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From: "msebor at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug c++/18888] New: unable to evaluate NULL
Date: Fri, 28 Aug 2015 21:49:00 -0000
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X-Bugzilla-Product: gdb
X-Bugzilla-Component: c++
X-Bugzilla-Version: 7.8
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https://sourceware.org/bugzilla/show_bug.cgi?id=18888

            Bug ID: 18888
           Summary: unable to evaluate NULL
           Product: gdb
           Version: 7.8
            Status: NEW
          Severity: normal
          Priority: P2
         Component: c++
          Assignee: unassigned at sourceware dot org
          Reporter: msebor at redhat dot com
  Target Milestone: ---

GNU gdb (GDB) Fedora 7.8.2-38.fc21 doesn't seem to know about GCC's __null
extension used to implement the NULL macro.  When the NULL macro is used in the
definition of a function like macro as happens to be the case with some of the
GCC tree checking macros such as DECL_ASSEMBLER_NAME_SET_P, GDB issues the
cryptic error 'No symbol "__null" in current context.' when an attempt is to
evaluate an invocation of the macro.  This makes GCC debugging harder than it
would be otherwise.

(gdb) p DECL_ASSEMBLER_NAME_SET_P(expr)
No symbol "__null" in current context.

Test case:

$ cat t.c && g++ -g3 t.c && gdb -batch -ex 'b main' -ex 'r' -ex 'macro expand
FOO(p, 1, 2)' -ex 'p FOO(p, 1, 2)' a.out
#include <stdlib.h>

#define FOO(a, b, c)  a == NULL ? b : c

int main ()
{
    void *p = NULL;
    int n;
    n = FOO (p, 1, 2);
    return n;
}
Breakpoint 1 at 0x4005ba: file t.c, line 7.

Breakpoint 1, main () at t.c:7
7           void *p = NULL;
expands to: p == __null ? 1 : 2
No symbol "__null" in current context.

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