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The enclosed patch removes the gratuitous ACK which is emitted from
set_debug_traps() in most of the sample debug stubs.
Athough there's a comment (``In case GDB is started before us, ack any
packets (presumably "$?#xx") sitting there'') that describes the code,
I believe this is very wrong. A stub should not ACK a packet it has
not received (or may not have even existed).
The failure mode is as follows:
* GDB is started, sends a qC packet.
* target program is started and calls set_debug_traps() which emits
ACK.
* GDB listens for the response for the qC packet, which will
never be sent.
* GDB times out with a "Ignoring packet error, continuing..." error.
I used a target system which used a stub derived from i386-stub.c
(including the gratuitous ACK) and have experienced the scenerio
described above firsthand.
Had the ACK not been sent, at the very least GDB would have retried
the command multiple times before failing. One can argue that GDB's
behavior should be more robust in the face of timed out commands. I
agree. But in my opinion, any policy with regards to timeouts or lost
packets belong wholely within GDB; the stub should not be ACKing
packets it didn't receive.
1999-03-01 J.T. Conklin <jtc@redbacknetworks.com>
* i386-stub.c, m32r-stub.c, sparc-stub.c, sparcl-stub.c,
sparclite-stub.c (set_debug_traps): Removed code that sent a
gratuitous ACK.
Index: i386-stub.c
===================================================================
RCS file: /usr/rback/release/tools-src/gdb/gdb/i386-stub.c,v
retrieving revision 1.1.1.4
diff -c -r1.1.1.4 i386-stub.c
*** i386-stub.c 1999/02/02 23:31:15 1.1.1.4
--- i386-stub.c 1999/02/27 00:10:06
***************
*** 893,904 ****
exceptionHook = remcomHandler;
}
- /* In case GDB is started before us, ack any packets (presumably
- "$?#xx") sitting there. */
- putDebugChar ('+');
-
initialized = 1;
-
}
/* This function will generate a breakpoint exception. It is used at the
--- 893,899 ----
Index: m32r-stub.c
===================================================================
RCS file: /usr/rback/release/tools-src/gdb/gdb/m32r-stub.c,v
retrieving revision 1.1.1.3
diff -c -r1.1.1.3 m32r-stub.c
*** m32r-stub.c 1999/02/15 20:57:10 1.1.1.3
--- m32r-stub.c 1999/02/27 00:10:39
***************
*** 1514,1523 ****
exceptionHandler (16, _catchException16);
/* exceptionHandler (17, _catchException17); */
- /* In case GDB is started before us, ack any packets (presumably
- "$?#xx") sitting there. */
- putDebugChar ('+');
-
initialized = 1;
}
--- 1514,1519 ----
Index: sparc-stub.c
===================================================================
RCS file: /usr/rback/release/tools-src/gdb/gdb/sparc-stub.c,v
retrieving revision 1.1.1.2
diff -c -r1.1.1.2 sparc-stub.c
*** sparc-stub.c 1999/02/02 23:31:47 1.1.1.2
--- sparc-stub.c 1999/02/27 00:10:46
***************
*** 485,495 ****
for (ht = hard_trap_info; ht->tt && ht->signo; ht++)
exceptionHandler(ht->tt, trap_low);
- /* In case GDB is started before us, ack any packets (presumably
- "$?#xx") sitting there. */
-
- putDebugChar ('+');
-
initialized = 1;
}
--- 485,490 ----
Index: sparcl-stub.c
===================================================================
RCS file: /usr/rback/release/tools-src/gdb/gdb/sparcl-stub.c,v
retrieving revision 1.1.1.1
diff -c -r1.1.1.1 sparcl-stub.c
*** sparcl-stub.c 1998/12/03 00:05:23 1.1.1.1
--- sparcl-stub.c 1999/02/27 00:10:56
***************
*** 602,612 ****
if (ht->tt != 4 || ! (read_psr () & 0x1000))
exceptionHandler(ht->tt, trap_low);
- /* In case GDB is started before us, ack any packets (presumably
- "$?#xx") sitting there. */
-
- putDebugChar ('+');
-
initialized = 1;
}
--- 602,607 ----
Index: sparclet-stub.c
===================================================================
RCS file: /usr/rback/release/tools-src/gdb/gdb/sparclet-stub.c,v
retrieving revision 1.1.1.2
diff -c -r1.1.1.2 sparclet-stub.c
*** sparclet-stub.c 1999/02/02 23:31:48 1.1.1.2
--- sparclet-stub.c 1999/02/27 00:11:04
***************
*** 649,658 ****
for (ht = hard_trap_info; ht->tt && ht->signo; ht++)
exceptionHandler(ht->tt, trap_low);
- /* In case GDB is started before us, ack any packets (presumably
- "$?#xx") sitting there. */
- putDebugChar ('+');
-
initialized = 1;
}
--- 649,654 ----
--
J.T. Conklin
RedBack Networks