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

In article <a28f28eb-52b0-4fd3-b045-4b29f46161b3@o77g2000hsf.googlegroups.com>,
Boltar <boltar2003@yahoo.co.uk> 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?
>
>B2003

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

On 27 Feb, 09:44, gaze...@xmission.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

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;
here you assign 1 to a, always, as 1 always equals 1

> {
> a = 2;
here you assign 2 to a, always

> }
> printf("a = %d\n",a);
> }
>
> When run it will print "a = 2". Should it compile at all or is GCCs
> parser broken?
Why do you think it is broken or should print something else?

Bye, Jojo

Boltar <boltar2003@yahoo.co.uk> writes:

> 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.

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("Earthquake 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

Boltar said:

> On 27 Feb, 09:44, gaze...@xmission.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

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>

In article <a28f28eb-52b0-4fd3-b045-4b29f46161b3@o77g2000hsf.googlegroups.com>,
Boltar <boltar2003@yahoo.co.uk> wrote:
>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

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("Earthquake 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_FEATURE
if ( new_feature_is_enabled() )
{
do_it_the_new_way();
do_another_thing_the_new_way();
}
else
#endif
{
do_it_the_old_way();
do_the_other_thing_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:ThisIsASpamTrap@gmail.com>

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

On 27 Feb, 09:56, Richard Heathfield <r...@see.sig.invalid> wrote:
> 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

Kenneth Brody <kenbrody@spamcop.net> writes:
[...]
> In case the OP (or anyone else) is wondering where this would be
> useful, consider:
>
> #if ENABLE_SOME_FEATURE
> if ( new_feature_is_enabled() )
> {
> do_it_the_new_way();
> do_another_thing_the_new_way();
> }
> else
> #endif
> {
> do_it_the_old_way();
> do_the_other_thing_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_FEATURE
if ( new_feature_is_enabled() )
{
do_it_the_new_way();
do_another_thing_the_new_way();
}
else
{
#endif
do_it_the_old_way();
do_the_other_thing_the_old_way();
#if ENABLE_SOME_FEATURE
}
#endif

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

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

Boltar <boltar2003@yahoo.co.uk> writes:
> 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_statement_added_later;

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

--
Keith Thompson (The_Other_Keith) <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 wrote:
>
> Kenneth Brody <kenbrody@spamcop.net> writes:
> [...]
> > In case the OP (or anyone else) is wondering where this would be
> > useful, consider:
> >
> > #if ENABLE_SOME_FEATURE
> > if ( new_feature_is_enabled() )
> > {
> > do_it_the_new_way();
> > do_another_thing_the_new_way();
> > }
> > else
> > #endif
> > {
> > do_it_the_old_way();
> > do_the_other_thing_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_FEATURE
> if ( new_feature_is_enabled() )
> {
> do_it_the_new_way();
> do_another_thing_the_new_way();
> }
> else
> {
> #endif
> do_it_the_old_way();
> do_the_other_thing_the_old_way();
> #if ENABLE_SOME_FEATURE
> }
> #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_ANOTHER_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_FEATURE
> ...
> #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 < LengthOfSomething ; i++ )
{
DoMyLogging(something[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:ThisIsASpamTrap@gmail.com>

> if (1 == 1) a = 1;

The semicolon brings the if to an end.

MisterE wrote:
>
>> if (1 == 1) a = 1;
>
> The semicolon brings the if to an end.

In Pascal. Not in C. In C the definitive thing is the presence of
a following 'else'.

--
[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

CBFalconer said:

> MisterE wrote:
>>
>>> if (1 == 1) a = 1;
>>
>> The semicolon brings the if to an end.
>
> In Pascal. Not in C. In C the definitive thing is the presence of
> a following 'else'.

If that is true, the following code will compile, and will *not* print
"Hmmm". Otherwise, the compilation will fail.

#include <stdio.h>

int main(void)
{
int a = 0;
if(1 == 1)
a = 1;
printf("Hmmm\n");
else
NULL;
return 0;
}

--
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 wrote:
> CBFalconer said:
>
>> MisterE wrote:
>>>> if (1 == 1) a = 1;
>>> The semicolon brings the if to an end.
>> In Pascal. Not in C. In C the definitive thing is the presence of
>> a following 'else'.
>
> If that is true,

It's presumably true for the version of C that Chuck imagines...

I _think_ his intentions are reasonably sound, but his bombastic
attitude combined with his being simply wrong far too often means
that I've "tuned him out".

In article <fq6o0g$it7$1@aioe.org>,
Mark Bluemel <mark_bluemel@pobox.com> wrote:
>Richard Heathfield wrote:
>> CBFalconer said:
>>
>>> MisterE wrote:
>>>>> if (1 == 1) a = 1;
>>>> The semicolon brings the if to an end.
>>> In Pascal. Not in C. In C the definitive thing is the presence of
>>> a following 'else'.
>>
>> If that is true,
>
>It's presumably true for the version of C that Chuck imagines...
>
>I _think_ his intentions are reasonably sound, but his bombastic
>attitude combined with his being simply wrong far too often means
>that I've "tuned him out".

Ooooooh. It seems Chuck is not clicking with the Clique anymore.

Richard Heathfield wrote:
> CBFalconer said:
>
>> MisterE wrote:
>>>> if (1 == 1) a = 1;
>>> The semicolon brings the if to an end.
>> In Pascal. Not in C. In C the definitive thing is the presence of
>> a following 'else'.
>
> If that is true, the following code will compile, and will *not* print
> "Hmmm". Otherwise, the compilation will fail.

...."presence of [an immediately] following 'else'."...

Howzat?

--
Micah J. Cowan
Programmer, musician, typesetting enthusiast, gamer...
http://micah.cowan.name/

In article <47C6C7CB.49D7DBBC@yahoo.com>,
CBFalconer <cbfalconer@maineline.net> wrote:

>>> if (1 == 1) a = 1;

>> The semicolon brings the if to an end.

>In Pascal. Not in C. In C the definitive thing is the presence of
>a following 'else'.

In C, the definitive thing is matching the grammar, which in this case
means that you can't just look for an else, a semicolon, or a closing
brace. You have to look for the end of the "then" part, then look to
see if there's an else immediately following.

-- Richard
--
:wq

Richard Heathfield wrote:
> CBFalconer said:
>> MisterE wrote:
>>>
>>>> if (1 == 1) a = 1;
>>>
>>> The semicolon brings the if to an end.
>>
>> In Pascal. Not in C. In C the definitive thing is the presence of
>> a following 'else'.
>
> If that is true, the following code will compile, and will *not* print
> "Hmmm". Otherwise, the compilation will fail.
>
> #include <stdio.h>
>
> int main(void)
> {
> int a = 0;
> if(1 == 1)
> a = 1;
> printf("Hmmm\n");
> else
> NULL;
> return 0;
> }

That has a following printf, not a following else. As you knew
anyhow. However, in Pascal:

IF true THEN a := 1;
ELSE (* anything *)

will bring up large compiler screams at the ELSE. Remove the semi
and all is well.

--
[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

CBFalconer said:

> Richard Heathfield wrote:
>> CBFalconer said:
>>> MisterE wrote:
>>>>
>>>>> if (1 == 1) a = 1;
>>>>
>>>> The semicolon brings the if to an end.
>>>
>>> In Pascal. Not in C. In C the definitive thing is the presence of
>>> a following 'else'.
>>
<snip>

>> if(1 == 1)
>> a = 1;
>> printf("Hmmm\n");
>> else
>> NULL;
>> return 0;
>> }
>
> That has a following printf, not a following else.

Er, that else sure looks like a following else to me.

Here's another counter-example:

int main(void)
{
if(1 == 1)
return 0;
}

According to you, that 'if' has no end (because there is no following
'else' present).

--
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 wrote:

>>> if(1 == 1)
>>> a = 1;
>>> printf("Hmmm\n");
>>> else
>>> NULL;
>>> return 0;
>>> }
>> That has a following printf, not a following else.
>
> Er, that else sure looks like a following else to me.

No, the else is following the printf.