3

The "screen" window manager allows specifying the desired size of the scrollback buffer.

E.g. when starting a new session: (source)

‘-h num’
    Set the history scrollback buffer to be num lines high.
    Equivalent to the defscrollback command (see Copy).

Or when already inside a screen session with these commands: (source)

12.1.2 Scrollback
— Command: defscrollback num
           Same as the scrollback command except that the default
           setting for new windows is changed. Defaults to 100. 
— Command: scrollback num
           Set the size of the scrollback buffer for the current 
           window to num lines. The default scrollback is 100 lines.
           Use C-a i to view the current setting.

But I cannot seem to find documentation stating the maximum value for num for any of above approaches.

So the question is: how to determine the maximum scrollback length for the screen utility?

3

I'm not sure where to find it documented, but digging into the source a bit gives some clues. When you pass in -h it sets the histheight (see screen.c). In main it parses -h as follows:

case 'h':
    if (--argc == 0)
        exit_with_usage(myname, NULL, NULL);
    nwin_options.histheight = atoi(*++argv);
    if (nwin_options.histheight < 0)
        exit_with_usage(myname, "-h: %s: negative scrollback size?", *argv);
    break;

The nwin_options struct is an instance of NewWindow which is defined in window.h:

struct NewWindow {
    int StartAt;    /* where to start the search for the slot */
    char    *aka;       /* aka string */
    char    **args;     /* argv vector */
    char    *dir;       /* directory for chdir */
    char    *term;      /* TERM to be set instead of "screen" */
    bool    aflag;
    bool    dynamicaka;
    int flowflag;
    int lflag;
    int histheight;
    int monitor;
    int wlock;      /* default writelock setting */
    int silence;
    bool    wrap;
    bool    Lflag;      /* logging */
    int slow;       /* inter character milliseconds */
    int gr;
    bool    c1;
    int bce;
    int encoding;
    char    *hstatus;
    char    *charset;
    int poll_zombie_timeout;
};

where we can see that histheight is an int, so presumably the max value you can set it to is maxint for a signed int.

  • :-) crosspost. Thanks Eric, yous is a bit more factual answer. – Paul van Leeuwen Jun 24 '17 at 11:16
  • It seems the maximum value can differ per system. Here is a Q&A that deals with actually determining it on a system. – Paul van Leeuwen Jun 24 '17 at 11:19
  • 1
    @PaulvanLeeuwen and make sure you're using the same compiler as screen was compiled with. For example, if you did the test with a 64-bit compiler, but screen was compiled with 32-bit one, your value would be way off – Eric Renouf Jun 24 '17 at 11:21
1

While trying to answer my own question, this is what I found on my own system via trial-and-error:

answer: there is a hard-limit (somewhere between 50,000,000 and 1,000,000,000), but performance will probably be your bottleneck (as such I was not able to determine the exact hard-limit)

My experiments included:

Start a new screen session without having any ~/.screenrc configuration file:

screen -a

Inside screen open the screen command prompt by pressing ctrl+a,: and enter the command:

scrollback 1000000

which resulted in the message: scrollback set to 1000000 (1,000,000).

Trying scrollback 1000000000 (1,000,000,000) quite quickly resulted in the message scrollback set to 0. I assume this means that 1,000,000 is accepted and 1,000,000,000 is too much.

Trying scrollback100000000` (100,000,000) made screen hang. A second terminal session and some patience later I was able to kill screen. This behavior turned out to be consistent when trying the same thing again.

Trying scrollback 10000000 (10,000,000) with 2 seconds delay resulted in the message scrollback set to 10000000.

Trying scrollback 50000000 (50,000,000) with 60 seconds delay resulted in the message scrollback set to 50000000.

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