Consider this short C example:
struct inner
{
unsigned x;
unsigned y : 3;
unsigned z : 3;
};
struct outer
{
unsigned char o : 3;
struct inner i __attribute__ ((packed));
};
When I use "ptype/o" on this, I get:
(gdb) ptype/o struct outer
/* offset | size */ type = struct outer {
/* 0: 5 | 1 */ unsigned char o : 3;
/* XXX 5-bit hole */
/* 1 | 8 */ struct inner {
/* 1 | 4 */ unsigned int x;
/* 5:29 | 4 */ unsigned int y : 3;
/* 5:26 | 4 */ unsigned int z : 3;
/* XXX 2-bit padding */
/* XXX 3-byte padding */
/* total size (bytes): 8 */
} i;
/* total size (bytes): 9 */
}
In the location of "o" ("0: 5"), the "5" means "there are 5 bits left
relative to the size of the underlying type.
I find this very difficult to follow. On irc, Sergio said that this
choice came because it is what pahole does. However, I think it's not
very useful, and maybe is just an artifact of the way that
DW_AT_bit_offset was defined in DWARF 3.
This patch changes ptype/o to print the offset of a bitfield in a more
natural way, that is, using the bit number according to the platform's
bit numbering.
With this patch, the output is now:
(gdb) ptype/o struct outer
/* offset | size */ type = struct outer {
/* 0: 0 | 1 */ unsigned char o : 3;
/* XXX 5-bit hole */
/* 1 | 8 */ struct inner {
/* 1 | 4 */ unsigned int x;
/* 5: 0 | 4 */ unsigned int y : 3;
/* 5: 3 | 4 */ unsigned int z : 3;
/* XXX 2-bit padding */
/* XXX 3-byte padding */
/* total size (bytes): 8 */
} i;
/* total size (bytes): 9 */
}
This is better, IMO, because now the "offset" of a bitfield is
consistent with the offset of an ordinary member, referring to its
offset from the start of the structure.
gdb/ChangeLog
2019-05-08 Tom Tromey <tromey@adacore.com>
* typeprint.c (print_offset_data::update): Print the bit offset,
not the number of bits remaining.
gdb/doc/ChangeLog
2019-05-08 Tom Tromey <tromey@adacore.com>
* gdb.texinfo (Symbols): Document change to ptype/o.
gdb/testsuite/ChangeLog
2019-05-08 Tom Tromey <tromey@adacore.com>
* gdb.base/ptype-offsets.exp: Update tests.
9d3421afbb
