Can't define variables after any operation?

Question

My C language teacher claims that all variables must be defined before any operation. I can somehow recall it's a very old feature of C (no later than 1990) but I can't reproduce it with GCC 7.2.0.

My teacher claims this:

int main(){
    int a; /* Valid */
    a = 1; /* An operation */
    int b; /* Invalid because an operation has already occurred */
    return 0;
}

I tried compiling with

gcc test.c -std=c89 -Wall -Wextra -pedantic

but it gives no error, not even a warning.

How can I verify (or prove wrong) that statement?

Fyi, those `// comments` are invalid in strict c89 as well. I'm surprised if they, too, are not flagged by gcc with pedantic errors enabled. clang definitely catches them. — WhozCraig, Sep 12 '17 at 07:39
[There you go](http://coliru.stacked-crooked.com/a/6c82a8b20a2959e8) — StoryTeller - Unslander Monica, Sep 12 '17 at 07:40
I don't have GCC 7.2.0 so I cannot confirm, but my gcc 6.3.0 Ubuntu spits out quite a few warnings. — Antti Haapala -- Слава Україні, Sep 12 '17 at 07:40
as far as i remember , when compiler generates object code, he moves all declarings to the top of it's visible scope — bobra, Sep 12 '17 at 07:42
@AnttiHaapala - The online compiler at coliru (my link) is GCC 7.2.0. This is most probably a mistake on the OP's part. — StoryTeller - Unslander Monica, Sep 12 '17 at 07:42
@StoryTeller also, here's [godbolt](https://godbolt.org/g/wstW2o) — Antti Haapala -- Слава Україні, Sep 12 '17 at 07:46
@AnttiHaapala Where can I see compiler output? I only see one line with ``. — iBug, Sep 12 '17 at 08:13
Tell your teacher that several things have happened since 1989. — Art, Sep 12 '17 at 08:16
@iBug Teaching ANSI C today is like teaching the Berlin wall as current affairs. — Art, Sep 12 '17 at 08:18
@iBug click on the button with the /!\ symbol on the lower right... — Antti Haapala -- Слава Україні, Sep 12 '17 at 08:28
@Art Well, it's a mandatory course in [my university](https://chinese.stackexchange.com/users/16738). If I could I would have opt in for an up-to-date C++ course (like [tag:c++1z]). — iBug, Sep 12 '17 at 08:29
@iBug Or C11. Or at least C99 (which is old enough to vote in most countries). I keep seeing questions here that suggest that there are professors out there that learned something 30 years ago and think that nothing worth knowing has happened since then. — Art, Sep 12 '17 at 08:49

gsamaras · Accepted Answer · 2017-09-12T07:46:18.857

4

Compile with -pedantic-errors, like this:

gcc test.c -std=c89 -Wall -Wextra -pedantic-errors

and you should see this error (among the unused variable warnings):

test.c:4:5: error: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
     int b;
     ^~~

in GCC 7.20.

PS: The comments are invalid too, I removed them before compiling.

edited Sep 12 '17 at 07:46

answered Sep 12 '17 at 07:40

gsamaras

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There should also be an error for the attempted use of `//` comments,which are not valid in C89 – M.M Sep 12 '17 at 07:49
@M.M I already have it as a PM, since I thought it was not the intention of the OP. Should I higligh it by including that in the compiler erros? – gsamaras Sep 12 '17 at 07:52
How can I make GCC warn me about double-slash `//` comments? – iBug Sep 12 '17 at 07:53
1

If you compile the code in your question with the command I provide in my answer, you will see that you receive an error for these comments @iBug. – gsamaras Sep 12 '17 at 08:01

Can't define variables after any operation?

1 Answers1