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From: "ed at catmur dot co.uk" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug exp/17904] New: "Cannot access memory at address 0x0" for
 function returning reference evaluated within EVAL_AVOID_SIDE_EFFECTS
Date: Thu, 29 Jan 2015 18:33:00 -0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=17904

            Bug ID: 17904
           Summary: "Cannot access memory at address 0x0" for function
                    returning reference evaluated within
                    EVAL_AVOID_SIDE_EFFECTS
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: exp
          Assignee: unassigned at sourceware dot org
          Reporter: ed at catmur dot co.uk

Given a function returning a reference:

    int& g() { static int i; return i; }

Evaluating the function within a EVAL_AVOID_SIDE_EFFECTS context gives an
error:

    (gdb) p 0 || +g()
    Cannot access memory at address 0x0

A workaround is to convert the reference to a pointer and indirect it:

    (gdb) p 0 || +*&g()
    $1 = true

This appears to be because OP_FUNCALL with EVAL_AVOID_SIDE_EFFECTS calls
allocate_value(), which for reference types returns a value with
location.address = 0; promotion on the reference argument then calls
coerce_ref() which gives a lazy value pointing to 0x0.

I'm not sure of a fix other than going through evaluate_subexp_standard()
putting in checks for noside == EVAL_AVOID_SIDE_EFFECTS.

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