Paul Eggert <egg...@cs.ucla.edu> writes:
>> +After printing each page, print an alert (bell) to standard error and
>> +wait for a newline to be read from @file{/dev/tty} before printing the
>> +next page.
>
> This sentence should start "Before" not "After", with the rest of the
> sentence changed accordingly. There is no bell printed after the last
> page.
>
> Looking at the code, it seems that this mistake has leaked into the
> code as well, in that sometimes 'pr' will print an alert and wait for
> '\n' at end of file, when there is no page to print. If I'm right,
> that needs to be fixed; pr should wait for '\n' only if it will
> actually print something afterward.
Fixing this required a bit more understanding of the code on my part,
but I think the attached patch should address it.
The existing code uses a 'pad_vertically' variable to keep track of
whether the current page has text or a header that needs to be
printed. Therefore, any time we set it to true, we must also pause. But
only once, at the start of the page.
Collin
>From 8afb79bbf294687fd4bdd3c92d1599f5021f703a Mon Sep 17 00:00:00 2001
Message-ID: <8afb79bbf294687fd4bdd3c92d1599f5021f703a.1753850358.git.collin.fu...@gmail.com>
From: Collin Funk <collin.fu...@gmail.com>
Date: Sun, 27 Jul 2025 15:00:15 -0700
Subject: [PATCH v5] pr: implement '-p' and modify '-f' conforming to POSIX
* src/pr.c (last_paused_page_number, pause_option, pause_on_first_page)
(tty_fd): New variables.
(short_options): Add '-p'.
(long_options): Add '--pause'.
(main): Add the option. Open a file descriptor to /dev/tty. Disable
pausing if POSIXLY_CORRECT is set.
(pause_maybe): New function.
(read_line, print_stored): Call it whenever pad_vertically is set to
true.
(usage): Mention the new option.
* doc/coreutils.texi (pr invocation): Document the new option. Document
the behaviors of POSIXLY_CORRECT on -p and -f.
* NEWS: Mention the new option. Mention the new behavior of 'pr -f' with
the POSIXLY_CORRECT environment variable set.
---
NEWS | 7 ++++
doc/coreutils.texi | 17 ++++++++++
src/pr.c | 80 ++++++++++++++++++++++++++++++++++++++++++++--
3 files changed, 101 insertions(+), 3 deletions(-)
diff --git a/NEWS b/NEWS
index 0b2be7116..3034ce718 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,9 @@ GNU coreutils NEWS -*- outline -*-
'factor' is now much faster at identifying large prime numbers,
and significantly faster on composite numbers greater than 2^128.
+ 'pr -f' with the POSIXLY_CORRECT environment variable set will pause
+ until a newline is read from /dev/tty before printing the first page.
+
** Bug fixes
cksum was not compilable by Apple LLVM 10.0.0 x86-64, which
@@ -64,6 +67,10 @@ GNU coreutils NEWS -*- outline -*-
Iranian locale (fa_IR) and for the Ethiopian locale (am_ET), and also
does so more consistently for the Thailand locale (th_TH.UTF-8).
+ pr now supports the -p option, to pause before printing each page
+ until a newline character is read from /dev/tty, as required by POSIX.
+ The corresponding long option is --pause.
+
* Noteworthy changes in release 9.7 (2025-04-09) [stable]
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 40ecf3126..5df11f910 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -2726,6 +2726,11 @@ @node pr invocation
Use a form feed instead of newlines to separate output pages. This does
not alter the default page length of 66 lines.
+@vindex POSIXLY_CORRECT
+This option will also pause until a newline is read from @file{/dev/tty}
+before printing the first page if the @env{POSIXLY_CORRECT} environment
+variable is set and standard output is associated with a terminal.
+
@item -h @var{header}
@itemx --header=@var{header}
@opindex -h
@@ -2825,6 +2830,18 @@ @node pr invocation
set with the @option{-W/-w} option. A limited overflow may occur with
numbered single column output (compare @option{-n} option).
+@item -p
+@itemx --pause
+@opindex -p
+@opindex --pause
+Before printing each page, print an alert (bell) to standard error and
+wait for a newline to be read from @file{/dev/tty} before printing the
+next page.
+
+@vindex POSIXLY_CORRECT
+This option is ignored if the @env{POSIXLY_CORRECT} environment variable
+is set and standard output is not associated with a terminal.
+
@item -r
@itemx --no-file-warnings
@opindex -r
diff --git a/src/pr.c b/src/pr.c
index e7081a059..5d945910c 100644
--- a/src/pr.c
+++ b/src/pr.c
@@ -419,6 +419,7 @@ typedef struct COLUMN COLUMN;
static int char_to_clump (char c);
static bool read_line (COLUMN *p);
+static void pause_maybe (void);
static bool print_page (void);
static bool print_stored (COLUMN *p);
static bool open_file (char *name, COLUMN *p);
@@ -618,6 +619,9 @@ static int files_ready_to_read = 0;
/* Current page number. Displayed in header. */
static uintmax_t page_number;
+/* The last page number that we paused on. */
+static uintmax_t last_paused_page_number;
+
/* Current line number. Displayed when -n flag is specified.
When printing files in parallel (-m flag), line numbering is as follows:
@@ -711,6 +715,15 @@ static char *custom_header;
/* (-D) Date format for the header. */
static char const *date_format;
+/* If true, pause before each page until a newline is read from /dev/tty. */
+static bool pause_option;
+
+/* If true, pause before the first page. */
+static bool pause_on_first_page;
+
+/* Used to read from /dev/tty if --p/--pause is in use. */
+static int tty_fd;
+
/* The local time zone rules, as per the TZ environment variable. */
static timezone_t localtz;
@@ -738,7 +751,7 @@ enum
};
static char const short_options[] =
- "-0123456789D:FJN:S::TW:abcde::fh:i::l:mn::o:rs::tvw:";
+ "-0123456789D:FJN:S::TW:abcde::fh:i::l:mn::po:rs::tvw:";
static struct option const long_options[] =
{
@@ -758,6 +771,7 @@ static struct option const long_options[] =
{"number-lines", optional_argument, nullptr, 'n'},
{"first-line-number", required_argument, nullptr, 'N'},
{"indent", required_argument, nullptr, 'o'},
+ {"pause", no_argument, nullptr, 'p'},
{"no-file-warnings", no_argument, nullptr, 'r'},
{"separator", optional_argument, nullptr, 's'},
{"sep-string", optional_argument, nullptr, 'S'},
@@ -868,6 +882,8 @@ main (int argc, char **argv)
idx_t n_digits = 0;
idx_t n_alloc = 0;
+ bool posixly_correct = (getenv ("POSIXLY_CORRECT") != nullptr);
+
initialize_main (&argc, &argv);
set_program_name (argv[0]);
setlocale (LC_ALL, "");
@@ -958,6 +974,8 @@ main (int argc, char **argv)
untabify_input = true;
break;
case 'f':
+ pause_on_first_page = posixly_correct;
+ FALLTHROUGH;
case 'F':
use_form_feed = true;
break;
@@ -999,6 +1017,9 @@ main (int argc, char **argv)
chars_per_margin = getoptnum (optarg, 0,
_("'-o MARGIN' invalid line offset"));
break;
+ case 'p':
+ pause_option = true;
+ break;
case 'r':
ignore_failed_opens = true;
break;
@@ -1061,7 +1082,7 @@ main (int argc, char **argv)
}
if (! date_format)
- date_format = (getenv ("POSIXLY_CORRECT") && !hard_locale (LC_TIME)
+ date_format = (posixly_correct && !hard_locale (LC_TIME)
? "%b %e %H:%M %Y"
: "%Y-%m-%d %H:%M");
@@ -1079,6 +1100,22 @@ main (int argc, char **argv)
error (EXIT_FAILURE, 0,
_("cannot specify both printing across and printing in parallel"));
+ /* POSIX states that the pausing behavior of -f and -p should occur only if
+ standard output is associated with a terminal. Only behave this way is
+ POSIXLY_CORRECT is set since the GNU Coding Standards discourages changing
+ program behavior based on output device type. */
+ if ((pause_option || pause_on_first_page) && posixly_correct
+ && ! isatty (STDOUT_FILENO))
+ pause_option = pause_on_first_page = false;
+
+ if (pause_option || pause_on_first_page)
+ {
+ tty_fd = open ("/dev/tty", O_RDONLY);
+ if (tty_fd < 0)
+ error (EXIT_FAILURE, errno, _("cannot open %s for reading"),
+ quoteaf ("/dev/tty"));
+ }
+
/* Translate some old short options to new/long options.
To meet downward compatibility with other UNIX pr utilities
and some POSIX specifications. */
@@ -1640,6 +1677,36 @@ print_files (int number_of_files, char **av)
;
}
+/* Pause until reading a newline from /dev/tty if needed. This function should
+ not be called until we know that we have text or a header to print for a
+ page. This means it must be called when pad_vertically is set to true. */
+
+static void
+pause_maybe (void)
+{
+ if ((pause_option || (pause_on_first_page && page_number == 1))
+ && last_paused_page_number < page_number)
+ {
+ last_paused_page_number = page_number;
+ if (pause_option
+ && (fputc ('\a', stderr) == EOF || fflush (stderr) != 0))
+ write_error ();
+ while (true)
+ {
+ char ch;
+ ssize_t bytes_read = read (tty_fd, &ch, sizeof ch);
+ if (bytes_read < 0)
+ error (EXIT_FAILURE, errno, _("error reading %s"),
+ quoteaf ("/dev/tty"));
+ /* Just exit if the user presses Ctrl-D. */
+ if (bytes_read == 0)
+ return;
+ if (ch == '\n')
+ break;
+ }
+ }
+}
+
/* Initialize header information.
If DESC is non-negative, it is a file descriptor open to
FILENAME for reading. */
@@ -2452,6 +2519,7 @@ read_line (COLUMN *p)
if (print_a_header && !storing_columns)
{
pad_vertically = true;
+ pause_maybe ();
print_header ();
}
else if (keep_FF)
@@ -2476,6 +2544,7 @@ read_line (COLUMN *p)
if (p->char_func != store_char)
{
pad_vertically = true;
+ pause_maybe ();
if (print_a_header && !storing_columns)
print_header ();
@@ -2586,7 +2655,10 @@ print_stored (COLUMN *p)
pad_vertically = true;
if (print_a_header)
- print_header ();
+ {
+ pause_maybe ();
+ print_header ();
+ }
if (p->status == FF_FOUND)
{
@@ -2824,6 +2896,8 @@ Paginate or columnate FILE(s) for printing.\n\
-o, --indent=MARGIN\n\
offset each line with MARGIN (zero) spaces, do not\n\
affect -w or -W, MARGIN will be added to PAGE_WIDTH\n\
+ -p, --pause pause at the beginning of each page until a newline\n\
+ is read from /dev/tty.\n\
-r, --no-file-warnings\n\
omit warning when a file cannot be opened\n\
"), stdout);
--
2.50.1
