Package: gdb
Version: 8.2-1
Severity: normal
Dear Maintainer,
I'm trying to debug a complex program and came across this
reproducible behaviour. On an i.MX6 QuadPlus CPU (Boundary Devices
Nitrogen6_Max board) hardware watchpoints are not triggered in my
complex case and in a trivial test case:
============= t.c =============
#include <stdio.h>
int main ()
{
int a = 1;
a = 2;
printf("a: %i\n", a);
return 0;
}
===============================
Compiled with `cc -g -O0 t.c -o t' using GCC (version (Debian
6.3.0-18+deb9u1) 6.3.0 20170516) on an amd64 box gives the following
successful triggers:
===============================
(gdb) l
1 #include <stdio.h>
2
3 int main ()
4 {
5 int a = 1;
6
7 a = 2;
8
9 printf("a: %i\n", a);
10
(gdb)
11 return 0;
12 }
(gdb) break 5
Breakpoint 1 at 0x6b8: file t.c, line 5.
(gdb) run
Starting program: /store-f/rah/proj/gdb-watchpoint/t
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Breakpoint 1, main () at t.c:5
5 int a = 1;
(gdb) p &a
$1 = (int *) 0x7fffffffe08c
(gdb) watch *0x7fffffffe08c
Hardware watchpoint 2: *0x7fffffffe08c
(gdb) c
Continuing.
Hardware watchpoint 2: *0x7fffffffe08c
Old value = 0
New value = 1
main () at t.c:7
7 a = 2;
(gdb) c
Continuing.
Hardware watchpoint 2: *0x7fffffffe08c
Old value = 1
New value = 2
main () at t.c:9
9 printf("a: %i\n", a);
(gdb) c
Continuing.
a: 2
===============================
However, compiling with GCC (version (Debian 8.2.0-12) 8.2.0) on the
i.MX6 board results in no trigger:
===============================
(gdb) break 5
Breakpoint 1 at 0x512: file t.c, line 5.
(gdb) run
Starting program: /srv/proj/chive/gdb-watchpoint/t
Breakpoint 1, main () at t.c:5
5 int a = 1;
(gdb) p &a
$1 = (int *) 0xbefff46c
(gdb) watch *0xbefff46c
Hardware watchpoint 2: *0xbefff46c
(gdb) c
Continuing.
a: 2
[Inferior 1 (process 1435) exited normally]
===============================
The program contains writes to the `a' variable, as shown in the
following assembler extract:
===============================
516: 2302 movs r3, #2
518: 607b str r3, [r7, #4]
===============================
from the disassembled main function:
===============================
0000050c <main>:
50c: b580 push {r7, lr}
50e: b082 sub sp, #8
510: af00 add r7, sp, #0
512: 2301 movs r3, #1
514: 607b str r3, [r7, #4]
516: 2302 movs r3, #2
518: 607b str r3, [r7, #4]
51a: 6879 ldr r1, [r7, #4]
51c: 4b04 ldr r3, [pc, #16] ; (530 <main+0x24>)
51e: 447b add r3, pc
520: 4618 mov r0, r3
522: f7ff ef54 blx 3cc <printf@plt>
526: 2300 movs r3, #0
528: 4618 mov r0, r3
52a: 3708 adds r7, #8
52c: 46bd mov sp, r7
52e: bd80 pop {r7, pc}
530: 00000062 .word 0x00000062
===============================
and as demonstrated in the following transcript:
===============================
(gdb) break 5
Breakpoint 1 at 0x512: file t.c, line 5.
(gdb) run
Starting program: /srv/proj/chive/gdb-watchpoint/t
Breakpoint 1, main () at t.c:5
5 int a = 1;
(gdb) p &a
$1 = (int *) 0xbefff46c
(gdb) watch *0xbefff46c
Hardware watchpoint 2: *0xbefff46c
(gdb) display /x *((int *) 0xbefff46c)
1: /x *((int *) 0xbefff46c) = 0x0
(gdb) n
7 a = 2;
1: /x *((int *) 0xbefff46c) = 0x1
(gdb) n
9 printf("a: %i\n", a);
1: /x *((int *) 0xbefff46c) = 0x2
(gdb) c
Continuing.
a: 2
[Inferior 1 (process 1484) exited normally]
===============================
Regards,
Bob Ham
-- System Information:
Debian Release: buster/sid
APT prefers testing-debug
APT policy: (500, 'testing-debug'), (500, 'testing')
Architecture: armhf (armv7l)
Kernel: Linux 4.15.0-g837bff1 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8),
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Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
Versions of packages gdb depends on:
ii libbabeltrace1 1.5.6-2
ii libc6 2.28-2
ii libexpat1 2.2.6-1
ii liblzma5 5.2.2-1.3
ii libncursesw6 6.1+20181013-1
ii libpython3.7 3.7.2~rc1-1
ii libreadline7 7.0-5
ii libtinfo6 6.1+20181013-1
ii zlib1g 1:1.2.11.dfsg-1
Versions of packages gdb recommends:
pn libc-dbg <none>
Versions of packages gdb suggests:
pn gdb-doc <none>
ii gdbserver 8.2-1
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