From Newsgroup: comp.lang.c

On 31/05/2025 12:53, Richard Heathfield wrote:

On 31/05/2025 12:02, Janis Papanagnou wrote:

Probably not, as my C90/C99 test more or less illustrated. It 'knew' of
a syntax rule, but failed to apply it correctly.

I just tried again, this time asking for a C90 program to produce a
frequency count of colours in a 24-bit bitmap.

It took three laps to make it C90-legal.

To be fair, the number of times I cycle { compile, read syntax errors,
edit my code } is usually more than 3. When I use Eclipse, I compile it
and receive errors every line or so.


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From Newsgroup: comp.lang.c

Am 30.05.2025 um 22:01 schrieb Richard Heathfield:

This evening I accidentally bumped into a chatbot, and I wondered
whether it was any good at C programming.

I was impressed when it solved Goldbach's Conjecture (admittedly for
small n) and 5-disc Hanoi, but then I thought back to a recent thread
here (which I have no intent of resurrecting), so I asked it if it could have saved me a job if I'd only asked it a few hours ago.

Judge for yourselves!

I asked: "Write a program that is valid C90 but invalid C99."

ChatGPT said:

Certainly! To illustrate this, I'll write a C program that is valid in
C90 but invalid in C99. The key difference between C90 and C99 revolves around variable declaration and initialization.
Key Issue:

    In C90, variables can only be declared at the beginning of a block (before any executable code).

    In C99, variables can be declared anywhere within a block, even between statements.

C Program Example:

This program is valid in C90 but invalid in C99 because the variable x
is declared after executable code.

#include <stdio.h>

int main() {
    printf("Hello, world!\n");

    int x = 5;  // Declaration after executable code (invalid in C99)

    printf("x = %d\n", x);
    return 0;
}

Is this relevant to know today ?

Why it’s Valid in C90:

    In C90, all variable declarations must appear at the beginning of a block (before any code). This program would be valid in C90 because x is declared after the printf statement, but only the declaration of x
itself breaks the rule in C99.

Why it’s Invalid in C99:

    In C99, variables can be declared anywhere within a block, even after executable statements (not just at the beginning of a block). This
is why the above code would cause an error in C99, as the variable x is declared after a statement (printf).

Compilation Example:

If you compile this program with a C99 compiler (e.g., GCC), you'll get
an error similar to this:

error: variable 'x' declared after statement

To summarize:

    C90 allows this style (declaration after code).

    C99 does not allow it, so the program becomes invalid.

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