Re: c99/c++ localised variable definition

From: Robert Watson (
Date: 01/29/05

  • Next message: Sam Leffler: "Re: c99/c++ localised variable definition"
    Date: Sat, 29 Jan 2005 18:04:52 +0000 (GMT)
    To: Garance A Drosihn <>

    On Fri, 28 Jan 2005, Garance A Drosihn wrote:

    > At 5:33 PM +0000 1/28/05, Paul Richards wrote:
    > >
    > >People used to programming in C++ or Perl (and many others) are
    > >used to defining variables as near to use as possible. This has
    > >never been possible before in C, but now with c99 it is.
    > Well, you could get a similar effect by creating a new scope.

    And, FWIW, I've been hoping we could eliminate some use of "new scopes" in
    the current code, since they're typically used to hide the fact that
    excessive code is ifdef'd, or that a function should really be two
    functions. For example, you used to find a moderate amount (and still
    find some) in places like ip_input(), where people would arbitrarily add a
    block of code conditional on a semi-obscure kernel option, and then
    realize they needed variables and make it into a code block. Where it
    does occur, it's almost always a sign of a problem with the code
    structure. Here's the sort of thing I mean:


            /* lots of code */

                 struct foo *foo;
                 int x;

                 stuff(foo, x);

    One of the main situations in which I've found the declaration of
    variables close to their use helpful, as opposed to at the head of the
    function, is for temporary values relating to list or array iteration as
    part of a for loop. I.e.,

        for (int i = 0; i < 100; i)) {


    In this scenario, the re-use of i as part of a broader scope actually
    makes C warnings less useful, since you lose the benefit of stuff like
    "used but not initialized" warnings as the variables are reused. Maybe
    what we should be doing is identifying a couple of places where we are
    willing to take this approach, and it offers a clear benefit, and
    specifically pointing at those. I have to say that the type of coding
    style that annoys me somewhat to read in blended C/C++ code is this sort
    of thing:

        struct big foo;

        // ... large block of code

        struct another_big bar;

        // ... large block of code

    In environments with constrained stacks, especially in the kernel or
    threaded applications, having all the serious declarations up front makes
    it much easier to decide if things are getting out of hand. That's one
    reason why things like ancillary counter variables seem reasonable, but
    more extensive use can be problematic.

    Robert N M Watson

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