Legal C or bug in gcc

**Kenny McCormack** · Feb 27 '08, 09:45 AM

Re: Legal C or bug in gcc

In article <a28f28eb-52b0-4fd3-b045-4b29f46161b3@o7 7g2000hsf.googl egroups.com>,
Boltar <boltar2003@yah oo.co.ukwrote:

>By accident I came across a bug like this in some Linux code I'd
>written today. All that had happened is I forgot the "else" yet the
>code still compiled and ran. A simplified example is below.
>
>#include <stdio.h>
>
>main()
>{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
>}
>
>When run it will print "a = 2". Should it compile at all or is GCCs
>parser broken?
>
>B2003

Time for me to re-post my "I wrote this hello, world program and it
compiled and ran just fine; why?" post.

**Boltar** · Feb 27 '08, 09:55 AM

Re: Legal C or bug in gcc

On 27 Feb, 09:44, gaze...@xmissio n.xmission.com (Kenny McCormack)
wrote:

Time for me to re-post my "I wrote this hello, world program and it
compiled and ran just fine; why?" post.

I should've known better than to expect a sensible answer from this
group.

Anyway , I just realised its treating the bracketed part as a seperate
block so you can save anymore sarcastic responses.

B2003

**Ben Bacarisse** · Feb 27 '08, 09:55 AM

Re: Legal C or bug in gcc

Boltar <boltar2003@yah oo.co.ukwrites:

By accident I came across a bug like this in some Linux code I'd
written today. All that had happened is I forgot the "else" yet the
code still compiled and ran. A simplified example is below.
>
#include <stdio.h>
>
main()
{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
}
>
When run it will print "a = 2". Should it compile at all or is GCCs
parser broken?

It is entirely legal. A compound statement (braces containing other
statements) is just another valid option where a statement is
required.

--
Ben.

**Richard Heathfield** · Feb 27 '08, 09:55 AM

Re: Legal C or bug in gcc

Boltar said:

By accident I came across a bug like this in some Linux code I'd
written today. All that had happened is I forgot the "else" yet the
code still compiled and ran. A simplified example is below.
>
#include <stdio.h>
>
main()
{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
}
>
When run it will print "a = 2". Should it compile at all or is GCCs
parser broken?

It's legal.

{
a = 2;
}

is a compound statement. You can have these pretty much anywhere you can
have an ordinary statement. If you like, you can do this:

#include <stdio.h>
int main(void)
{
{ printf("Hello, world!\n"); }
{ puts("How are you today?"); }
{ puts("Earthquak e in UK - film at 11"); }
{ return 0; }
}

Although the above is a pastiche, this facility is nevertheless useful and
powerful, but does have the unfortunate consequence you have noted -
forgetting an "else" doesn't necessarily give you the syntax error you'd
have hoped for!

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**Richard Heathfield** · Feb 27 '08, 09:55 AM

Re: Legal C or bug in gcc

Boltar said:

On 27 Feb, 09:44, gaze...@xmissio n.xmission.com (Kenny McCormack)
wrote:

>Time for me to re-post my "I wrote this hello, world program and it
>compiled and ran just fine; why?" post.

>
I should've known better than to expect a sensible answer from this
group.

Mr McCormack is a troll. You get them in any group.

I have already posted a sensible answer.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**santosh** · Feb 27 '08, 10:35 AM

Re: Legal C or bug in gcc

Boltar wrote:

By accident I came across a bug like this in some Linux code I'd
written today. All that had happened is I forgot the "else" yet the
code still compiled and ran. A simplified example is below.
>
#include <stdio.h>
>
main()
{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
}
>
When run it will print "a = 2". Should it compile at all or is GCCs
parser broken?

No this is legal C. A opening { starts a compound statement or block,
closed by the next }. It's sometimes useful for data confinement.

<OT>
There *is* a related GCC extension which allows a compound statement
enclosed in parenthesis as an expression, but this is not an
illustration of one. This is pure Standard C.
</OT>

**Richard Tobin** · Feb 27 '08, 10:35 AM

Re: Legal C or bug in gcc

In article <a28f28eb-52b0-4fd3-b045-4b29f46161b3@o7 7g2000hsf.googl egroups.com>,
Boltar <boltar2003@yah oo.co.ukwrote:

>By accident I came across a bug like this in some Linux code I'd

>main()
>{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
>}

Perhaps you should run it through an indenting tool for enlightenment:

main()
{
int a;

if (1 == 1)
a = 1;

{
a = 2;
}

printf("a = %d\n",a);
}

-- Richard
--
:wq

**Kenneth Brody** · Feb 27 '08, 03:05 PM

Re: Legal C or bug in gcc

Richard Heathfield wrote:

>
Boltar said:
>

By accident I came across a bug like this in some Linux code I'd
written today. All that had happened is I forgot the "else" yet the
code still compiled and ran. A simplified example is below.

#include <stdio.h>

main()
{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
}

When run it will print "a = 2". Should it compile at all or is GCCs
parser broken?

>
It's legal.
>
{
a = 2;
}
>
is a compound statement. You can have these pretty much anywhere you can
have an ordinary statement. If you like, you can do this:
>
#include <stdio.h>
int main(void)
{
{ printf("Hello, world!\n"); }
{ puts("How are you today?"); }
{ puts("Earthquak e in UK - film at 11"); }
{ return 0; }
}
>
Although the above is a pastiche, this facility is nevertheless useful and
powerful, but does have the unfortunate consequence you have noted -
forgetting an "else" doesn't necessarily give you the syntax error you'd
have hoped for!

In case the OP (or anyone else) is wondering where this would be
useful, consider:

#if ENABLE_SOME_FEA TURE
if ( new_feature_is_ enabled() )
{
do_it_the_new_w ay();
do_another_thin g_the_new_way() ;
}
else
#endif
{
do_it_the_old_w ay();
do_the_other_th ing_the_old_way ();
}

Compare this "clean" version to how you would have to write it if
compound statements weren't legal on their own.

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

**Boltar** · Feb 27 '08, 05:15 PM

Re: Legal C or bug in gcc

On 27 Feb, 09:56, Richard Heathfield <r...@see.sig.i nvalidwrote:

I have already posted a sensible answer.

Yes , thanks for that. I realised what it was happening with the code
about 10 seconds after I posted. That'll teach me to post when I'm
half asleep instead of attempting to think.

B2003

**CBFalconer** · Feb 27 '08, 05:25 PM

Re: Legal C or bug in gcc

Richard Heathfield wrote:

>

.... snip ...

>
Although the above is a pastiche, this facility is nevertheless
useful and powerful, but does have the unfortunate consequence
you have noted - forgetting an "else" doesn't necessarily give
you the syntax error you'd have hoped for!

Which is where the Pascal syntax of never preceding an else with a
semi is useful.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

**Keith Thompson** · Feb 27 '08, 05:45 PM

Re: Legal C or bug in gcc

Kenneth Brody <kenbrody@spamc op.netwrites:
[...]

In case the OP (or anyone else) is wondering where this would be
useful, consider:
>
#if ENABLE_SOME_FEA TURE
if ( new_feature_is_ enabled() )
{
do_it_the_new_w ay();
do_another_thin g_the_new_way() ;
}
else
#endif
{
do_it_the_old_w ay();
do_the_other_th ing_the_old_way ();
}
>
Compare this "clean" version to how you would have to write it if
compound statements weren't legal on their own.

It's not *that* bad:

#if ENABLE_SOME_FEA TURE
if ( new_feature_is_ enabled() )
{
do_it_the_new_w ay();
do_another_thin g_the_new_way() ;
}
else
{
#endif
do_it_the_old_w ay();
do_the_other_th ing_the_old_way ();
#if ENABLE_SOME_FEA TURE
}
#endif

It also has the advantage that if you delete all the
#if ENABLE_SOME_FEA TURE
...
#endif
sections, the remaining code still looks ok (apart from the
indentation).

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Keith Thompson** · Feb 27 '08, 05:55 PM

Re: Legal C or bug in gcc

Boltar <boltar2003@yah oo.co.ukwrites:

By accident I came across a bug like this in some Linux code I'd
written today. All that had happened is I forgot the "else" yet the
code still compiled and ran. A simplified example is below.
>
#include <stdio.h>
>
main()
{
int a;
if (1 == 1) a = 1;
{
a = 2;
}
printf("a = %d\n",a);
}
>
When run it will print "a = 2". Should it compile at all or is GCCs
parser broken?

Your question has already been answered to death (and BTW "Kenny
McCormack" is at least annoying to us as he is to you), but let me
offer a suggestion that can avoid such problems.

Whenever I write an if, while, for, or do-while statement, I always
use blocks for the controlled statements. For example:

if (1 == 1) {
a = 1;
}
else {
a = 2;
}

It's a habit I picked up from Perl, which requires it, but I find that
it's useful in C as well.

Strictly speaking you'll still have the same problem if you drop the
"else", but with a more uniform layout I think you're less liketly to
do so. This style also avoids problems like:

if (condition)
statement;
another_stateme nt_added_later;

Of course, plenty of smart people don't like this style.

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Kenneth Brody** · Feb 27 '08, 09:15 PM

Re: Legal C or bug in gcc

Keith Thompson wrote:

>
Kenneth Brody <kenbrody@spamc op.netwrites:
[...]

In case the OP (or anyone else) is wondering where this would be
useful, consider:

#if ENABLE_SOME_FEA TURE
if ( new_feature_is_ enabled() )
{
do_it_the_new_w ay();
do_another_thin g_the_new_way() ;
}
else
#endif
{
do_it_the_old_w ay();
do_the_other_th ing_the_old_way ();
}

Compare this "clean" version to how you would have to write it if
compound statements weren't legal on their own.

>
It's not *that* bad:
>
#if ENABLE_SOME_FEA TURE
if ( new_feature_is_ enabled() )
{
do_it_the_new_w ay();
do_another_thin g_the_new_way() ;
}
else
{
#endif
do_it_the_old_w ay();
do_the_other_th ing_the_old_way ();
#if ENABLE_SOME_FEA TURE
}
#endif

I've seen code like that, and it looks "ugly" to me. Plus, I think
mine is earier to maintain. Consider what happens with you start to
#define ENABLE_ANOTHER_ FEATURE and ENABLE_YET_ANOT HER_FEATURE, and
start having multiple if/elseif's along the way.

It also has the advantage that if you delete all the
#if ENABLE_SOME_FEA TURE
...
#endif
sections, the remaining code still looks ok (apart from the
indentation).

Then what about debugging code, with local variables?

#if DO_MY_LOGGING
{
int i;
for ( i=0 ; i < LengthOfSomethi ng ; i++ )
{
DoMyLogging(som ething[i]);
}
}
#endif

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

**MisterE** · Feb 28 '08, 07:25 AM

Re: Legal C or bug in gcc

if (1 == 1) a = 1;

The semicolon brings the if to an end.

Legal C or bug in gcc

Legal C or bug in gcc

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