Is it ok to define sv inside the curlies?

That shouldn't be of much concern portability-wise. Variable declarations at the beginning of a block (compound statement) have pretty much always been supported, at least since ANSI/ISO C (aka C89/C90), and - AFAIK - even before that time (i.e. old K&R-like variants).

As I wasn't entirely sure about definitions (as opposed to declarations), I tried the following snippet with the oldest compilers I could find (up to 12 years old) on various Unix platforms:

int main() {
    int x = 42;
    if (x==42) {
        int y = 99;
        x += y;
    }
    return 0;
}
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It did compile fine on all of them. Both in ANSI mode and when explicitly requesting the respective K&R emulation modes, i.e. cc -qlanglvl=classic (AIX), cc -Ac (HP-UX), cc -Xs (SunOS), cc -cckr (IRIX), gcc -traditional (Linux), etc. (note that -traditional is no longer supported with modern versions of gcc — I used a 10-year old gcc-2.95)

What isn't supported in strict ANSI C is mixing declarations/definitions and statements within the same block, such as

    if (x==42) {
        x++;
        int y = 99;
        ...
    }
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