Re: [Libguestfs] [PATCH nbdkit 4/4] common/protocol: Install <nbd-protocol.h> as a public header.
On 9/24/19 4:07 PM, Richard W.M. Jones wrote:
...
> @@ -140,19 +146,19 @@ struct nbd_fixed_new_option_reply_info_export {
> uint16_t info; /* NBD_INFO_EXPORT */
> uint64_t exportsize; /* size of export */
> uint16_t eflags; /* per-export flags */
> -} __attribute__((packed));
> +} NBD_ATTRIBUTE_PACKED;
and here's one that gets member offsets wrong, if packed is not used.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
Some further small changes are made to allow this header to be used
from arbitrary ISO C compilers.
It can now be used from other projects.
---
+#if defined(__GNUC__) || defined(__clang__)
+#define NBD_ATTRIBUTE_PACKED __attribute__((__packed__))
+#else
+#define NBD_ATTRIBUTE_PACKED
+#endif
+
Actually, I just realized that this fallback is wrong - it induces
padding bytes, and thus creates a struct incompatible with the wire
format. We're better off making the #else use
#error Please port to your compiler's notion of a packed struct
so that the file serves as a reference, but does not get mistakenly used
with incorrect padding.
#define NBD_MAX_STRING 4096 /* Maximum length of a string field */
/* Old-style handshake. */
@@ -50,7 +56,7 @@ struct nbd_old_handshake {
uint16_t gflags; /* global flags */
uint16_t eflags; /* per-export flags */
char zeroes[124]; /* must be sent as zero bytes */
-} __attribute__((packed));
+} NBD_ATTRIBUTE_PACKED;
This one actually gets wire alignment without packing.
#define NBD_OLD_VERSION UINT64_C(0x420281861253)
@@ -59,7 +65,7 @@ struct nbd_new_handshake {
char nbdmagic[8]; /* "NBDMAGIC" */
uint64_t version; /* NBD_NEW_VERSION */
uint16_t gflags; /* global flags */
-} __attribute__((packed));
+} NBD_ATTRIBUTE_PACKED;
This one would get tail padding, messing up anyone using sizeof(struct
nbd_new_handshake).
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