Lots of programs are perfectly in their rights to expect the traditional
"next free fd" behaviour. The most troublesome will be the shells: not
only do they themselves use it, they pass on that behaviour to shell
scripts. I'd expect that any program which uses mildly complex fd
manipulation takes advantage of this behaviour.
The idiom which is most likely to break is
close_all();
open("/dev/ttyX", O_RDWR);
dup();
dup();
A newly execd program could reasonably expect this to work:
int main()
{
int fd = dup(2);
assert(fd == 3);
...
In other words, I really don't think you can get away without
reproducing this behaviour.
J