: buffer pointer 0x7f2c4a613620, buffer data pointer: 0x7f2c49e24b50

Is it a normal behaviour that the buffer data ptr is changed by avformat_open_input? since I would like to keep the initial pointer address given I am using it in another function, and have malloced it the required memory.

Answer 1

It was something to do with my read function, nothing to do with the avformat_open_input. I was writing over the pointer as per described here (see read_packet function). After thinking about it more yes it made sense to do, given the the read function should increment the pointer to the beginning of the next frame.