static
enum mad_flow output(void *data,
             struct mad_header const *header,
             struct mad_pcm *pcm)
...

char fbuffer[8192];
unsigned int fsize;


int main(void)
{
  fsize=4000000; //Сколько-то там
  decode((unsigned char const *)fbuffer, fsize);
  return 0;
}

/*
* This is a private message structure. A generic pointer to this structure
* is passed to each of the callback functions. Put here any data you need
* to access from within the callbacks.
*/

struct buffer {
  unsigned char const *start;
  unsigned long length;
};

/*
* This is the input callback. The purpose of this callback is to (re)fill
* the stream buffer which is to be decoded. In this example, an entire file
* has been mapped into memory, so we just call mad_stream_buffer() with the
* address and length of the mapping. When this callback is called a second
* time, we are finished decoding.
*/

static
enum mad_flow input(void *data,
            struct mad_stream *stream)
{
  struct buffer *buffer = data;

  if (!buffer->length)
    return MAD_FLOW_STOP;

  fread(infile,fbuffer,sizeof(fbuffer));


  if (buffer->length>=sizeof(fbuffer))
  {
    mad_stream_buffer(stream, buffer->start, sizeof(fbuffer));
    buffer->length-=sizeof(fbuffer);
  }
  else
  {
    mad_stream_buffer(stream, buffer->start, buffer->length);
    buffer->length=0;
  }
  return MAD_FLOW_CONTINUE;
}