Newbie question about C variable declarations

Question

I'm reading an introducty book about C and I came across the following paragraph:

But the following code compiles with expected output:

#include <stdio.h>

int main()
{
    for(int i = 0; i<=10; i++)
    {
        int val = i + 1;
        
        int x = val * val;
        
        printf("%d\n", x);

        int y = x;

    }

    return 0;
}

I use https://www.onlinegdb.com/ and in the above code I declared many variables after the first executable statement. And this is to me does not match what the section from the book tells.

Am I misunderstanding what the book is telling?

Answer 1

In strictly conforming C 1990, declarations could appear only at file scope (outside of function definitions) or at the start of a compound statement. The grammar for a compound statement in C 1990 6.6.2 was:

compound-statement
    { declaration-list_opt statement-list_opt }

That says a compound statement is { followed by zero or more declarations, then zero or more statements, then }. So the declarations had to come first.

In C 1999 6.8.2, this changed to:

compound-statement
    { block-item-list_opt }

A block-item-list is a list of block-item, each of which may be a declaration or a statement, so declarations and statements could be freely mixed.

In your example, the declarations int val = i + 1; and int x = val * val; do not appear after executable statements in their compound statement. The compound statement starts with the { immediately before int val = i + 1;, so that declaration is at the start of the compound statement.

Another change was that the for grammar was changed from this in C 1990 6.6.5:

for ( expression_opt ; expression_opt ; expression_opt ) statement

to this choice of two forms in C 1999 6.8.5:

for ( expression_opt ; expression_opt ; expression_opt ) statement
for ( declaration expression_opt ; expression_opt ) statement

(Note the declaration includes a terminating ;.)

That explains why you can have int i = 0 in for(int i = 0; i<=10; i++).