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NNTP-Posting-Date: Fri, 30 Sep 2016 16:09:57 -0500
From: Gareth Owen <gwowen@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: volatile in struct member
References: <dc811de3-cad5-4f57-b68d-dd8b0252e507@googlegroups.com> <aa598ef6-1a14-443d-b1f7-50a20cecef0c@googlegroups.com> <87ponl8pxf.fsf@bsb.me.uk>
Date: Fri, 30 Sep 2016 22:09:57 +0100
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Xref: csiph.com comp.lang.c:94335

Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> But I suspect the question is more about the placement of the keyword.
> I'd put the volatile on the object declaration:
>
>   struct small { int i; };
>   volatile struct small sm;

But one could easily imagine a struct like

struct register
{
   volatile int * address;
   int last_written_value;
}

void write_reg(struct register*reg,int val)
{
  *reg->address = val;
  last_written_value = val;
}

int read_reg(struct register*reg,int val)
{
  return reg->last_written_value;
}

I wouldn't need to have the cached value marked as volatile (though in a
trivial example like this it would make little difference to performance).

I've programmed FPGAs with "write only" registers like this.