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core: use terminal_get_size_by_csi18 to query terminal size

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This allows us to query the window size without moving the cursor. We have
various reports about the cursor being in an unexpected position and/or state.

Closes https://github.com/systemd/systemd/issues/37526.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek
2025-12-18 10:47:19 +01:00
parent bfb8e9c575
commit d91a184509
1 changed files with 18 additions and 13 deletions
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src/basic/terminal-util.c
+18 -13
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@@ -2375,17 +2375,12 @@ int terminal_get_size_by_dsr(
assert(input_fd >= 0);
assert(output_fd >= 0);
/* Tries to determine the terminal dimension by means of ANSI sequences rather than TIOCGWINSZ
* ioctl(). Why bother with this? The ioctl() information is often incorrect on serial terminals
* (since there's no handshake or protocol to determine the right dimensions in RS232), but since the
* ANSI sequences are interpreted by the final terminal instead of an intermediary tty driver they
* should be more accurate.
/* Tries to determine the terminal dimension by means of ANSI sequences.
*
* Unfortunately there's no direct ANSI sequence to query terminal dimensions. But we can hack around
* it: we position the cursor briefly at an absolute location very far down and very far to the
* right, and then read back where we actually ended up. Because cursor locations are capped at the
* terminal width/height we should then see the right values. In order to not risk integer overflows
* in terminal applications we'll use INT16_MAX-1 as location to jump to — hopefully a value that is
* We position the cursor briefly at an absolute location very far down and very far to the right,
* and then read back where we actually ended up. Because cursor locations are capped at the terminal
* width/height we should then see the right values. In order to not risk integer overflows in
* terminal applications we'll use INT16_MAX-1 as location to jump to — hopefully a value that is
* large enough for any real-life terminals, but small enough to not overflow anything or be
* recognized as a "niche" value. (Note that the dimension fields in "struct winsize" are 16bit only,
* too). */
@@ -2650,8 +2645,13 @@ int terminal_fix_size(int input_fd, int output_fd) {
unsigned rows, columns;
int r;
/* Tries to update the current terminal dimensions to the ones reported via ANSI sequences */
/* Tries to update the current terminal dimensions to the ones reported via ANSI sequences.
*
* Why bother with this? The ioctl() information is often incorrect on serial terminals (since
* there's no handshake or protocol to determine the right dimensions in RS232), but since the ANSI
* sequences are interpreted by the final terminal instead of an intermediary tty driver they should
* be more accurate.
*/
r = terminal_verify_same(input_fd, output_fd);
if (r < 0)
return r;
@@ -2660,7 +2660,12 @@ int terminal_fix_size(int input_fd, int output_fd) {
if (ioctl(output_fd, TIOCGWINSZ, &ws) < 0)
return log_debug_errno(errno, "Failed to query terminal dimensions, ignoring: %m");
r = terminal_get_size_by_dsr(input_fd, output_fd, &rows, &columns);
r = terminal_get_size_by_csi18(input_fd, output_fd, &rows, &columns);
if (IN_SET(r, -EOPNOTSUPP, -EINVAL))
/* We get -EOPNOTSUPP if the query fails and -EINVAL when the received answer is invalid.
* Try the fallback method. It is more involved and moves the cursor, but seems to have wider
* support. */
r = terminal_get_size_by_dsr(input_fd, output_fd, &rows, &columns);
if (r < 0)
return log_debug_errno(r, "Failed to acquire terminal dimensions via ANSI sequences, not adjusting terminal dimensions: %m");