scr_dump - curses screen dump

The curses library provides applications with the ability to write the contents of a window to an external file using scr_dump or putwin, and read it back using scr_restore or getwin.

The putwin and getwin functions do the work; while scr_dump and scr_restore conveniently save and restore the whole screen, i.e., stdscr.

ncurses6

A longstanding implementation of screen-dump was revised with ncurses6 to remedy problems with the earlier approach:

• A magic number is written to the beginning of the dump file, allowing applications (such as file(1)) to recognize curses dump files.

  Because ncurses6 uses a new format, that requires a new magic number was unused by other applications. This 16-bit number was unused:

  0x8888 (octal \210\210)

  but to be more certain, this 32-bit number was chosen:

  0x88888888 (octal \210\210\210\210)

  This is the pattern submitted to the maintainers of the file program:

  #
  # ncurses5 (and before) did not use a magic number,
  # making screen dumps "data".
  #
  # ncurses6 (2015) uses this format, ignoring byte-order
  0    string    \210\210\210\210ncurses    ncurses6 screen image
  #

• The screen dumps are written in textual form, so that internal data sizes are not directly related to the dump-format, and enabling the library to read dumps from either narrow- or wide-character- configurations.

  The narrow library configuration holds characters and video attributes in a 32-bit chtype, while the wide-character library stores this information in the cchar_t structure, which is much larger than 32-bits.

• It is possible to read a screen dump into a terminal with a different screen-size, because the library truncates or fills the screen as necessary.

• The ncurses6 getwin reads the legacy screen dumps from ncurses5.

ncurses5 (Legacy)

The screen-dump feature was added to ncurses in June 1995. While there were fixes and improvements in succeeding years, the basic scheme was unchanged:

• The WINDOW structure was written in binary form.

• The WINDOW structure refers to lines of data, which were written as an array of binary data following the WINDOW.

• When getwin restored the window, it would keep track of offsets into the array of line-data and adjust the WINDOW structure which was read back into memory.

This is similar to Unix System V, but does not write a magic number to identify the file format.

Given a simple program which writes text to the screen (and for the sake of example, limiting the screen-size to 10x20):

#include <curses.h>

int
main(void)
{
    putenv("LINES=10");
    putenv("COLUMNS=20");
    initscr();
    start_color();
    init_pair(1, COLOR_WHITE, COLOR_BLUE);
    init_pair(2, COLOR_RED, COLOR_BLACK);
    bkgd(COLOR_PAIR(1));
    move(4, 5);
    attron(A_BOLD);
    addstr("Hello");
    move(5, 5);
    attroff(A_BOLD);
    attrset(A_REVERSE | COLOR_PAIR(2));
    addstr("World!");
    refresh();
    scr_dump("foo.out");
    endwin();
    return 0;
}

When run using ncurses6, the output looks like this:

\210\210\210\210ncurses 6.0.20170415
_cury=5
_curx=11
_maxy=9
_maxx=19
_flags=14
_attrs=\{REVERSE|C2}
flag=_idcok
_delay=-1
_regbottom=9
_bkgrnd=\{NORMAL|C1}\s
rows:
1:\{NORMAL|C1}\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
2:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
3:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
4:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
5:\s\s\s\s\s\{BOLD}Hello\{NORMAL}\s\s\s\s\s\s\s\s\s\s
6:\s\s\s\s\s\{REVERSE|C2}World!\{NORMAL|C1}\s\s\s\s\s\s\s\s\s
7:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
8:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
9:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s
10:\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s\s

The first four octal escapes are actually nonprinting characters, while the remainder of the file is printable text. You may notice:

• The actual color pair values are not written to the file.

• All characters are shown in printable form; spaces are \s to ensure they are not overlooked.

• Attributes are written in escaped curly braces, e.g., \{BOLD}, and may include a color pair (C1 or C2 in this example).

• The parameters in the header are written out only if they are nonzero. When reading back, order does not matter.

Running the same program with Solaris xpg4 curses gives this dump:

MAX=10,20
BEG=0,0
SCROLL=0,10
VMIN=1
VTIME=0
FLAGS=0x1000
FG=0,0
BG=0,0,
0,0,0,1,
0,19,0,0,
1,0,0,1,
1,19,0,0,
2,0,0,1,
2,19,0,0,
3,0,0,1,
3,19,0,0,
4,0,0,1,
4,5,0x20,0,Hello
4,10,0,1,
4,19,0,0,
5,0,0,1,
5,5,0x4,2,World!
5,11,0,1,
5,19,0,0,
6,0,0,1,
6,19,0,0,
7,0,0,1,
7,19,0,0,
8,0,0,1,
8,19,0,0,
9,0,0,1,
9,19,0,0,
CUR=11,5

Solaris getwin requires that all parameters are present, and in the same order. The xpg4 curses library does not know about the bce (back color erase) capability, and does not color the window background.

On the other hand, the SVr4 curses library does know about the background color. However, its screen dumps are in binary. Here is the corresponding dump (using od -t x1):

0000000 1c 01 c3 d6 f3 58 05 00 0b 00 0a 00 14 00 00 00
0000020 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00
0000040 00 00 b8 1a 06 08 cc 1a 06 08 00 00 09 00 10 00
0000060 00 00 00 80 00 00 20 00 00 00 ff ff ff ff 00 00
0000100 ff ff ff ff 00 00 00 00 20 80 00 00 20 80 00 00
0000120 20 80 00 00 20 80 00 00 20 80 00 00 20 80 00 00
*
0000620 20 80 00 00 20 80 00 00 20 80 00 00 48 80 00 04
0000640 65 80 00 04 6c 80 00 04 6c 80 00 04 6f 80 00 04
0000660 20 80 00 00 20 80 00 00 20 80 00 00 20 80 00 00
*
0000740 20 80 00 00 20 80 00 00 20 80 00 00 57 00 81 00
0000760 6f 00 81 00 72 00 81 00 6c 00 81 00 64 00 81 00
0001000 21 00 81 00 20 80 00 00 20 80 00 00 20 80 00 00
0001020 20 80 00 00 20 80 00 00 20 80 00 00 20 80 00 00
*
0001540 20 80 00 00 20 80 00 00 00 00 f6 d1 01 00 f6 d1
0001560 08 00 00 00 40 00 00 00 00 00 00 00 00 00 00 07
0001600 00 04 00 01 00 01 00 00 00 01 00 00 00 00 00 00
0001620 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0002371

scr_dump(3NCURSES), util(3NCURSES)