Re: [PATCH v5] Add negative repeat count to 'x' command

[Pedro Alves <palves at redhat dot com>]

On 08/08/2016 03:45 PM, Thomas Preudhomme wrote:
> Sorry for the late reply but I noticed an issue in the tests added by
> this patch for arm-none-eabi:
> 
> 
> FAIL: gdb.base/examine-backward.exp: address zero boundary: examine 3
> bytes forward from 0x0
> NA->FAIL: gdb.base/examine-backward.exp char-width=1, print-max=20 take
> 1 string backward (6/6)
> NA->FAIL: gdb.base/examine-backward.exp char-width=1, print-max=20 take
> 6 strings backward (pattern 1)
> 
> 
> The first one is a testism:
> 
> we match for: 0x\[0-9a-f\]+00.*:${byte}${byte}${byte}
> but we get: 0x0:    0x7f    0x45    0x4c
> 
> I'd thus suggest to match 0x\(\[0-9a-f\]+0\)?0 instead, with the right
> amount of escaping (I don't know whether ? needs escaping and why + does
> not need escaping in current pattern).

We're examining at 0x0.  Shouldn't we expect literally "0x0"
instead then?

> 
> 
> The other two seems like genuine errors. take 6 strings backward
> (pattern 1) gives:
> 
> gdb_expect_list pattern: /"ABCDEFGHIJKLMNOPQRST".../
> x/-6s^M
> 0x9aff <_fini+2>: "\277\370\274\b\274\236FpGABCDEFGHIJK"...^M
> 0x9b13 <TestStrings+11>:        "LMNOPQRSTUVWXYZ"^M
> 0x9b23 <TestStrings+27>:        ""^M
> 0x9b24 <TestStrings+28>:        ""^M
> 0x9b25 <TestStrings+29>:
> "\343\201\273\343\201\222\343\201\273\343\201\222"^M
> 0x9b32 <TestStrings+42>:        "01234567890123456789"...^M
> (gdb) FAIL: gdb.base/examine-backward.exp: char-width=1, print-max=20:
> take 6 strings backward (pattern 1)
> 
> 
> take 1 string backward (6/6) gives:
> 
> 0x9aff <_fini+2>: "\277\370\274\b\274\236FpGABCDEFGHIJK"...
> 
> 
> Please let me know what I can do to help diagnose this error.

The find-strings-backwards algorithm just looks back for '\0' to
find string boundaries.  Looks like it just happens that in your
case, the TestStrings array is immediately preceded by the
tail of _fini, with no gap in between.

Try this.  It's not strictly correct to assume that the linker
places the objects consecutively, but it's probably safe
in practice.

diff --git c/gdb/testsuite/gdb.base/examine-backward.c w/gdb/testsuite/gdb.base/examine-backward.c
index b338503..187fad1 100644
--- c/gdb/testsuite/gdb.base/examine-backward.c
+++ w/gdb/testsuite/gdb.base/examine-backward.c
@@ -32,6 +32,14 @@ literals.  The content of each array is the same as followings:
   };
 */
 
+/* This is here just to ensure we have a null character before
+   TestStrings, to avoid showing garbage when we look for strings
+   backwards from TestStrings.  */
+
+const char NullString[] = {
+  0x00
+};
+
 const char TestStrings[] = {
   0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
   0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50,