why does setjmp return 1 after longjmp(buf,0)?

[Don Libes]

I asked why setjmp returns 1 after longjmp(buf,0).

I got some imaginative answers, only one (below) which struck me as
having half a chance of being right.  I also did a little research.
Yes, I really have all these manuals in front of me as I type this.)

V6 didn't have setjmp.  It had setexit/reset, neither of which took
  arguments, and which used a single static buffer.  ugh.

V7 and 4.0BSD setjmp returned 0 after longjmp(buf,0) (according to the manual).
  Guy Harris reports that the V7 source shows it returned 1 after
  longjmp(buf,0).  Either my manuals predate his sources, or Berkeley
  coded from documentation that was incorrect.  (Probably both true!)

S3 (S4, S5) setjmp returned 1 after longjmp(buf,0).

ANSI C setjmp returns 1 after longjmp(buf,0).


Some people agreed with me that the Standard behavior is stooopid,
while admittedly being existing practice.  No one disagreed or claimed
this is a useful feature.  A few people told me they had also been
bitten by this.

The responses I suspect are correct are a paraphrase of the
self-described "wild guesses" of Karl Heuer <karl at kelp.ima.isc.com>
and Scott David Daniels <daniels at cse.ogi.edu>:

Often the only information wanted in a setjmp call is whether it is
from the initialization or a longjmp.  If it had been possible, what
would have been defined was "longjmp with one arg causes a non-zero
return from the corresponding setjmp; with two args it causes the
setjmp to return the second arg." However, as long as longjmp was not
treated specially, such semantics were not possible.

After the "improvement", programmers didn't have to "bother" with
coding the 2nd argument at the price of not being able to return 0.
You may find this hard to swallow, but then you probably never passed
uncasted 0's either.  I think both of these are the mark of
thoughtless programming.

The end result is that the Standard accepted this behavior without
realizing that it was created for a lousy reason.  (Yes, I should've
complained sooner.)

I still claim that no one in their right mind has or ever will write
longjmp(buf,0) knowing that it will return 1.  What's even more
ironic, is that the entire reason for defining it this way (if this
theory is correct), is that being able to write "longjmp(buf)" is now
illegal!

By the way, no one answered my second question either.  Although Karl
Heuer evidentally knows, he wouldn't tell.

>>Oh, and as long as I'm getting my burning questions answered, what
>>does PJ stand for?

>If you knew, you'd respect his decision to always use his initials.  :-)

Don Libes          libes at cme.nist.gov      ...!uunet!cme-durer!libes