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Re: SIGSEGV on gdb 6.7*
Daniel Jacobowitz wrote:
On Mon, Feb 04, 2008 at 08:49:48PM +0000, Greg Law wrote:
yet there appear to be pointers to the regcache squirrelled away in
various other places, notably the prologue_cache member of the
frame_info structure.
Could you give a specific example? I don't think there should be such
pointers.
When a register is examined, we (eventually) get to frame_register_unwind.
This does:
frame->unwind->prev_register (frame->next, &frame->prologue_cache, regnum,
optimizedp, lvalp, addrp, realnump, bufferp);
which is actually a function pointer to (on plain old x86 Linux)
sentinel_frame_prev_register, which goes:
static void
sentinel_frame_prev_register (struct frame_info *next_frame,
void **this_prologue_cache,
int regnum, int *optimized,
enum lval_type *lvalp, CORE_ADDR *addrp,
int *realnum, gdb_byte *bufferp)
{
struct frame_unwind_cache *cache = *this_prologue_cache;
/* Describe the register's location. A reg-frame maps all registers
onto the corresponding hardware register. */
*optimized = 0;
*lvalp = lval_register;
*addrp = register_offset_hack (current_gdbarch, regnum);
*realnum = regnum;
/* If needed, find and return the value of the register. */
if (bufferp != NULL)
{
/* Return the actual value. */
/* Use the regcache_cooked_read() method so that it, on the fly,
constructs either a raw or pseudo register from the raw
register cache. */
regcache_cooked_read (cache->regcache, regnum, bufferp);
}
}
note the first line which is an assignment to 'cache' from
*this_prologue_cache. The regcache_cooked_read is called, passing
cache->regcache. If flushregs has been called, the regcache appears
to be garbage. If I've not got myself terribly confused, this is
not especially surprising, since we freed the regcache at
registers_changed(), leaving the frame_info structure pointing at
invalid memory.
I guess this might result in some "unnecessary" fetches of the register
state, but that has to be favourable to a SEGV :)
Not really - we go to a lot of trouble to avoid this.
Yeah, I guess this more important on remote targets over slow links,
right? In that case, do we not need to take care from
registers_changed() to go and invalidate all the pointers to cached
registers in the frame structures? Hmm, although thinking about it
now, I guess the frame structures all need throwing away/recalculating
if the registers have changed anyway? I can't find what causes that
to happen though.
g