[metadata]
id = "DCL22-C"
type = "recommendation"
category = "DCL"
number = 22
title = "Use volatile for data that cannot be cached"
description = """
An object that has volatile-qualified type may be modified in ways unknown to
theimplementationor have other unknown side effects. Asynchronous signal
handling, for example, may cause objects to be modified in a manner unknown to
the compiler. Without this type qualifier, unintended optimizations may occur.
These optimizations may cause race conditions because a programmer may write
code that prevents a race condition, yet the compiler is not aware of the
programmer's data model and may modify the code during compilation to permit
race conditions. Thevolatilekeyword eliminates this confusion by imposing
restrictions on access and caching. According to the C99 Rationale [C99
Rationale 2003], Type qualifying objects as volatile does not guarantee
synchronization between multiple threads, protect against simultaneous memory
accesses, or, unless used to declare objects of typesig_atomic_t, guarantee
atomicity of accesses to the object. For restrictions specific to signal
handlers, seeSIG31-C. Do not access shared objects in signal handlers. However,
type qualifying objects as volatile does ensure that a conforming compiler will
not elide or reorder access to the object.
"""
severity = "Low"
likelihood = "Probable"
priority = "P4"
level = "L3"
cert_version = "2016 Edition (Wiki)"
last_modified = "May 19, 2025"
[rules.cert_c.DCL22-C]
enabled = true
[references]
wiki = "https://wiki.sei.cmu.edu/confluence/display/c/DCL22-C.+Use+volatile+for+data+that+cannot+be+cached"