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Date: Mon, 04 Jan 2010 15:25:35 +0100
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Atomic file creation
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Date: 2010-01-04T15:25:35+01:00
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vlc schrieb:

> But this is not completely safe as it is not atomic. A higher-priority
> task e.g. could intercept my task between lines 1 and 2 and create the
> same file. In this case the file would be overwritten.

A multitasking program can use a protected object
for this.  Have all tasks perform I/O if and only if
it is their turn as decided by the queuing control of
the protected object.

> Is there something like C's O_EXCL flag for "open" in Ada?

Strictly speaking, O_EXCL is not C, but it is provided by Unix's
open(2) call. fopen(3), which is C, does not offer exclusive access
IIRC.

The Form string parameter of your Ada implementation might
provide sharing control.  The docs will then explain how a
Form value affects files opened by many tasks; possibly even
explain whether the form strings have an effect on using the
external file in other programs.