Re: Files opened for exclusive read

[Mark H Johnson <Mark_H_Johnson@raytheon.com>]

Mark H Johnson wrote:

> Preben Randhol wrote:
> 
>>
>> Will shared=yes be silently ignored on a linux/unix OS where this is not
>> an issue?
>>
> It has been a couple years since I asked that question of ACT, but I 
> believe the answer is NO. When shared=yes, the GNAT run time will share 
> the file pointer (buffers, etc.). This was NOT what we wanted - we 
> happened to have more than one task reading some initialization data 
> from a file and were expecting each task to read the whole file (not 
> just pieces).
>   --Mark
> 

In retrospect, my explanation is perhaps a little too brief. The 
following is a summary of a recent (5.02a) copy of the GNAT User's Guide 
(section 8.8, Shared Files)

In the absence of a shared=xxx, attempting to open two or more files w/ 
the same full name is an error (raises USE_ERROR).

If shared=no, the file can be opened with separate stream identifiers w/ 
the exact effect dependent on how the C stream routines do this.

If shared=yes and two or more files are opened with the same full name, 
the same stream is shared between the files w/ the semantics in section 
A.14 of the Ada Reference Manual.

So, to recap - shared=yes is not ignored, the semantics in the ARM are 
followed. If you want to open the same file more than once w/o error and 
without sharing, use shared=no. [at least this is how it works on x86 
Linux, I can't comment on other machine/OS combinations]
   --Mark