11:39 am, 30 Nov 04
shadowing fun
I've mentioned this quirk before, but I still find it amusing: variable initializations in C can refer the the variable itself. In other words, every let works like a letrec.
So I can do stuff like:
char *crazy = malloc(strlen(crazy) + 1); /* make crazy exactly as big as it ought to be. */
This has the effect that you can't shadow a variable with a variable defined in terms of the shadowee.
Code like this:
int t = 0;
{
int t = t + 1;
printf("t %d\n", t);
}
Doesn't print out 1 unless you're lucky. While this seems like a crazy thing to do, I find it's a pretty common idiom in ML (where you don't need to create a nested scope to shadow) for when you're functionally updating something.
A C analogy might be something like, for example,
p = sprintf(buf, [...]);
p = sprintf(p, [...]);
p = sprintf(p, [...]);
where mutating p isn't really the goal.
So I can do stuff like:
char *crazy = malloc(strlen(crazy) + 1); /* make crazy exactly as big as it ought to be. */
This has the effect that you can't shadow a variable with a variable defined in terms of the shadowee.
Code like this:
int t = 0;
{
int t = t + 1;
printf("t %d\n", t);
}
Doesn't print out 1 unless you're lucky. While this seems like a crazy thing to do, I find it's a pretty common idiom in ML (where you don't need to create a nested scope to shadow) for when you're functionally updating something.
A C analogy might be something like, for example,
p = sprintf(buf, [...]);
p = sprintf(p, [...]);
p = sprintf(p, [...]);
where mutating p isn't really the goal.
void *ret = &ret + sizeof(void*); /* or some similar stupid thing */I suppose it would be easier to code the compiler if you didn't have to perform that check, but it would be easier to code the compiler if it didn't perform printf type conversion checks, too, and that's often done.
Re: emacs to the rescue
ctl-p in vim. ;)I still can't think of a good use that actually uses the randomly initialized data stored at the lvalue -- but then, C isn't known for making it easy on programmers.
I'm lucky!
Doesn't print out 1 unless you're lucky.I was lucky. However, I changed the "int t = 0;" to "int t = 7;" and it still printed out 1... Nifty.
make crazy exactly as big as it ought to be?
char *crazy = malloc(strlen(crazy) + 1); /* make crazy exactly as big as it ought to be. */This doesn't actually do anything useful, right? Your comment confuses me. In fact, it segfaults for me... definitely not useful. =]