On Fri, Nov 24, 2000 at 05:10:08PM -0600, Karl Fogel wrote: > Greg Stein <[EMAIL PROTECTED]> writes: > > So... by stating the parameter is "const char * const *argv", we are saying > > what we intend to do (or not do) with the arguments. And I think we really > > ought to treat it as if those const qualifiers were on there. > > I confess I don't understand the nuance of these `const' usages.
"Come over to the Const Side, Luke..." > The code needs to permute argv; if it can't, we'll have to change the > interface, right? If Greg H has to cast internally to get something > he can permute, then we might as well not require a const on the > castee in the first place. If we add const to the interface, then we can't cast away the const. We have to make a copy of the array [that we'll be permuting]. This is what the code does now. If we attempted to cast away the const and then change it, we could core dump. See my example with the "static const * const cmdline[]". Everything about that is const, so No Changes Allowed. :-) > Does the above declaration still permit permutation of the array? Nope. > (Apologies; I'm rereading my K&R and still not fully understanding > what the above does.) As said elsewhere, K&R doesn't have "const", so I don't know that you'd find it in there :-) [ of course, if they updated for ANSI... well. ] > That is (using the invocation "svn -d foo bar"), we have > > argv --------+ > | > | > V > [pointer1 , pointer2 , pointer3 , pointer4, pointer5] > | | | | | > | | | | | > | | | | | > V V V V V > [svn\0] [-d\0] [foo\0] [bar\0] <NULL> > > > So there are three things that could be constified: > > 1. The variable `argv', which holds a pointer to an array of > pointers. > > 2. The elements of that array, each of which points to a C string. > > 3. The elements of a given C string. > > If we're constifying #2, then that's a problem for permutation. We also need #3 to be const. If you attempt to modify those on Linux, you get a core dump. The kernel places the string contents in a read-only segment. Strangely, the array of pointers (#2) is modifiable. There is not much sense in adding const to argv itself (#1) since that is call-by-value anyways. The function (implementation) can change that value at will, without any impact on the caller. > Please feel free to bean me if I'm totally missing the point here... Consider yourself beaned :-). But it is understandable. As Brane points out, this stuff can be a bit tricky to wrap your head around. Following the "read from right to left" rule is the easiest way to read something declared as "const char * const *argv". When [] gets in there, too, then duct tape your head before reading it; otherwise, your head could blow up. I've lost two friends that way. Cheers, -g -- Greg Stein, http://www.lyra.org/
