现象
在gdb中，如果想看一个struct的某个field的偏移量，和C语言一样可以通过对一个0地址变量取地址，然后取成员的偏移量获得。更神奇的地方在于和C语言一样，这里也不会触发内存访问异常。
另外还有一个奇怪的现象：对于x取地址的时候没有问题，但是对于c字段取地址之后会有一个莫名其妙的错误提示"<error: Cannot access memory at address 0x8>"。
tsecer@harry: cat -n main.cpp
1 struct tsecer
2 {
3 double x;
4 char c;
5 double d;
6 };
7
8 int main()
9 {
10 tsecer x;
11 return x.c;
12 }
tsecer@harry: gcc -g main.cpp
tsecer@harry: gdb -quiet ./a.out
Reading symbols from ./a.out...
(gdb) p &((tsecer*)0)->x
$1 = (double *) 0x0
(gdb) p &((tsecer*)0)->c
$2 = 0x8 <error: Cannot access memory at address 0x8>
(gdb)
lazy evaluate
在取一个struct的特定域(field)时，gdb其实只是计算了它的相对地址(offset)而没有真正取这个field的绝对地址。这也意味着，一个变量是由两部分(基地址+offset)而不是一部分(绝对地址)组成。
在前面的例子中，x的地址其实是通过0作为基地址加上0作为offset两部分表示。
下面是计算一个结构field值(value)的函数。可以看到，它主要是计算了offset偏移量而不是计算了绝对地址。
/// @file: gdb-10.1\gdb\value.c
/* Given a value ARG1 (offset by OFFSET bytes)
of a struct or union type ARG_TYPE,
extract and return the value of one of its (non-static) fields.
FIELDNO says which field. */
struct value *
value_primitive_field (struct value *arg1, LONGEST offset,
int fieldno, struct type *arg_type)
{
///....
else
{
/* Plain old data member */
offset += (TYPE_FIELD_BITPOS (arg_type, fieldno)
/ (HOST_CHAR_BIT * unit_size));
/* Lazy register values with offsets are not supported. */
if (VALUE_LVAL (arg1) == lval_register && value_lazy (arg1))
value_fetch_lazy (arg1);
if (value_lazy (arg1))
v = allocate_value_lazy (type);
else
{
v = allocate_value (type);
value_contents_copy_raw (v, value_embedded_offset (v),
arg1, value_embedded_offset (arg1) + offset,
type_length_units (type));
}
v->offset = (value_offset (arg1) + offset
+ value_embedded_offset (arg1));
}
set_value_component_location (v, arg1);
return v;
}
这也意味着一个value的内存存储结构相对单独存储一个绝度地址会稍微复杂一些。下面的value结构中存储了基地址address和偏移量offset两个字段。
/// @file:
/* Note that the fields in this structure are arranged to save a bit
of memory. */
struct value
{
///...
/* Location of value (if lval). */
union
{
/* If lval == lval_memory, this is the address in the inferior */
CORE_ADDR address;
/*If lval == lval_register, the value is from a register. */
struct
{
/* Register number. */
int regnum;
/* Frame ID of "next" frame to which a register value is relative.
If the register value is found relative to frame F, then the
frame id of F->next will be stored in next_frame_id. */
struct frame_id next_frame_id;
} reg;
/* Pointer to internal variable. */
struct internalvar *internalvar;
/* Pointer to xmethod worker. */
struct xmethod_worker *xm_worker;
/* If lval == lval_computed, this is a set of function pointers
to use to access and describe the value, and a closure pointer
for them to use. */
struct
{
/* Functions to call. */
const struct lval_funcs *funcs;
/* Closure for those functions to use. */
void *closure;
} computed;
} location {};
/* Describes offset of a value within lval of a structure in target
addressable memory units. Note also the member embedded_offset
below. */
LONGEST offset = 0;
///...
取地址
对于&这种操作，通过value_address取到值的内存地址即可，没必要真正访问内容。再强调一遍，只是计算内存地址。
下面代码是返回“基地址+offset”获得地址。
CORE_ADDR
value_address (const struct value *value)
{
if (value->lval != lval_memory)
return 0;
if (value->parent != NULL)
return value_address (value->parent.get ()) + value->offset;
if (NULL != TYPE_DATA_LOCATION (value_type (value)))
{
gdb_assert (PROP_CONST == TYPE_DATA_LOCATION_KIND (value_type (value)));
return TYPE_DATA_LOCATION_ADDR (value_type (value));
}
return value->location.address + value->offset;
}
为什么有错误提示
正如注释
/* For a pointer to a textual type, also print the string
pointed to, unless pointer is null. */
所说，对于一个指向文本(字符)结构的指针，除了输出地址之外，还会输出指向的字符串内容(除非指针为0)，此时就会真正访问内存地址。
而对于通常的double指针就不会，所以double取地址就不会报错。
/// @file: gdb-10.1\gdb\c-valprint.c
/* Print a pointer based on the type of its target.
Arguments to this functions are roughly the same as those in c_val_print.
A difference is that ADDRESS is the address to print, with embedded_offset
already added. UNRESOLVED_ELTTYPE and ELTTYPE represent the pointed type,
respectively before and after check_typedef. */
static void
print_unpacked_pointer (struct type *type, struct type *elttype,
struct type *unresolved_elttype,
const gdb_byte *valaddr, int embedded_offset,
CORE_ADDR address, struct ui_file *stream, int recurse,
const struct value_print_options *options)
{
int want_space = 0;
struct gdbarch *gdbarch = get_type_arch (type);
if (elttype->code () == TYPE_CODE_FUNC)
{
/* Try to print what function it points to. */
print_function_pointer_address (options, gdbarch, address, stream);
return;
}
if (options->symbol_print)
want_space = print_address_demangle (options, gdbarch, address, stream,
demangle);
else if (options->addressprint)
{
fputs_filtered (paddress (gdbarch, address), stream);
want_space = 1;
}
/* For a pointer to a textual type, also print the string
pointed to, unless pointer is null. */
if (c_textual_element_type (unresolved_elttype, options->format)
&& address != 0)
{
if (want_space)
fputs_filtered (" ", stream);
val_print_string (unresolved_elttype, NULL, address, -1, stream, options);
}
else if (cp_is_vtbl_member (type))
{
/* Print vtbl's nicely. */
CORE_ADDR vt_address = unpack_pointer (type, valaddr + embedded_offset);
struct bound_minimal_symbol msymbol =
lookup_minimal_symbol_by_pc (vt_address);
/* If 'symbol_print' is set, we did the work above. */
if (!options->symbol_print
&& (msymbol.minsym != NULL)
&& (vt_address == BMSYMBOL_VALUE_ADDRESS (msymbol)))
{
if (want_space)
fputs_filtered (" ", stream);
fputs_filtered (" <", stream);
fputs_filtered (msymbol.minsym->print_name (), stream);
fputs_filtered (">", stream);
want_space = 1;
}
if (vt_address && options->vtblprint)
{
struct value *vt_val;
struct symbol *wsym = NULL;
struct type *wtype;
if (want_space)
fputs_filtered (" ", stream);
if (msymbol.minsym != NULL)
{
const char *search_name = msymbol.minsym->search_name ();
wsym = lookup_symbol_search_name (search_name, NULL,
VAR_DOMAIN).symbol;
}
if (wsym)
{
wtype = SYMBOL_TYPE (wsym);
}
else
{
wtype = unresolved_elttype;
}
vt_val = value_at (wtype, vt_address);
common_val_print (vt_val, stream, recurse + 1, options,
current_language);
if (options->prettyformat)
{
fprintf_filtered (stream, "\n");
print_spaces_filtered (2 + 2 * recurse, stream);
}
}
}
}
什么时候真正从地址取值
例如，在最后print变量的时候必然是需要取内存中内容的；反之，对于&这种只取地址的操作就没有必要了。
/* See valprint.h. */
void
print_value (value *val, const value_print_options &opts)
{
int histindex = record_latest_value (val);
annotate_value_history_begin (histindex, value_type (val));
printf_filtered ("$%d = ", histindex);
annotate_value_history_value ();
print_formatted (val, 0, &opts, gdb_stdout);
printf_filtered ("\n");
annotate_value_history_end ();
}
/* Access to the value history. */
/* Record a new value in the value history.
Returns the absolute history index of the entry. */
int
record_latest_value (struct value *val)
{
/* We don't want this value to have anything to do with the inferior anymore.
In particular, "set $1 = 50" should not affect the variable from which
the value was taken, and fast watchpoints should be able to assume that
a value on the value history never changes. */
if (value_lazy (val))
value_fetch_lazy (val);
/* We preserve VALUE_LVAL so that the user can find out where it was fetched
from. This is a bit dubious, because then *&$1 does not just return $1
but the current contents of that location. c'est la vie... */
val->modifiable = 0;
value_history.push_back (release_value (val));
return value_history.size ();
}
更直观的例子
如果把char* 类型强转为int，因为print看到的是int类型而不是char * 类型，所以gdb不会尝试输出字符串的内容，进而不会有这个报错了。
(gdb) p (char*)1
$5 = 0x1 <error: Cannot access memory at address 0x1>
(gdb) p (int)(char*)1
$6 = 1
(gdb)