How to get the cursor position in a C program using termcap, without writing a character?

Question

I would like to know how to get the cursor position (x, y) in my program, without writing anything on the screen neither tracking it all the time.

I found out a way t

Answer 1

There are several problems:

• canonical mode is buffered (see below)

• the read is done on the file-descriptor for standard output (that may happen to work — sometimes — but don't count on it)

• the read does not read enough characters to get a typical response

• the response would have two decimal integers, separated by semicolon ;

• the response would have a final character (which would become an issue if the read actually asked for enough characters...)

Further reading:

• General Terminal Interface The Single UNIX ® Specification, Version 2

In canonical mode input processing, terminal input is processed in units of lines. A line is delimited by a newline character (NL), an end-of-file character (EOF), or an end-of-line (EOL) character. See Special Characters for more information on EOF and EOL. This means that a read request will not return until an entire line has been typed or a signal has been received. Also, no matter how many bytes are requested in the read() call, at most one line will be returned. It is not, however, necessary to read a whole line at once; any number of bytes, even one, may be requested in a read() without losing information.

• XTerm Control Sequences

    CSI Ps n  Device Status Report (DSR).
                Ps = 5  -> Status Report.
              Result ("OK") is CSI 0 n
                Ps = 6  -> Report Cursor Position (CPR) [row;column].
              Result is CSI r ; c R

That is, your program should be prepared to read Escape[ followed by two decimal integers (with no fixed limit on their length), and two other characters ; and R.

By the way, termcap by itself will do little for your solution. While ncurses has some relevant capabilities defined in the terminal database:

#       u9      terminal enquire string (equiv. to ANSI/ECMA-48 DA)
#       u8      terminal answerback description
#       u7      cursor position request (equiv. to VT100/ANSI/ECMA-48 DSR 6)
#       u6      cursor position report (equiv. to ANSI/ECMA-48 CPR)

few programs use those, and in any case you would find it difficult to use the cursor position report in a termcap application.