On Sat, Aug 14, 2021 at 03:59:22PM +0200, Christophe JAILLET wrote:

Hi all,

Le 05/08/2021 à 12:43, Dan Carpenter a écrit :

quoted

The "= {};" style empty struct initializer is preferred over = {0}.
It avoids the situation where the first struct member is a pointer and
that generates a Sparse warning about assigning using zero instead of
NULL.  Also it's just nicer to look at.

Some people complain that {} is less portable but the kernel has
different portability requirements from userspace so this is not a
issue that we care about.

Signed-off-by: Dan Carpenter <redacted>
---
  scripts/checkpatch.pl | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 461d4221e4a4..32c8a0ca6fd0 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl

@@ -4029,6 +4029,12 @@ sub process {
  			     "Using $1 is unnecessary\n" . $herecurr);
  		}
+# prefer = {}; to = {0};
+		if ($line =~ /= \{ *0 *\}/) {
+			WARN("ZERO_INITIALIZER",
+			     "= {} is preferred over = {0}\n" . $herecurr);
+		}
+
  # Check for potential 'bare' types
  		my ($stat, $cond, $line_nr_next, $remain_next, $off_next,
  		    $realline_next);

[1] and [2] state that {} and {0} don't have the same effect. So if correct,
this is not only a matter of style.

When testing with gcc 10.3.0, I arrived at the conclusion that both {} and
{0} HAVE the same behavior (i.e the whole structure and included structures
are completely zeroed) and I don't have a C standard to check what the rules
are.
gcc online doc didn't help me either.

To test, I wrote a trivial C program, compiled it with gcc -S and looked at
the assembly files.


Maybe, if it is an undefined behavior, other compilers behave differently
than gcc.


However, the 2 persons listed bellow have a much better Linux and C
background than me. So it is likely that my testings were too naive.

There are number of reasons why you didn't notice any difference.
1. {} is GCC extension
2. {} was adopted in latest C standards, so need to check which one GCC 10
is using by default.
3. Main difference will be in padding - {0} will set to zero fields but
won't touch padding, while {} will zero everything.

Can someone provide some rational or compiler output that confirms that {}
and {0} are not the same?

Because if confirmed, I guess that there is some clean-up work to do all
over the code, not only to please Sparse!


Thanks in advance.
CJ



[1]: Russell King - https://lore.kernel.org/netdev/YRFGxxkNyJDxoGWu@shredder/T/#efe1b6c7862b7ca9588c2734f04be5ef94e03d446 (local)

[2]: Leon Romanovsky - https://lore.kernel.org/netdev/YRFGxxkNyJDxoGWu@shredder/T/#efe1b6c7862b7ca9588c2734f04be5ef94e03d446 (local)