Libguestfs - Oct 2021 - [PATCH 2/3] lib/proto: suppress "may be used uninitialized" in send_file_complete()

gcc emits the following warning:
> proto.c: In function ?send_file_complete?:
> proto.c:437:10: error: ?buf? may be used uninitialized
> [-Werror=maybe-uninitialized]
>   437 |   return send_file_chunk (g, 0, buf, 0);
>       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In theory, passing the 1-byte array "buf", with indeterminate
contents, to
xdr_bytes() ultimately, could be fine -- assuming xdr_bytes() never reads
the contents of the buffer, due to the buffer size being zero. However,
the xdr_bytes() manual does not seem to guarantee this (it also does not
explicitly permit passing a NULL buffer alongside size=0, which would be
even simpler for the caller).

In order to shut up the compiler, just zero-initialize the buffer --
that's simpler than adding diagnostics pragmas. The
"maybe-uninitialized"
warning is otherwise very useful, so keep it globally enabled (per
WARN_CFLAGS / WERROR_CFLAGS).

Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---
 lib/proto.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/proto.c b/lib/proto.c
index 3976e98b56d0..f798ece05e32 100644
--- a/lib/proto.c
+++ b/lib/proto.c
@@ -433,7 +433,7 @@ send_file_cancellation (guestfs_h *g)
 static int
 send_file_complete (guestfs_h *g)
 {
-  char buf[1];
+  char buf[1] = { '\0' };
   return send_file_chunk (g, 0, buf, 0);
 }
 
-- 
2.19.1.3.g30247aa5d201

On Tue, Oct 12, 2021 at 12:36:26AM +0200, Laszlo Ersek
wrote:
> In order to shut up the compiler, just zero-initialize the buffer --
> that's simpler than adding diagnostics pragmas. The
"maybe-uninitialized"
> warning is otherwise very useful, so keep it globally enabled (per
> WARN_CFLAGS / WERROR_CFLAGS).
...
> +++ b/lib/proto.c
> @@ -433,7 +433,7 @@ send_file_cancellation (guestfs_h *g)
>  static int
>  send_file_complete (guestfs_h *g)
>  {
> -  char buf[1];
> +  char buf[1] = { '\0' };
>    return send_file_chunk (g, 0, buf, 0);
If it were me writing this, I would have done this to shave typing:

  char c = '\0';
  return send_file_chunk (g, 0, &c, 0);

or even abbreviated with:

  char c = 0;

In fact, since send_file_chunk takes a const char *, we could get away
with:


  return send_file_chunk (g, 0, "", 0);

But your way is fine, too, and we aren't in a code golf competition.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org