- number is outside the window. Note that ERR is distinct from
- <STRONG>TRUE</STRONG> and <STRONG>FALSE</STRONG>, which are the normal return values of this
- function.
+ number is outside the window.
+
+ The constant <STRONG>ERR</STRONG> is distinct from <STRONG>TRUE</STRONG> and <STRONG>FALSE</STRONG>, which are the
+ normal return values of this function. Because the function
+ returns a <STRONG>bool</STRONG>, returning <STRONG>ERR</STRONG> (which is neither <STRONG>TRUE</STRONG> nor <STRONG>FALSE</STRONG>)
+ may not be supported by the compiler.
+
+ To provide error-checking and also match the X/Open function
+ prototype, the <STRONG>ERR</STRONG> is provided by a macro named <STRONG>is_linetouched</STRONG>.
+ The actual function returns <STRONG>FALSE</STRONG> when it detects an error.