A terminal description has two capabilities, `co' and `li', that describe the screen size in columns and lines. But there is more to the question of screen size than this.
On some operating systems the "screen" is really a window and the
effective width can vary. On some of these systems, tgetnum
uses the actual width of the window to decide what value to return for
the `co' capability, overriding what is actually written in the
terminal description. On other systems, it is up to the application
program to check the actual window width using a system call. For
example, on BSD 4.3 systems, the system call ioctl with code
TIOCGWINSZ will tell you the current screen size.
On all window systems, termcap is powerless to advise the application
program if the user resizes the window. Application programs must
deal with this possibility in a system-dependent fashion. On some
systems the C shell handles part of the problem by detecting changes
in window size and setting the TERMCAP environment variable
appropriately. This takes care of application programs that are
started subsequently. It does not help application programs already
running.
On some systems, including BSD 4.3, all programs using a terminal get
a signal named SIGWINCH whenever the screen size changes.
Programs that use termcap should handle this signal by using
ioctl TIOCGWINSZ to learn the new screen size.
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