Packing structures?

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1

What is the exact way to pack structures (no extra spaces) in QNX 6? I’ve
tried to find help on the #pragma pack command in the help files but there
is nothing.

Thanks,

Jim

P.S. The docs for gcc/qcc are pretty weak

2

Jim Lambert <jlambert@futurex.com> wrote:
: What is the exact way to pack structures (no extra spaces) in QNX 6? I’ve
: tried to find help on the #pragma pack command in the help files but there
: is nothing.

: Thanks,

From the gcc info docs:

Specifying Attributes of Variables

The keyword __attribute__' allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Eight attributes are currently defined for variables: aligned’, mode', nocommon’,
packed', section’, transparent_union', unused’, and `weak’. Other
attributes are available for functions (*note Function Attributes::slight_smile: and
for types (*note Type Attributes::).

…

packed' 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, unless you specify a larger
value with the `aligned’ attribute.

Here is a structure in which the field x' is packed, so that it immediately follows a’:

struct foo
{
char a;
int x[2] attribute ((packed));
};

3

This is in the QNX docs? I searched on packed and attribute and it did
not come up. Thanks for the info, it works.

Where are these gcc info docs? I’d like to get a copy.

Thanks,

Jim

“Alain Magloire” <alain@qnx.com> wrote in message
news:a0i3ag$j10$2@nntp.qnx.com…

Jim Lambert <> jlambert@futurex.com> > wrote:
: What is the exact way to pack structures (no extra spaces) in QNX 6?
I’ve
: tried to find help on the #pragma pack command in the help files but
there
: is nothing.

: Thanks,

From the gcc info docs:

Specifying Attributes of Variables

The keyword __attribute__' allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Eight attributes are currently defined for variables: aligned’, mode', nocommon’,
packed', section’, transparent_union', unused’, and `weak’. Other
attributes are available for functions (*note Function Attributes::slight_smile: and
for types (*note Type Attributes::).

…

packed' 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, unless you specify a larger
value with the `aligned’ attribute.

Here is a structure in which the field x' is packed, so that it immediately follows a’:

struct foo
{
char a;
int x[2] attribute ((packed));
};

4

Jim Lambert <jlambert@futurex.com> wrote:
: This is in the QNX docs? I searched on packed and attribute and it did
: not come up. Thanks for the info, it works.

: Where are these gcc info docs? I’d like to get a copy.

http://gcc.gnu.org/ (search for manuals)

Or in the src distribution, I believe we use 2.95.2.

You can also order them from the FSF (nicely covered etc …)
http://www.gnu.org

5

On 28 Dec 2001 21:30:44 GMT, Alain Magloire <alain@qnx.com> wrote:


(…)

  1. /usr/info/gcc.info-* files (attributes, pragmas), _pack*.h
    headers in /usr/include, and -fpack-struct compiler param.

ako

6

Jim Lambert <jlambert@futurex.com> wrote:

What is the exact way to pack structures (no extra spaces) in QNX 6? I’ve
tried to find help on the #pragma pack command in the help files but there
is nothing.

Warning: unaligned access to data-types can result in pogram failure
(SIGBUS error) on non-x86 CPU platforms.

-David

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