Ubuntu Manpage: scr_dump - format of curses screen-dumps.

NAME

       scr_dump - format of curses screen-dumps.

SYNOPSIS

       scr_dump

DESCRIPTION

       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 SystemV, but does not write a “magic number” to identify the file
       format.

PORTABILITY

There is no standard format for putwin. This section gives a brief description of the
existing formats.

X/Open Curses
Refer to X/Open Curses, Issue 7 (2009).

X/Open's documentation for enhanced curses says only:

The getwin( ) function reads window-related data stored in the file by putwin( ). The
function then creates and initializes a new window using that data.

The putwin( ) function writes all data associated with win into the stdio(3) stream to
which filep points, using an unspecified format. This information can be retrieved
later using getwin( ).

In the mid-1990s when the X/Open Curses document was written, there were still systems
using older, less capable curses libraries (aside from the BSD curses library which was
not relevant to X/Open because it did not meet the criteria for base curses). The
document explained the term “enhanced” as follows:

• Shading is used to identify X/Open Enhanced Curses material, relating to interfaces
included to provide enhanced capabilities for applications originally written to be
compiled on systems based on the UNIX operating system. Therefore, the features
described may not be present on systems that conform to XPG4 or to earlier XPG
releases. The relevant reference pages may provide additional or more specific
portability warnings about use of the material.

In the foregoing, emphasis was added to unspecified format and to XPG4 or to earlier XPG
releases, for clarity.

Unix SystemV
Unix SystemV curses identified the file format by writing a “magic number” at the
beginning of the dump. The WINDOW data and the lines of text follow, all in binary form.

The Solaris curses source has these definitions:

/* terminfo magic number */
#define MAGNUM 0432

/* curses screen dump magic number */
#define SVR2_DUMP_MAGIC_NUMBER 0433
#define SVR3_DUMP_MAGIC_NUMBER 0434

That is, the feature was likely introduced in SVr2 (1984), and improved in SVr3 (1987).
The Solaris curses source has no magic number for SVr4 (1989). Other operating systems
(AIX and HPUX) use a magic number which would correspond to this definition:

/* curses screen dump magic number */
#define SVR4_DUMP_MAGIC_NUMBER 0435

That octal number in bytes is 001, 035. Because most Unix vendors use big-endian
hardware, the magic number is written with the high-order byte first, e.g.,

01 35

After the magic number, the WINDOW structure and line-data are written in binary format.
While the magic number used by the Unix systems can be seen using od(1), none of the Unix
systems documents the format used for screen-dumps.

The Unix systems do not use identical formats. While collecting information for for this
manual page, the savescreen test-program produced dumps of different size (all on 64-bit
hardware, on 40x80 screens):

• AIX (51817 bytes)

• HPUX (90093 bytes)

• Solaris 10 (13273 bytes)

• ncurses5 (12888 bytes)

Solaris
As noted above, Solaris curses has no magic number corresponding to SVr4 curses. This is
odd since Solaris was the first operating system to pass the SVr4 guidelines. Solaris has
two versions of curses:

• The default curses library uses the SVr3 magic number.

• There is an alternate curses library in /usr/xpg4. This uses a textual format with no
magic number.

According to the copyright notice, the xpg4 Solaris curses library was developed by
MKS (Mortice Kern Systems) from 1990 to 1995.

Like ncurses6, there is a file-header with parameters. Unlike ncurses6, the contents
of the window are written piecemeal, with coordinates and attributes for each chunk of
text rather than writing the whole window from top to bottom.

PDCurses
PDCurses added support for screen dumps in version 2.7 (2005). Like Unix SystemV and
ncurses5, it writes the WINDOW structure in binary, but begins the file with its three-
byte identifier “PDC”, followed by a one-byte version, e.g.,

“PDC\001”

NetBSD
As of April 2017, NetBSD curses does not support scr_dump and scr_restore (or scr_init,
scr_set), although it has putwin and getwin.

Like ncurses5, NetBSD putwin does not identify its dumps with a useful magic number. It
writes

• the curses shared library major and minor versions as the first two bytes (e.g., 7 and
1),

• followed by a binary dump of the WINDOW,

• some data for wide-characters referenced by the WINDOW structure, and

• finally, lines as done by other implementations.

EXAMPLE

       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

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