RFC: Fix "break *EXP thread NUM"

Daniel Jacobowitz
2009-11-23T21:27:44Z

This issue has come up several times over the years. I've CC'd a
couple of folks from the last discussions I found in the list
archives. Here's an example:

(gdb) # OK
(gdb) break main thread 999
Unknown thread 999.
(gdb) # NG
(gdb) break *main thread 999
A syntax error in expression, near `thread 999'.

References:

http://sourceware.org/ml/gdb-patches/2005-04/msg00092.html
http://sourceware.org/ml/gdb-patches/2005-04/msg00116.html
http://sourceware.org/ml/gdb-patches/2006-07/msg00412.html

The problem is with the expression parser, which has no way to detect
the "end" of an expression (a poorly defined concept). Since "if" can
not be a valid C identifier, and we do not parse C statements (just
expressions), we have a special technique in the lexer to detect that
token and stop. So both of these work:

(gdb) break main if 999
Breakpoint 1 at 0x44cce0
(gdb) break *main if 999
Note: breakpoint 1 also set at pc 0x44cce0.
Breakpoint 2 at 0x44cce0

In this patch, I have taken advantage of the fact that even if you
have a local variable named "thread", there is no valid C expression
which has a variable name, whitespace, and an integer or floating
point constant. So "thread [0-9]" can also safely be treated
as an expression terminator.

Does anyone see a problem with this approach?

I updated the manual, and don't think a NEWS entry is required.
Tested on x86_64-linux, with no regressions.

--
Daniel Jacobowitz
CodeSourcery

2009-11-23 Daniel Jacobowitz <dan@codesourcery.com>

* c-exp.y (yylex): Stop before "thread N".

* gdb.texinfo (Thread-Specific Breakpoints): Thread specifiers
are allowed after the breakpoint condition.

* gdb.base/condbreak.exp: Test combinations of "break *EXP",
"if", and "thread". Correct matching in the previous test.

Index: c-exp.y
===================================================================
RCS file: /cvs/src/src/gdb/c-exp.y,v
retrieving revision 1.65
diff -u -p -r1.65 c-exp.y
--- c-exp.y 11 Nov 2009 16:45:46 -0000 1.65
+++ c-exp.y 23 Nov 2009 21:26:06 -0000
@@ -2250,6 +2250,22 @@ yylex (void)
return 0;
}

+ /* For the same reason (breakpoint conditions), "thread N"
+ terminates the expression. "thread" could be an identifier, but
+ an identifier is never followed by a number without intervening
+ punctuation. */
+ if (namelen == 6
+ && strncmp (tokstart, "thread", 6) == 0
+ && (tokstart[6] == ' ' || tokstart[6] == '\t')
+ && ! scanning_macro_expansion ())
+ {
+ char *p = tokstart + 7;
+ while (*p == ' ' || *p == '\t')
+ p++;
+ if (*p >= '0' && *p <= '9')
+ return 0;
+ }
+
lexptr += namelen;

tryname:
Index: doc/gdb.texinfo
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v
retrieving revision 1.642
diff -u -p -r1.642 gdb.texinfo
--- doc/gdb.texinfo 23 Nov 2009 18:44:10 -0000 1.642
+++ doc/gdb.texinfo 23 Nov 2009 21:26:13 -0000
@@ -5214,8 +5214,8 @@ breakpoint, the breakpoint applies to @e
program.

You can use the @code{thread} qualifier on conditional breakpoints as
-well; in this case, place @samp{thread @var{threadno}} before the
-breakpoint condition, like this:
+well; in this case, place @samp{thread @var{threadno}} before or
+after the breakpoint condition, like this:

@smallexample
(@value{GDBP}) break frik.c:13 thread 28 if bartab > lim
Index: testsuite/gdb.base/condbreak.exp
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.base/condbreak.exp,v
retrieving revision 1.13
diff -u -p -r1.13 condbreak.exp
--- testsuite/gdb.base/condbreak.exp 3 Jan 2009 05:58:03 -0000 1.13
+++ testsuite/gdb.base/condbreak.exp 23 Nov 2009 21:26:13 -0000
@@ -207,10 +207,10 @@ gdb_expect {
setup_xfail hppa2.0w-*-* 11512CLLbs
send_gdb "continue\n"
gdb_expect {
- -re "Continuing\\..*Breakpoint \[0-9\]+, marker2 \\(a=43\\) at .*$srcfile1:($bp_location8|$bp_location9).*($bp_location8|$bp_location9)\[\t \]+.*" {
+ -re "Continuing\\..*Breakpoint \[0-9\]+, marker2 \\(a=43\\) at .*$srcfile1:($bp_location8|$bp_location9).*($bp_location8|$bp_location9)\[\t \]+.*$gdb_prompt $" {
pass "run until breakpoint at marker2"
}
- -re "Continuing\\..*Breakpoint \[0-9\]+, $hex in marker2 \\(a=43\\) at .*$srcfile1:($bp_location8|$bp_location9).*($bp_location8|$bp_location9)\[\t \]+.*" {
+ -re "Continuing\\..*Breakpoint \[0-9\]+, $hex in marker2 \\(a=43\\) at .*$srcfile1:($bp_location8|$bp_location9).*($bp_location8|$bp_location9)\[\t \]+.*$gdb_prompt $" {
xfail "run until breakpoint at marker2"
}
-re "$gdb_prompt $" {
@@ -220,3 +220,16 @@ gdb_expect {
fail "(timeout) run until breakpoint at marker2"
}
}
+
+# Test combinations of conditional and thread-specific breakpoints.
+gdb_test "break main if (1==1) thread 999" \
+ "Unknown thread 999\\."
+gdb_test "break main thread 999 if (1==1)" \
+ "Unknown thread 999\\."
+
+# Verify that both if and thread can be distinguished from a breakpoint
+# address expression.
+gdb_test "break *main if (1==1) thread 999" \
+ "Unknown thread 999\\."
+gdb_test "break *main thread 999 if (1==1)" \
+ "Unknown thread 999\\."