sscanf always generates error condition

[Greg Hunt]

In article <1991May1.193252.16232 at agate.berkeley.edu>, ilan343 at violet.berkeley.edu (Geraldo Veiga) writes:
> First of all, thank you for all postings and mail helping me  with 
> this question.

You're welcome.

Me>In the test case, errno is being used without having a value assigned to
Me>it, so the "9" is just garbage that happened to be in the memory
Me>associated with errno at the time.  On the other system you tried, the
Me>garbage happened to have been zero, so everything worked.

GV> This was not the case.   In the ISC system call errno=0 (or whatever I
GV> set it to) before and errno=9 after.  Also, sscanf returns 1 
GV> (as it should in my example).

That's wierd.  Since the return value is correct, then I would guess
that the errno being set to 9 is meaningless.  I would have expected
it to not be set at all, however.  Obviously on the ISC system, it's
being handled differently.

GV> This problem came up because I have a generic error message function
GV> used for both system and user errors.  This function always checks the
GV> value of errno and prints whatever system error message was indicated 
GV> (with a perror() call).
GV> 
GV> From the discussion in this and one previous posting, this doesn't seem
GV> a like very good idea, since a system call could set the value of errno
GV> to some misleading value that would generate a non-related message
GV> next time I print a user error message.

The common error handling is a great idea!  Don't throw it away!  Just
change the code after a system call or library call to check the return
value to detect if an error occurred first, then call your common error
handler to print the error out (and shutdown the process, or whatever
else it does).  This is exactly how I use the common error handler that
I have in my own code, and it works great.

GV> I was aware that the it wouldn't set errno=0, but setting it to a
GV> non-zero value on normal termination still bothers me.

It would bother me, too.  It might be a bug in the ISC sscanf library
call (albeit a fairly harmless one since the function works and does
return the proper value).  If you're inclined to do so, you might want
to check with them about it.  Giving them your test case that shows
errno is being reset would help them track it down to determine if it
really is a bug, or whether it's supposed to be happening.

-- 
Greg Hunt                        Internet: hunt at dg-rtp.rtp.dg.com
DG/UX Kernel Development         UUCP:     {world}!mcnc!rti!dg-rtp!hunt
Data General Corporation
Research Triangle Park, NC, USA  These opinions are mine, not DG's.