[metadata]
id = "EXP11-C"
type = "recommendation"
category = "EXP"
number = 11
title = "Do not make assumptions regarding the layout of structures with bit-fields"
description = """
The internal representations of bit-field structures have several properties
(such as internal padding) that areimplementation-defined. Additionally, bit-
field structures have several implementation-defined constraints: Consequently,
it is impossible to write portable safe code that makes assumptions regarding
the layout of bit-field structure members. Bit-fields can be used to allow flags
or other integer values with small ranges to be packed together to save storage
space. Bit-fields can improve the storage efficiency of structures. Compilers
typically allocate consecutive bit-field structure members into the sameint-
sized storage, as long as they fit completely into that storage unit. However,
the order of allocation within a storage unit is implementation-defined.
Someimplementationsareright-to-left: the first member occupies the low-order
position of the storage unit. Others areleft-to-right: the first member occupies
the high-order position of the storage unit. Calculations that depend on the
order of bits within a storage unit may produce different results on different
implementations.
"""
severity = "Medium"
likelihood = "Probable"
priority = "P4"
level = "L3"
cert_version = "2016 Edition (Wiki)"
last_modified = "May 20, 2025"
[rules.cert_c.EXP11-C]
enabled = true
[references]
wiki = "https://wiki.sei.cmu.edu/confluence/display/c/EXP11-C.+Do+not+make+assumptions+regarding+the+layout+of+structures+with+bit-fields"