Very Computer

by **alois egge** » Tue, 13 Apr 1999 04:00:00

Hello,
I want to redirect the output of two or more processes to the same file.
What happens if the information is written at the same time.
Can I rely on the Operating System (AIX 4.x), that all information is
written into the output file.

Thanks

by **Jens-Uwe Mag** » Tue, 13 Apr 1999 04:00:00

>I want to redirect the output of two or more processes to the same file.
>What happens if the information is written at the same time.
>Can I rely on the Operating System (AIX 4.x), that all information is
>written into the output file.

If you open the file for append (>> using the shell) then the OS will make
sure that output is always appended at the end of the file.

--
Jens-Uwe Mager <pgp-mailto:62CFDB25>

by **Scott L. Field** » Tue, 13 Apr 1999 04:00:00

Yes, but you can't guarantee that the data will be in the order you want.

Further, it will be by "write()" and not by any standard you imply.

IE, if you have a sentence but it takes 2 write() system calls to print it,
you
might have a "write()" from the other process interrupt them.

Further, the "printf()" functions are buffered, and by default are fully
buffered.
You either have to force the write or depend on it writing to the actual
file
every 4K of data.

>Hello,
>I want to redirect the output of two or more processes to the same file.
>What happens if the information is written at the same time.
>Can I rely on the Operating System (AIX 4.x), that all information is
>written into the output file.

>Thanks

by **Flemming Josephse** » Tue, 13 Apr 1999 04:00:00

This is true .. I have a series of scripts that depends on a common log
file .... I managed to get around the problem of 'funny sequences and
broken lines ' by observing the process' and putting in a strategically
placed sleep here and there.... by the grace of the great CPU this
worked, so, even if it's nothing to brag about .. it did the trick and
I'm suitably thankful.

> Yes, but you can't guarantee that the data will be in the order you want.

> Further, it will be by "write()" and not by any standard you imply.

> IE, if you have a sentence but it takes 2 write() system calls to print it,
> you
> might have a "write()" from the other process interrupt them.

> Further, the "printf()" functions are buffered, and by default are fully
> buffered.
> You either have to force the write or depend on it writing to the actual
> file
> every 4K of data.

> >Hello,
> >I want to redirect the output of two or more processes to the same file.
> >What happens if the information is written at the same time.
> >Can I rely on the Operating System (AIX 4.x), that all information is
> >written into the output file.

> >Thanks

by **Juergen Gmeine** » Wed, 14 Apr 1999 04:00:00

> > Further, the "printf()" functions are buffered, and by default are fully
> > buffered.
> > You either have to force the write or depend on it writing to the actual
> > file
> > every 4K of data.

my "solution" to the problem is not to redirect the
output of the scripts (which would make the output buffered
and therefore lead to scrambled output), but
in the scripts via "exec >> mylogfileorwhatever 2>&1".
this redirects the output, yet it remains line-buffered.

i only used this with scripts started interactively, though.
it probably won't work if you start them via cron or at.

and it only affects the scripts output ... programs called
within the script still do buffered io, unless you use
setlinebuf()

i still find it useful, sometimes

regards,
juergen

by **Tony R. Benne** » Thu, 15 Apr 1999 04:00:00

>> > Further, the "printf()" functions are buffered, and by default are fully
>> > buffered.
>> > You either have to force the write or depend on it writing to the actual
>> > file
>> > every 4K of data.

>my "solution" to the problem is not to redirect the
>output of the scripts (which would make the output buffered
>and therefore lead to scrambled output), but
>in the scripts via "exec >> mylogfileorwhatever 2>&1".
>this redirects the output, yet it remains line-buffered.

>i only used this with scripts started interactively, though.
>it probably won't work if you start them via cron or at.

>and it only affects the scripts output ... programs called
>within the script still do buffered io, unless you use
>setlinebuf()

>i still find it useful, sometimes

>regards,
>juergen

The only solution I have found is that is guaranteed to not 'interleave'
lines from two processes is to controll access to the print file with a
semaphore... this of course will not work from a shell-script.

--

by **Joerg Brueh** » Fri, 23 Apr 1999 04:00:00

Hi !

Jens-Uwe Mager schrieb:

> >I want to redirect the output of two or more processes to the same file.
> >What happens if the information is written at the same time.
> >Can I rely on the Operating System (AIX 4.x), that all information is
> >written into the output file.

> If you open the file for append (>> using the shell) then the OS will make
> sure that output is always appended at the end of the file.

I am sure this depends on the shell used, I had bad luck when
I tried that (sorry - I do not remember the environment used).
It seems that some shells do not set the O_APPEND flag for a file
opened with '>>', which IMHO should be done.

I take it that the original question refers to concurrent processes.

My approach would be:
1) Explicitly 'open()' the file in the program, using the O_APPEND
flag, maybe even O_SYNC (probably overkill - try both with and
without).
2) Explicitly use 'write()', not the STDIO-functions.

If you want to use STDIO-functions like 'fprintf()', also use
'fflush()' at proper places - before the buffer fills !
You would then need to combine 'open (... , O_APPEND)' with
'fdopen()', I have never done that.

HTH
Joerg Bruehe

--
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
(speaking only for himself)

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