Re: tmpnam() not unique

Many thanks for the information.


-----Original Message-----
From: RPG400-L [mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Scott Hanson
Sent: 14 October 2014 17:29
To: rpg400-l@xxxxxxxxxxxx
Subject: Re: tmpnam() not unique


The filename is generated using a hash function based on the current job number and the system clock. Because the IFS filename which is generated contains only six characters for uniqueness, the hash function is not a perfect hash and thus has potential collisions (more than one set of input values that maps to the same hash value and thus the same filename). The collisions should be very, very unlikely but are possible and may be within the same job or a different job.

Scott Hanson

message: 4
date: Tue, 14 Oct 2014 14:52:21 +0000
from: "McGovern, Sean" <Sean.McGovern@xxxxxxxxxxxx>
subject: RE: tmpnam() not unique

Thanks Scott.

You say tmpnam() already uses current job number but it is still
possible for a collision to occur (with another job) ?


-----Original Message-----
From: RPG400-L [mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of
Scott Hanson
Sent: 14 October 2014 15:40
To: rpg400-l@xxxxxxxxxxxx
Subject: Re: tmpnam() not unique


The code for the IFS version of tmpnam() uses both the current job
number and the current clock (timestamp) to generate a unique
filename. Once the name is generated, the code uses stat() to make
sure a filename does not already exist with that name. If the file
already exists, it will generate another name. The name generated by
tmpnam() is unique to the system at time of generation.

It is possible for a collision to occur with the filename such that
the same filename is generated twice. The caller of the tmpnam()
routine should test that some other process has not created an
equivalently named file in the lag time between name generation and
file creation. Some applications can use the tmpfile() function to
avoid this issue.

Your solution of adding the job number as a prefix/suffix should work
well and avoid any potential issues.

Scott Hanson

message: 5
date: Tue, 14 Oct 2014 13:19:57 +0000
from: "McGovern, Sean" <Sean.McGovern@xxxxxxxxxxxx>
subject: tmpnam() not unique

I've believe I have encountered an issue with my use of tmpnam().

If 2 jobs use tmpnam() at the same timestamp moment, I *think* the
same filename is returned (though I don't know how to test this

scenario).

I haven't coded to handle this possibility, and end up with two jobs
trying to create the same temporary IFS file.

I was thinking that a solution could be just prefix/suffix the job
number to the filename returned by tmpnam().

Does that seem like a feasible solution ?

--

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