[doc] attribute packed does not apply to variables

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  • [doc] attribute packed does not apply to variables
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Martin Sebor Oct. 12, 2018, 3:03 a.m.
The manual says that:

   The packed attribute specifies that a variable or structure
   field should have the smallest possible alignment--one byte
   for a variable, and one bit for a field...

The variable part doesn't actually reflect reality: GCC and all
other compilers I tested ignore the attribute with a warning on
non-member objects, and only honor it on members.

The part that reads "and one bit for a field" is also incorrect:
it most likely meant to say bit-field.

The attached change fixes both of these issues.

Martin

Comments

Jeff Law Oct. 12, 2018, 3:06 a.m. | #1
On 10/11/18 9:03 PM, Martin Sebor wrote:
> The manual says that:
> 
>   The packed attribute specifies that a variable or structure
>   field should have the smallest possible alignment--one byte
>   for a variable, and one bit for a field...
> 
> The variable part doesn't actually reflect reality: GCC and all
> other compilers I tested ignore the attribute with a warning on
> non-member objects, and only honor it on members.
> 
> The part that reads "and one bit for a field" is also incorrect:
> it most likely meant to say bit-field.
> 
> The attached change fixes both of these issues.
> 
> Martin
> 
> gcc-doc-attr-packed.diff
> 
> gcc/ChangeLog:
> 
> 	* doc/extend.texi (attribute packed): Correct typos.
OK.
jeff
Eric Gallager Oct. 12, 2018, 4:43 a.m. | #2
On 10/11/18, Martin Sebor <msebor@gmail.com> wrote:
> The manual says that:
>
>    The packed attribute specifies that a variable or structure
>    field should have the smallest possible alignment--one byte
>    for a variable, and one bit for a field...
>
> The variable part doesn't actually reflect reality: GCC and all
> other compilers I tested ignore the attribute with a warning on
> non-member objects, and only honor it on members.
>
> The part that reads "and one bit for a field" is also incorrect:
> it most likely meant to say bit-field.
>
> The attached change fixes both of these issues.
>
> Martin
>

The word "one" is duplicated in there: "one one" should be just "one"
Martin Sebor Oct. 12, 2018, 3:03 p.m. | #3
On 10/11/2018 10:43 PM, Eric Gallager wrote:
> On 10/11/18, Martin Sebor <msebor@gmail.com> wrote:
>> The manual says that:
>>
>>    The packed attribute specifies that a variable or structure
>>    field should have the smallest possible alignment--one byte
>>    for a variable, and one bit for a field...
>>
>> The variable part doesn't actually reflect reality: GCC and all
>> other compilers I tested ignore the attribute with a warning on
>> non-member objects, and only honor it on members.
>>
>> The part that reads "and one bit for a field" is also incorrect:
>> it most likely meant to say bit-field.
>>
>> The attached change fixes both of these issues.
>>
>> Martin
>>
>
> The word "one" is duplicated in there: "one one" should be just "one"

Yes, thanks.  I spotted it last night just before committing
and removed it.

Martin

Patch

gcc/ChangeLog:

	* doc/extend.texi (attribute packed): Correct typos.

Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi	(revision 265073)
+++ gcc/doc/extend.texi	(working copy)
@@ -6220,13 +6220,13 @@  int f (struct Data *pd, const char *s)
 
 @item packed
 @cindex @code{packed} variable attribute
-The @code{packed} attribute specifies that a variable or structure field
-should have the smallest possible alignment---one byte for a variable,
-and one bit for a field, unless you specify a larger value with the
-@code{aligned} attribute.
+The @code{packed} attribute specifies that a structure member should have
+the smallest possible alignment---one one bit for a bit-field and one byte
+otherwise, unless a larger value is specified with the @code{aligned}
+attribute.  The attribute does not apply to non-member objects.
 
-Here is a structure in which the field @code{x} is packed, so that it
-immediately follows @code{a}:
+For example in the structure below, the member array @code{x} is packed
+so that it immediately follows @code{a} with no intervening padding:
 
 @smallexample
 struct foo