c - Placement of iterator declarations in C11 (coding style)

Where did you get this idea?

The C11 draft, section 6.8.5 (1), says:

iteration-statement:

while ( expression ) statement

do statement while ( expression ) ;

for ( expression_opt ; expression_opt ; expression_opt ) statement

for ( declaration expression_opt ; expression_opt ) statement

That last form makes it pretty clear that a declaration is still allowed in the first clause of the for statement.

[Update]

Note that declaration is something like int i = 0 ;. That is, it includes the semi-colon (see section 6.7). So for (int i = 0 ; i < 10 ; ++i) is definitely allowed by C11.

[Update 2]

As for when to use which, that is a matter of opinion. My opinion is that you should embed the declaration whenever you can, because it makes the code easier to read.

If I am reading your code and I see:

for (int i = 0 ; i < 10 ; ++i)
...

...then I know you are not relying on the value of i after the loop terminates, because it is no longer in scope.

If I see:

int i;
for (i = 0 ; i < 10 ; ++i)
...

...then I expect you to use the value of i later; e.g. maybe you break early and want to know what i was when that happened. If I do not see you use i later, I will probably wonder what you were thinking and/or what I am missing.

As usual, readability is the most important metric. Of course, what qualifies as "readable" is a matter of opinion.