Output colorization. Copyright 2011-2013 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1301, USA. Select Graphic Rendition (SGR, "\33[...m") strings. Also Erase in Line (EL) to Right ("\33[K") by default. Why have EL to Right after SGR? – The behavior of line-wrapping when at the bottom of the terminal screen and at the end of the current line is often such that a new line is introduced, entirely cleared with the current background color which may be different from the default one (see the boolean back_color_erase terminfo(5) capability), thus scrolling the display by one line. The end of this new line will stay in this background color even after reverting to the default background color with "\33[m', unless it is explicitly cleared again with "\33[K" (which is the behavior the user would instinctively expect from the whole thing). There may be some unavoidable background-color flicker at the end of this new line because of this (when timing with the monitor's redraw is just right). – The behavior of HT (tab, "\t") is usually the same as that of Cursor Forward Tabulation (CHT) with a default parameter of 1 ("\33[I"), i.e., it performs pure movement to the next tab stop, without any clearing of either content or screen attributes (including background color); try printf 'asdfqwerzxcv
' in a bash(1) shell to demonstrate this. This is not what the user would instinctively expect of HT (but is ok for CHT). The instinctive behavior would include clearing the terminal cells that are skipped over by HT with blank cells in the current screen attributes, including background color; the boolean dest_tabs_magic_smso terminfo(5) capability indicates this saner behavior for HT, but only some rare terminals have it (although it also indicates a special glitch with standout mode in the Teleray terminal for which it was initially introduced). The remedy is to add "\33K" after each SGR sequence, be it START (to fix the behavior of any HT after that before another SGR) or END (to fix the behavior of an HT in default background color that would follow a line-wrapping at the bottom of the screen in another background color, and to complement doing it after START). Piping GCC's output through a pager such as less(1) avoids any HT problems since the pager performs tab expansion.
Generic disadvantages of this remedy are: – Some very rare terminals might support SGR but not EL (nobody will use "gcc -fdiagnostics-color" on a terminal that does not support SGR in the first place). – Having these extra control sequences might somewhat complicate the task of any program trying to parse "gcc -fdiagnostics-color" output in order to extract structuring information from it. A specific disadvantage to doing it after SGR START is: – Even more possible background color flicker (when timing with the monitor's redraw is just right), even when not at the bottom of the screen. There are no additional disadvantages specific to doing it after SGR END.
It would be impractical for GCC to become a full-fledged terminal program linked against ncurses or the like, so it will not detect terminfo(5) capabilities.
