bionic (5) scr_dump.5.gz

Provided by: ncurses-bin_6.1-1ubuntu1.18.04.1_amd64 bug

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 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

SEE ALSO

       scr_dump(3NCURSES), util(3NCURSES).

AUTHORS

       Thomas E. Dickey
       extended screen-dump format for ncurses 6.0 (2015)

       Eric S. Raymond
       screen dump feature in ncurses 1.9.2d (1995)

                                                                                                     scr_dump(5)