reentrant vs. non-reentrant code

[Roy Smith]

In article <1670001 at hpcilzb.HP.COM> tedj at hpcilzb.HP.COM (Ted Johnson) writes:
> Can someone please explain the difference between "reentrant" and
> "non-reentrant" code?  Thanks!

	Reentrant code is, quite simply, code you can reenter without ill
effect.  Usually this means that each invocation of the code gets its own
set of variables, although this is not a sufficient condition to make
something reentrant.  Take a typical example in C, a recursve factorial
routine:

	fact (i)
	int i;
	{
		return (i<2 ? i : i*fact(i-1));
	}

	If you call fact(5), the first instance of i is 5.  Fact calls
itself with an argument of i=4.  Now you have two instances of fact in the
process of being executed.  This works because i is automatic; each
invocation of fact gets a new piece of memory in which to store i.

	Now, change the routine to look like:

	fact (i)
	int i;
	{
		static j;

		j = i;
		return (j<2 ? j : j*fact(j-1));
	}

and you end up with a bit of non-reentrant code: each invocation of fact
still has its own instance of i, but they all share the same j.  The second
time fact gets called, it overwrites the value of j still being used.
Actually, what I've just shown is a bit of code which is not recursively
reentrant, which is not quite the same thing as not being reentrant.  A
better example would be something like:

	sayfoo()
	{
		static char *str = "foo\n";

		while (*str)
		{
			putchar (*str);
			str++;
		}
	}

	The first time you call sayfoo(), it prints "foo\n", but each other
time you call it, it just returns without printing anything.  Any code with
static variables in it is likely to be non-reentrant.  This generally means
Fortran code is non-reentrant since Fortran uses static storage.  Self
modifying code is also almost certainly non-reentrant.  Sometimes code is
reentrant except for certain windows of vulnerability which much be locked
around.  A typical example is a Unix device driver.
-- 
Roy Smith, System Administrator
Public Health Research Institute
{allegra,philabs,cmcl2,rutgers}!phri!roy -or- phri!roy at uunet.uu.net
"The connector is the network"