Standard input and output are connected to a terminal that implements ANSI escape sequences, but is of unknown dimensions.
I need to know how big the terminal so to facilitate drawing a full-screen text UI on it. How can I get the size?
The correct size is not loaded into environment variables. I cannot use TIOCGETS; the the call would return success but the values are not correct -- the kernel doesn't know the size either.
There are lots and lots of answers searching stackoverflow, but they all depend on the OS providing the answer one way or anther; but this time that is not true.
The best clue I can find is the DSR command which returns the current cursor position; but there's no move to bottom/right command.
The resize program does this by moving the cursor to a very large column and row; the terminal moves as far as it can, e.g.,
CUP 999 999
Then resize asks where the cursor is:
DSR 6
The terminal replies with the actual cursor position (i.e., the cursor position report CPR), from which resize knows the terminal's size: the cursor is on the lower-right corner.
That's all done using standard (ECMA-48 / VT100) escape sequences. In XTerm Control Sequences (which should apply to your "ANSI" terminal)
CSI Ps n Device Status Report (DSR).
Ps = 6 -> Report Cursor Position (CPR) [row;column].
Result is CSI r ; c R