From: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
Newsgroups: comp.os.msdos.djgpp
Subject: GCC/Info docs bug regarding __attribute__
Date: 1 Aug 1997 04:09:38 GMT
Organization: The National Capital FreeNet
Lines: 136
Message-ID: <5rrni2$48j@freenet-news.carleton.ca>
Reply-To: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
NNTP-Posting-Host: freenet5.carleton.ca
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk



In a program I am writing I am having problems using __attribute__ ((packed))
on a struct meant for reading the (unpacked) records of a binary file
generated from other software (a Visual Basic app to be precise).

As indicated in the documentation (info gcc) I have used this:

struct etmp {

...

} __attribute__ ((packed));

But, this generates the error:

 ../chem/element.cc(43) Error: semicolon missing after declaration of `etmp'

According to the GCC dox this should work!


I also tried ending with

}; __attribute__ ((packed));

This causes:

../chem/element.cc(43) Warning:  empty declaration

which indicates that the __attribute__ is now being treated as a seperate item
from the struct and is not working.

Lastly I tried:

struct etmp __attribute__ ((packed)) {

...

};

But that causes:

 ../chem/element.cc(27) Error: parse error before `{'
 ../chem/element.cc(43) Error: parse error before `}'
 In method `element::element(void *)':
 ../chem/element.cc(84) Error: aggregate `struct etmp e' has incomplete type and cannot be initialized

(27 is the struct line, 43 is the end of struct line, and 84 is the
first line to try to refer to the struct.)


Relevant clauses in info:

Specifying Attributes of Types
==============================

   The keyword `__attribute__' allows you to specify special attributes
of `struct' and `union' types when you define such types. This keyword
is followed by an attribute specification inside double parentheses.
Three attributes are currently defined for types: `aligned', `packed',
and `transparent_union'. ...

[therefore, __attribute__ ((packed)) is written correctly...]

   ... You may specify ... the `packed' attribute only past the closing
brace of a [struct] definition. ...

[therefore, struct foo { ... } __attribute__ ((packed)); is correct or
 struct foo { ... }; __attribute__ ((packed)); is correct, whereas the
 compiler barfs for both! This seems to be a contradiction.]

... For example, you could write:

          struct S { short f[3]; } __attribute__ ((aligned));
...

[This example implies the same syntax is correct for ((packed)), but that
 caused the missing semicolon error!]

...`packed'
     This attribute, attached to an `enum', `struct', or `union' type
     definition, specified that the minimum required memory be used to
     represent the type.

     Specifying this attribute for `struct' and `union' types is
     equivalent to specifying the `packed' attribute on each of the
     structure or union members.  Specifying the `-fshort-enums' flag
     on the line is equivalent to specifying the `packed' attribute on
     all `enum' definitions.

     You may only specify this attribute after a closing curly brace on
     an `enum' definition, not in a `typedef' declaration.

[Even if this also applies to 'struct', I do not have 'typedef struct
 etmp { ... } ETMP __attribute__ ((packed))', which would then be illegal,
 but merely 'struct etmp { ... } __attribute__ ((packed))'!]


That is all the dox say about the packed attribute for types.


I am forced to conclude one of three things is the case:

1. The syntax for the packed attribute differs from that for the aligned one,
   and the dox are incomplete in this area; and the __attribute__ ((packed))
   belongs elsewhere in the struct. If so, I need urgently to know where,
   and the docs need updating.
2. The syntax is correct, but GCC has a bug. In that case I need to know of
   a workaround urgently, and GCC needs to be debugged and redistributed.
3. There are certain situations (Such as what? Structs with four letter names?
   Structs with doubles? Structs with strings? Mine has a double
   and a couple of fixed length strings, char foo[N]; style... C++ structs?
   Structs compiled during a full moon?!) in which attributes are not
   allowed, and the dox are incomplete and a workaround is again required
   urgently. In this case there is a buglet in GCC also, in the way of an
   obtuse error message that could be made more informative.



Unfortunately, important work is at a virtual standstill until this bug gets
resolved. (Maybe someone compiled a virus with a "class werewolf"... maybe
it infected my compiler... Maybe my struct is undergoing lycanthropy, and
turns into a class, with different syntax, during full moon nights!)



...
C:\DJGPP> antivir -detect -clean -log -drives: ACD -memory -bootsectors
...


--
    .*.  Where feelings are concerned, answers are rarely simple [GeneDeWeese]
 -()  <  When I go to the theater, I always go straight to the "bag and mix"
    `*'  bulk candy section...because variety is the spice of life... [me]
Paul Derbyshire ao950 AT freenet DOT carleton DOT ca, http://chat.carleton.ca/~pderbysh