Assignment operator in C

Question

volatile int lhs = 1;
int rhs = 2;
int x = 3;

x = lhs = rhs;

Does an assigment operator return the (typeof lhs)rhs ? or Does it return new, just read value of lhs ? It is important to me since lhs is volatile and it can change between assigments.

I couldnt find the anwser at cpprefernce.

I think the question here is, suppose lhs is a memory location that, no matter what you write to it, a read always returns 42. Will x in the sample code end up containing 2 or 42? — Steve Summit
– Steve Summit, Commented May 11, 2023 at 19:30
Related, possibly duplicate: assignment expressions and volatile — Weather Vane
– Weather Vane, Commented May 11, 2023 at 19:30
@jarmod: Why should we see that page; what does it tell us about the volatile issue in this question? — Eric Postpischil
– Eric Postpischil, Commented May 11, 2023 at 19:33
It is very bad practice to write an expression with 4 side effects that are dubiously sequenced in relation to each other. — Lundin
– Lundin, Commented May 12, 2023 at 8:32

Eric Postpischil · Accepted Answer · 2023-05-11 19:31:15Z

8

The C standard does not specify this whether lhs is read or not; it permits either behavior.

C 2018 6.5.16 specifies the assignment operators. Paragraph 3 says:

… An assignment expression has the value of the left operand after the assignment,¹¹⁵⁾ but is not an lvalue…

Footnote 115 says:

The implementation is permitted to read the object to determine the value but is not required to, even when the object has volatile-qualified type.

Thus, the implementation is permitted to do either of:

read the stored value of lhs (presumably after the side effect of updating it is complete) to determine the value of lhs = rhs;, or
directly use the value resulting from converting rhs to the type of lhs.

answered May 11, 2023 at 19:31

Eric Postpischil

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1 Comment

OODAX Over a year ago

that's what i wanted to hear. thanks

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