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Hello,

What does this print:

printf("%d\n", 3 ?: 4);

For me it seems to print the number 3. It seems the expression says
if the first expression is true than use the second expression as the
result. But if there is no second expression than use the first
expression.

I don't see this syntax (with the missing second expression)
referenced in the C language standard, so I was wondering if this is
official C language syntax?

Cheers,
Ivan Novick

In article <59f5a3b1-6dce-4382-a82d->,
<> wrote:
>Hello,
>
>What does this print:
>
>printf("%d\n", 3 ?: 4);

CLC answer: A syntax error during compilation.

Real answer: It is a gcc extension.

--
> No, I haven't, that's why I'm asking questions. If you won't help me,
> why don't you just go find your lost manhood elsewhere.

CLC in a nutshell.

On 7/2/2010 10:47 AM, wrote:
> Hello,
>
> What does this print:
>
> printf("%d\n", 3 ?: 4);
>
> For me it seems to print the number 3. It seems the expression says
> if the first expression is true than use the second expression as the
> result. But if there is no second expression than use the first
> expression.
>
> I don't see this syntax (with the missing second expression)
> referenced in the C language standard, so I was wondering if this is
> official C language syntax?

It is not. A conforming C implementation must issue a
diagnostic message.

You're probably using the popular gcc compiler, whose default
mode compiles a language that isn't exactly C, but "C with extras."
One of those extras is the construct you ask about: In gcc-dialect,
`x ?: y' is equivalent to `x ? x : y' (except that `x' is evaluated
only once).

To compile standard C with the gcc compiler, use a command-line
flag to specify the C version you intend: `-ansi' or `-std=c99',
usually. Consider using the `-pedantic' flag if your code is clean
enough to withstand it. I also recommend `-Wall -W'.

--
Eric Sosman
lid

Eric Sosman <> writes:

> On 7/2/2010 10:47 AM, wrote:
<snip>
>> printf("%d\n", 3 ?: 4);
<snip>
> To compile standard C with the gcc compiler, use a command-line
> flag to specify the C version you intend: `-ansi' or `-std=c99',
> usually. Consider using the `-pedantic' flag if your code is clean
> enough to withstand it.

gcc needs -pedantic to turn off its extensions (such as the one being
discussed). This may depend on the gcc version, but I think this is how
it has been for some time.

--
Ben.