why function like macro doesn't work here?

**Keith Thompson** · Nov 15 '05, 12:28 AM

Re: why function like macro doesn't work here?

"baumann@pa n" <asmboozer@hotm ail.com> writes:[color=blue]
> hi all,
>
> i defined a macro like the below
>
>
> #define initSock() \
> do{\
> WSADATA ws_data;\
> WSAStartup(0x02 02,&ws_data);\
> }while(0)
>
> then in a function , i wrote
>
> if(!initSock())
> {
> ....
> }
>
> but the compiler complains
> main.c
> e:\project\scan ap\main.c(11) : error C2059: syntax error : 'do'
> e:\project\scan ap\main.c(11) : error C2065: 'ws_data' : undeclared
> identifier
> e:\project\scan ap\main.c(11) : warning C4133: 'function' : incompatible
> types - from 'int *' to 'struct WSAData *'
>
>
> if i change the macro to static function, it's ok.[/color]

The do { ... } while(0) macro definition trick lets you use a macro
invocation as if it were a function call, but only in a statement
context. It doesn't let you use it in an expression, because you
can't have a loop in an expression.

Even if it were allowed, your code probably wouldn't work. You're
trying to use the value "returned" by initSock(), but it doesn't yield
a value.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

**baumann@pan** · Nov 15 '05, 12:28 AM

Re: why function like macro doesn't work here?

Keith Thompson wrote:[color=blue]
> "baumann@pa n" <asmboozer@hotm ail.com> writes:[color=green]
> > hi all,
> >
> > i defined a macro like the below
> >
> >
> > #define initSock() \
> > do{\
> > WSADATA ws_data;\
> > WSAStartup(0x02 02,&ws_data);\
> > }while(0)
> >
> > then in a function , i wrote
> >
> > if(!initSock())
> > {
> > ....
> > }
> >
> > but the compiler complains
> > main.c
> > e:\project\scan ap\main.c(11) : error C2059: syntax error : 'do'
> > e:\project\scan ap\main.c(11) : error C2065: 'ws_data' : undeclared
> > identifier
> > e:\project\scan ap\main.c(11) : warning C4133: 'function' : incompatible
> > types - from 'int *' to 'struct WSAData *'
> >
> >
> > if i change the macro to static function, it's ok.[/color]
>
> The do { ... } while(0) macro definition trick lets you use a macro
> invocation as if it were a function call, but only in a statement
> context. It doesn't let you use it in an expression, because you
> can't have a loop in an expression.
>
> Even if it were allowed, your code probably wouldn't work. You're
> trying to use the value "returned" by initSock(), but it doesn't yield
> a value.
>
> --
> Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
> San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
> We must do something. This is something. Therefore, we must do this.[/color]

is there the way to solute it by macro?

**Chris Dollin** · Nov 15 '05, 12:29 AM

Re: why function like macro doesn't work here?

baumann@pan wrote:
[color=blue][color=green][color=darkred]
>> >
>> > i defined a macro like the below
>> >
>> >
>> > #define initSock() \
>> > do{\
>> > WSADATA ws_data;\
>> > WSAStartup(0x02 02,&ws_data);\
>> > }while(0)
>> >
>> > then in a function , i wrote
>> >
>> > if(!initSock())[/color][/color][/color]
[color=blue]
> is there the way to solute it by macro?[/color]

I don't think so; but why would you bother?

--
Chris "electric hedgehog" Dollin
It's called *extreme* programming, not *stupid* programming.

**John Bode** · Nov 15 '05, 12:29 AM

Re: why function like macro doesn't work here?

baumann@pan wrote:[color=blue]
> hi all,
>
> i defined a macro like the below
>
>
> #define initSock() \
> do{\
> WSADATA ws_data;\
> WSAStartup(0x02 02,&ws_data);\
> }while(0)
>
> then in a function , i wrote
>
> if(!initSock())
> {
> ....
> }
>[/color]

Whoa, hold on. Remeber that a macro is a text placeholder *only*;
after preprocessing, the above statement gets expanded to

if (!do{WSADATA ws_data;WSAStar tup(0x0202,&ws_ data);}while(0) )

Hence the following:
[color=blue]
> but the compiler complains
> main.c
> e:\project\scan ap\main.c(11) : error C2059: syntax error : 'do'
> e:\project\scan ap\main.c(11) : error C2065: 'ws_data' : undeclared
> identifier
> e:\project\scan ap\main.c(11) : warning C4133: 'function' : incompatible
> types - from 'int *' to 'struct WSAData *'
>
>
> if i change the macro to static function, it's ok.
>
>
> any help would be appreciated. thanks.[/color]

Don't do it as a macro. That's about the only help I can offer.
[color=blue]
>
> bauman@Pan[/color]

Me · Nov 15 '05, 12:30 AM

Re: why function like macro doesn't work here?

baumann@pan wrote:[color=blue]
> #define initSock() \
> do{\
> WSADATA ws_data;\
> WSAStartup(0x02 02,&ws_data);\
> }while(0)
>
> then in a function , i wrote
>
> if(!initSock())
> {
> ....
> }
>
> but the compiler complains
> main.c
> e:\project\scan ap\main.c(11) : error C2059: syntax error : 'do'
> e:\project\scan ap\main.c(11) : error C2065: 'ws_data' : undeclared
> identifier
> e:\project\scan ap\main.c(11) : warning C4133: 'function' : incompatible
> types - from 'int *' to 'struct WSAData *'
>
>
> if i change the macro to static function, it's ok.[/color]

"if" wants an expression that returns non-void, you're passing a
statement (minus the required semi-colon) to it. You have a few ways to
convert this to an expression (I'm assuming the return of WSAStartup is
the thing you want to test):

1. rewrite this macro:
#define initSock(v) ... v = WSAStartup ...

some_t good;
initSock(v);
if (v)
...

2. GNU C expression statement extension:
basically replace the do with ( and replace the while(0) with ).

3. rewrite it as a function:
you can figure this out

4. C99 compound literals:
#define initSock() WSAStartup(0x02 02, &(WSADATA){} )

**Kenneth Brody** · Nov 15 '05, 12:31 AM

Re: why function like macro doesn't work here?

"baumann@pa n" wrote:[color=blue]
>
> hi all,
>
> i defined a macro like the below
>
> #define initSock() \
> do{\
> WSADATA ws_data;\
> WSAStartup(0x02 02,&ws_data);\
> }while(0)
>
> then in a function , i wrote
>
> if(!initSock())
> {
> ....
> }
>
> but the compiler complains
> main.c
> e:\project\scan ap\main.c(11) : error C2059: syntax error : 'do'
> e:\project\scan ap\main.c(11) : error C2065: 'ws_data' : undeclared
> identifier
> e:\project\scan ap\main.c(11) : warning C4133: 'function' : incompatible
> types - from 'int *' to 'struct WSAData *'
>
> if i change the macro to static function, it's ok.
>
> any help would be appreciated. thanks.[/color]

If I declare initSock() as a function:

void initSock()
{
do{
WSDATA ws_data;
WSAStartup(0x02 02,&ws_data);
}while(0);
}

my compiler complains here as well, telling me on the "if(!initSock() )"
line:

foo.c(20) : error C2171: '!' : illegal on operands of type 'void '
foo.c(20) : error C2180: controlling expression has type 'void'
foo.c(20) : error C2180: controlling expression has type 'void'

Hint... what will be the result of expanding your macro?

--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer .h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:ThisIsA SpamTrap@gmail. com>

**Dave Thompson** · Nov 15 '05, 12:40 AM

Re: why function like macro doesn't work here?

On 8 Jul 2005 13:01:46 -0700, "Me" <anti_spam_emai l2003@yahoo.com >
wrote:

<snip>[color=blue]
> "if" wants an expression that returns non-void, you're passing a
> statement (minus the required semi-colon) to it. You have a few ways to[/color]

Actually, that yields scalar = not void and not struct or union;
arithmetic or pointer is OK, or array which decays to pointer.
[color=blue]
> convert this to an expression (I'm assuming the return of WSAStartup is
> the thing you want to test):
>
> 1. rewrite this macro:
> #define initSock(v) ... v = WSAStartup ...
>
> some_t good;
> initSock(v);
> if (v)
> ...
>[/color]
initSock(good);
if( good )

except that actually WSAStartup returning 0 is good and nonzero is
bad, so either
#define initSock(ok) ... ok = WSAStartup (...) == 0; ...
_Bool myok; initSock (myok); if( !myok ) /* error */

or
#define initSock(err) ... err = WSAStartup (...); ....
int myerr; initSock (myerr); if( myerr /* != 0 */ ) /* error */
[color=blue]
> 2. GNU C expression statement extension:
> basically replace the do with ( and replace the while(0) with ).
>
> 3. rewrite it as a function:
> you can figure this out
>
> 4. C99 compound literals:
> #define initSock() WSAStartup(0x02 02, &(WSADATA){} )[/color]

or even in C90 ... WSAStartup (0x0202, malloc(sizeof(W SADATA))) ...
leaving one fairly small memory leak per process.

- David.Thompson1 at worldnet.att.ne t

**Sensei** · Nov 15 '05, 12:45 AM

Re: why function like macro doesn't work here?

On 2005-07-07 21:27:51 -0500, "baumann@pa n" <asmboozer@hotm ail.com> said:
[color=blue]
> #define initSock() \
> do{\
> WSADATA ws_data;\
> WSAStartup(0x02 02,&ws_data);\
> }while(0)
>
> then in a function , i wrote
>
> if(!initSock())
> {
> ....
> }[/color]

Wrong.

Why don't you use

inline int initSock()
{
....
}
[color=blue]
> if i change the macro to static function, it's ok.[/color]

I make it easy: think the #define statement as just as a
Search-and-Replace facility given by the compiler.

--
Sensei <senseiwa@tin.i t>

cd /pub
more beer

why function like macro doesn't work here?

why function like macro doesn't work here?

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