[PATCH] gdb, gdbsupport, gdbserver: add support for printf 't' length modifier

simon.marchi
2026-01-12T18:24:43Z

From: Simon Marchi <simon.marchi@polymtl.ca>

Tom de Vries reported [1] that a use of the `t` printf length modifier
(used for ptrdiff_t) breaks "set debug dwarf-die 1":

+break -qualified main
Reading Reading compcomp unit at offset unit at offset 0x00x39

Reading comp unit at offset 0x1a8
Reading comp unit at offset 0x1e9
Reading comp unit at offset 0x2c5
Reading comp unit at offset 0x2a2
Reading comp unit at offset 0x904
Unrecognized format specifier 't' in printf

This use is in dwarf2/read.c:

gdb_printf (gdb_stdlog, "Read die from %s@0x%tx of %s:\n",
m_die_section->get_name (),
(begin_info_ptr - m_die_section->buffer),
bfd_get_filename (m_abfd));

Add support for the `t` length modifier in format_pieces, which
gdb_printf ultimately uses (through ui_out::vmessage). Modify the three
users of format_pieces: gdb's printf command, gdb's ui_out::vmessage
function and gdbserver's ax_printf function.

The implementation is mostly copied from what we do for size_t.

Since format_pieces is also used to implement the printf command, this
patch brings user-visible changes. Before:

(gdb) printf "%td\n", -123
❌️ Unrecognized format specifier 't' in printf

After:

(gdb) printf "%td\n", -123
-123

[1] https://inbox.sourceware.org/gdb-patches/d4ccce34-aad5-4e3d-8fc9-efb2fc11275c@suse.de/

Change-Id: Ie9fce78f5f48082d8f8a9ca2847b5ae26acaa60d
---
gdb/printcmd.c | 10 ++++++++++
gdb/testsuite/gdb.base/printcmds.c | 6 ++++++
gdb/testsuite/gdb.base/printcmds.exp | 4 ++++
gdb/ui-out.c | 18 ++++++++++++++++++
gdb/unittests/format_pieces-selftests.c | 12 ++++++++----
gdbserver/ax.cc | 8 ++++++++
gdbsupport/format.cc | 11 ++++++++++-
gdbsupport/format.h | 2 +-
8 files changed, 65 insertions(+), 6 deletions(-)

diff --git a/gdb/printcmd.c b/gdb/printcmd.c
index 783b83615d02..7c97fc719926 100644
--- a/gdb/printcmd.c
+++ b/gdb/printcmd.c
@@ -2815,6 +2815,16 @@ ui_printf (const char *arg, struct ui_file *stream)
{
size_t val = value_as_long (val_args[i]);

+ DIAGNOSTIC_PUSH
+ DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL
+ gdb_printf (stream, current_substring, val);
+ DIAGNOSTIC_POP
+ break;
+ }
+ case ptrdiff_t_arg:
+ {
+ ptrdiff_t val = value_as_long (val_args[i]);
+
DIAGNOSTIC_PUSH
DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL
gdb_printf (stream, current_substring, val);
diff --git a/gdb/testsuite/gdb.base/printcmds.c b/gdb/testsuite/gdb.base/printcmds.c
index 8445fcc1aa2a..82796932e1d3 100644
--- a/gdb/testsuite/gdb.base/printcmds.c
+++ b/gdb/testsuite/gdb.base/printcmds.c
@@ -2,6 +2,8 @@
It is explicitly unsigned to avoid differences due to native characters
being either signed or unsigned. */
#include <stdlib.h>
+#include <stddef.h>
+
unsigned char ctable1[256] = {
0000, 0001, 0002, 0003, 0004, 0005, 0006, 0007,
0010, 0011, 0012, 0013, 0014, 0015, 0016, 0017,
@@ -77,6 +79,10 @@ charptr teststring2 = "more contents";

const char *teststring3 = "this is a longer test string that we can use";

+/* For testing printf with %z (size_t) and %t (ptrdiff_t) length modifiers. */
+size_t test_size = 1234;
+ptrdiff_t test_ptrdiff = -5678;
+
/* Test printing of a struct containing character arrays. */

struct some_arrays {
diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
index bc100f93df9a..32a952f01a55 100644
--- a/gdb/testsuite/gdb.base/printcmds.exp
+++ b/gdb/testsuite/gdb.base/printcmds.exp
@@ -908,6 +908,10 @@ proc test_printf {} {
# PR cli/14977.
gdb_test "printf \"%s\\n\", 0" "\\(null\\)"

+ # Test %z (size_t) and %t (ptrdiff_t) length modifiers.
+ gdb_test "printf \"%zu\\n\", test_size" "1234"
+ gdb_test "printf \"%td\\n\", test_ptrdiff" "-5678"
+
with_max_value_size 20 {
gdb_test {printf "%s", teststring3} \
"^printed string requires 45 bytes, which is more than max-value-size"
diff --git a/gdb/ui-out.c b/gdb/ui-out.c
index 4f20e6b93477..87874e22530f 100644
--- a/gdb/ui-out.c
+++ b/gdb/ui-out.c
@@ -682,6 +682,24 @@ ui_out::vmessage (const ui_file_style &in_style, const char *format,
}
}
break;
+ case ptrdiff_t_arg:
+ {
+ ptrdiff_t val = va_arg (args, ptrdiff_t);
+ switch (piece.n_int_args)
+ {
+ case 0:
+ call_do_message (style, current_substring, val);
+ break;
+ case 1:
+ call_do_message (style, current_substring, intvals[0], val);
+ break;
+ case 2:
+ call_do_message (style, current_substring,
+ intvals[0], intvals[1], val);
+ break;
+ }
+ }
+ break;
case double_arg:
call_do_message (style, current_substring, va_arg (args, double));
break;
diff --git a/gdb/unittests/format_pieces-selftests.c b/gdb/unittests/format_pieces-selftests.c
index af574ccc900f..c5f3cf74f1dc 100644
--- a/gdb/unittests/format_pieces-selftests.c
+++ b/gdb/unittests/format_pieces-selftests.c
@@ -105,40 +105,44 @@ test_gdb_formats ()
static void
test_format_int_sizes ()
{
- check ("Hello\\t %hu%lu%llu%zu", /* ARI: %ll */
+ check ("Hello\\t %hu%lu%llu%zu%tu", /* ARI: %ll */
{
{"Hello\t ", literal_piece, 0},
{"%hu", int_arg, 0},
{"%lu", long_arg, 0},
{"%" LL "u", long_long_arg, 0},
{"%zu", size_t_arg, 0},
+ {"%tu", ptrdiff_t_arg, 0},
});

- check ("Hello\\t %hx%lx%llx%zx", /* ARI: %ll */
+ check ("Hello\\t %hx%lx%llx%zx%tx", /* ARI: %ll */
{
{"Hello\t ", literal_piece, 0},
{"%hx", int_arg, 0},
{"%lx", long_arg, 0},
{"%" LL "x", long_long_arg, 0},
{"%zx", size_t_arg, 0},
+ {"%tx", ptrdiff_t_arg, 0},
});

- check ("Hello\\t %ho%lo%llo%zo", /* ARI: %ll */
+ check ("Hello\\t %ho%lo%llo%zo%to", /* ARI: %ll */
{
{"Hello\t ", literal_piece, 0},
{"%ho", int_arg, 0},
{"%lo", long_arg, 0},
{"%" LL "o", long_long_arg, 0},
{"%zo", size_t_arg, 0},
+ {"%to", ptrdiff_t_arg, 0},
});

- check ("Hello\\t %hd%ld%lld%zd", /* ARI: %ll */
+ check ("Hello\\t %hd%ld%lld%zd%td", /* ARI: %ll */
{
{"Hello\t ", literal_piece, 0},
{"%hd", int_arg, 0},
{"%ld", long_arg, 0},
{"%" LL "d", long_long_arg, 0},
{"%zd", size_t_arg, 0},
+ {"%td", ptrdiff_t_arg, 0},
});
}

diff --git a/gdbserver/ax.cc b/gdbserver/ax.cc
index 44fcf3157291..4993bed2532c 100644
--- a/gdbserver/ax.cc
+++ b/gdbserver/ax.cc
@@ -908,6 +908,14 @@ ax_printf (CORE_ADDR fn, CORE_ADDR chan, const char *format,
break;
}

+ case ptrdiff_t_arg:
+ {
+ ptrdiff_t val = args[i];
+
+ printf (current_substring, val);
+ break;
+ }
+
case literal_piece:
/* Print a portion of the format string that has no
directives. Note that this will not include any
diff --git a/gdbsupport/format.cc b/gdbsupport/format.cc
index b515a95114e0..43995edf453e 100644
--- a/gdbsupport/format.cc
+++ b/gdbsupport/format.cc
@@ -111,6 +111,7 @@ format_pieces::format_pieces (const char **arg, bool gdb_extensions,
int seen_big_l = 0, seen_h = 0, seen_big_h = 0;
int seen_big_d = 0, seen_double_big_d = 0;
int seen_size_t = 0;
+ int seen_ptrdiff_t = 0;
int bad = 0;
int n_int_args = 0;
bool seen_i64 = false;
@@ -224,6 +225,11 @@ format_pieces::format_pieces (const char **arg, bool gdb_extensions,
seen_size_t = 1;
f++;
break;
+ case 't':
+ /* For ptrdiff_t. */
+ seen_ptrdiff_t = 1;
+ f++;
+ break;
case 'I':
/* Support the Windows '%I64' extension, because an
earlier call to format_pieces might have converted %lld
@@ -257,6 +263,8 @@ format_pieces::format_pieces (const char **arg, bool gdb_extensions,
case 'i':
if (seen_size_t)
this_argclass = size_t_arg;
+ else if (seen_ptrdiff_t)
+ this_argclass = ptrdiff_t_arg;
else if (lcount == 0)
this_argclass = int_arg;
else if (lcount == 1)
@@ -334,7 +342,8 @@ format_pieces::format_pieces (const char **arg, bool gdb_extensions,

if (lcount > 1 || seen_h || seen_big_h || seen_big_h
|| seen_big_d || seen_double_big_d || seen_size_t
- || seen_prec || seen_zero || seen_space || seen_plus)
+ || seen_ptrdiff_t || seen_prec || seen_zero || seen_space
+ || seen_plus)
bad = 1;

this_argclass = value_arg;
diff --git a/gdbsupport/format.h b/gdbsupport/format.h
index 91bcf1db55a6..ced10e46a716 100644
--- a/gdbsupport/format.h
+++ b/gdbsupport/format.h
@@ -36,7 +36,7 @@
enum argclass
{
literal_piece,
- int_arg, long_arg, long_long_arg, size_t_arg, ptr_arg,
+ int_arg, long_arg, long_long_arg, size_t_arg, ptrdiff_t_arg, ptr_arg,
string_arg, wide_string_arg, wide_char_arg,
double_arg, long_double_arg,
dec32float_arg, dec64float_arg, dec128float_arg,

base-commit: b9517052ce5c1d87bfd7030bcbf90a98f63fddf6
--
2.52.0