#define PTB_INIT(b) int _ptb_len=0,_ptb_left=sizeof(b)-1; char * _ptb_ptr=b;
#define PTB_ADD(a...) do { int _ptb_writ=snprintf(_ptb_ptr,_ptb_left,a ); if (_ptb_writ >=0 ) { _ptb_left-=_ptb_writ; _ptb_len+=_ptb_writ; _ptb_ptr+=_ptb_writ; } } while (0)
#define PTB_LEN _ptb_len