volatile

[00704a-Liber]

I've got a question on 'volatile'.  In the following code fragment:

/*...*/
extern volatile int foo;
int bar;
int int_function();
/*...*/
bar = foo++ + int_function();
/*...*/

how is foo incremented?  Is the value saved when it is read for the
addition, the value 1 added to it, and stored back in foo?  Is foo just
incremented whenever the compiler would normally increment a post-increment
non-volatile variable?  Is this just an error that a compiler should flag.
I've looked in the standard for the answer and I can't find one.
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