[metadata]
id = "CON40-C"
type = "rule"
category = "CON"
number = 40
title = "Do not refer to an atomic variable twice in an expression"
description = """
A consistent locking policy guarantees that multiple threads cannot
simultaneously access or modify shared data. Atomic variables eliminate the need
for locks by guaranteeing thread safety when certain operations are performed on
them. The thread-safe operations on atomic variables are specified in the C
Standard, subclauses 7.17.7 and 7.17.8 [ISO/IEC 9899:2024]. While atomic
operations can be combined, combined operations do not provide the thread safety
provided by individual atomic operations. Every time an atomic variable appears
on the left side of an assignment operator, including a compound assignment
operator such as*=, an atomic write is performed on the variable. The use of the
increment (++)or decrement(--)operators on an atomic variable constitutes an
atomic read-and-write operation and is consequently thread-safe. Any reference
of an atomic variable anywhere else in an expression indicates a distinct atomic
read on the variable. If the same atomic variable appears twice in an
expression, then two atomic reads, or an atomic read and an atomic write, are
required. Such a pair of atomic operations is not thread-safe, as another thread
can modify the atomic variable between the two operations. Consequently, an
atomic variable must not be referenced twice in the same expression.
"""
severity = "Medium"
likelihood = "Probable"
priority = "P8"
level = "L2"
cert_version = "2016 Edition (Wiki)"
last_modified = "Jul 02, 2025"
[rules.cert_c.CON40-C]
enabled = true
[references]
wiki = "https://wiki.sei.cmu.edu/confluence/display/c/CON40-C.+Do+not+refer+to+an+atomic+variable+twice+in+an+expression"
cwe = ["CWE-366"]