GDB syntax error when casting to base class

Marc Khouzam
Thu Sep 9 12:50:00 GMT 2010


I got an eclipse bug report that led me to a bizarre behavior from GDB.
In the very short session below, I'm getting a syntax error when casting
a variable to its base class.

If I remove the constructor (see comment in the code below)
the problem goes away.  Or if I add the word 'struct', it also goes away.

This is causing problems in Eclipse because -var-info-path-expression
returns the expression that causes a syntax error.

I tried to debug it but I ended up in c-exp.y and had to give up.

I was going to propose a patch that adds 'struct' to the 
-var-info-path-expression output, but I didn't think that was the 
greatest of solutions.  Currently, this is what we are planning to 
do in Eclipse to work around this problem.

I can open a PR if it makes sense?



> gdb.7.2 a.out
GNU gdb (GDB) 7.2
(gdb) l 1
1       class bar{};
3       class foo : public bar {
4         public:
5           foo() {}  // Comment this line to make things work
6           void test() { return; }
7       };
9       int main(void)
10      {
11          foo f;
12          f.test();
13          return 0;
14      }

(gdb) b 6
Breakpoint 1 at 0x8048457: file, line 6.
(gdb) r
Starting program: /local/lmckhou/testing/a.out 
Breakpoint 1, foo::test (this=0xbfffbfa3) at
6           void test() { return; }

(gdb) p (*(bar*) this)
A syntax error in expression, near `) this)'.

(gdb) p (*(struct bar*) this)
$1 = {<No data fields>}

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