The C Standard, 6.7.2.1 [ISO/IEC 9899:2011], states

There may be unnamed padding within a structure object, but not at its beginning. . . . There may be unnamed padding at the end of a structure or union.

Subclause 6.7.9, paragraph 9, states that

unnamed members of objects of structure and union type do not participate in initialization. Unnamed members of structure objects have indeterminate value even after initialization.

The only exception is that padding bits are set to zero when a static or thread-local object is implicitly initialized (paragraph10):

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:

— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

— if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

Because these padding values are unspecified, attempting a byte-by-byte comparison between structures can lead to incorrect results [Summit 1995]. 

Noncompliant Code Example

In this noncompliant code example, memcmp() is used to compare the contents of two structures, including any padding bytes:

#include <string.h>
 
struct s {
  char c;
  int i;
  char buffer[13];
};
 
void compare(const struct s *left, const struct s *right) {  
  if ((left && right) &&
      (0 == memcmp(left, right, sizeof(struct s)))) {
    /* ... */
  }
}

Compliant Solution

In this compliant solution, all of the fields are compared manually to avoid comparing any padding bytes:

#include <string.h>
 
struct s {
  char c;
  int i;
  char buffer[13];
};
 
void compare(const struct s *left, const struct s *right) {  
  if ((left && right) &&
      (left->c == right->c) &&
      (left->i == right->i) &&
      (0 == memcmp(left->buffer, right->buffer, 13))) {
    /* ... */
  }
}

Exceptions

EXP42-C-EX1: A structure can be defined such that the members are aligned properly or the structure is packed using implementation-specific packing instructions. This is true only when the members' data types have no padding bits of their own and when their object representations are the same as their value representations. This frequently is not true for the _Bool type or floating-point types and need not be true for pointers. In such cases, the compiler does not insert padding, and use of functions such as memcmp() is acceptable.

This compliant example uses the #pragma pack compiler extension from Microsoft Visual Studio to ensure the structure members are packed as tightly as possible:

#include <string.h>
 
#pragma pack(push, 1)
struct s {
  char c;
  int i;
  char buffer[13];
};
#pragma pack(pop)
 
void compare(const struct s *left, const struct s *right) {  
  if ((left && right) &&
      (0 == memcmp(left, right, sizeof(struct s)))) {
    /* ... */
  }
}

Risk Assessment

Comparing padding bytes, when present, can lead to unexpected program behavior.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP42-C

Medium

Probable

Medium

P8

L2

Automated Detection

Tool

Version

Checker

Description

Astrée

memcpy-with-paddingPartially checked
Axivion Bauhaus Suite

CertC-EXP42
CodeSonar

BADFUNC.MEMCMP

Use of memcmp

Helix QAC

DF4726, DF4727, DF4728, DF4729


Klocwork

MISRA.STDLIB.MEMCMP.PTR_ARG_TYPES
LDRA tool suite
618 SPartially implemented
Cppcheck
cert.py

Detected by the addon cert.py

Does not warn about global/static padding data as this is probably initialized to 0

Parasoft C/C++test
CERT_C-EXP42-a

Don't memcpy or memcmp non-PODs

PC-lint Plus

958, 959

Assistance provided: reports structures which require padding between members or after the last member

Polyspace Bug Finder

CERT C: Rule EXP42-C


Checks for memory comparison of padding data (rule fully covered)

PVS-Studio

V1103
RuleChecker

memcpy-with-paddingPartially checked
TrustInSoft Analyzer

comparable_char_blocks

Exhaustively verified (see the compliant and the non-compliant example).

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

ISO/IEC TS 17961Comparison of padding data [padcomp]Prior to 2018-01-12: CERT: Unspecified Relationship
CERT CEXP62-CPP. Do not access the bits of an object representation that are not part of the object's value representationPrior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

[ISO/IEC 9899:2011]6.7.2.1, "Structure and Union Specifiers"
6.7.9, "Initialization"
[Summit 1995]Question 2.8
Question 2.12