Do not convert a pointer value to a pointer type that is more strictly aligned than the referenced type. Different alignments are possible for different types of objects. If the type-checking system is overridden by an explicit cast or the pointer is converted to a void pointer (
void *) and then to a different type, the alignment of an object may be changed.
The C Standard, 126.96.36.199, paragraph 7 [ISO/IEC 9899:2011], states
A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined.
If the misaligned pointer is dereferenced, the program may terminate abnormally. On some architectures, the cast alone may cause a loss of information even if the value is not dereferenced if the types involved have differing alignment requirements.
Noncompliant Code Example
In this noncompliant example, the
&c is converted to the more strictly aligned
ip. On some implementations,
cp will not match
&c. As a result, if a pointer to one object type is converted to a pointer to a different object type, the second object type must not require stricter alignment than the first.
Compliant Solution (Intermediate Object)
In this compliant solution, the
char value is stored into an object of type
int so that the pointer's value will be properly aligned:
Noncompliant Code Example
The C Standard allows any object pointer to be cast to and from
void *. As a result, it is possible to silently convert from one pointer type to another without the compiler diagnosing the problem by storing or casting a pointer to
void * and then storing or casting it to the final type. In this noncompliant code example,
loop_function() is passed the
char_ptr but returns an object of type
This example compiles without warning using GCC 4.8 on Ubuntu Linux 14.04. However,
int_pointer can be more strictly aligned than an object of type
Because the input parameter directly influences the return value, and
loop_function() returns an object of type
int *, the formal parameter
v_pointer is redeclared to accept only an object of type
Noncompliant Code Example
Some architectures require that pointers are correctly aligned when accessing objects larger than a byte. However, it is common in system code that unaligned data (for example, the network stacks) must be copied to a properly aligned memory location, such as in this noncompliant code example:
Assigning an unaligned value to a pointer that references a type that needs to be aligned is undefined behavior. An implementation may notice, for example, that
header must be aligned and use an inline
memcpy() that uses instructions that assume aligned data.
This compliant solution avoids the use of the
EXP36-C-EX1: Some hardware architectures have relaxed requirements with regard to pointer alignment. Using a pointer that is not properly aligned is correctly handled by the architecture, although there might be a performance penalty. On such an architecture, improper pointer alignment is permitted but remains an efficiency problem.
The x86 32- and 64-bit architectures usually impose only a performance penalty for violations of this rule, but under some circumstances, noncompliant code can still exhibit undefined behavior. Consider the following program:
This code tries to read short ints (which are 16 bits long) from odd pairs in a character array, which violates this rule. On 32- and 64-bit x86 platforms, this program should run to completion without incident. However, the program aborts with a SIGSEGV due to the unaligned reads on a 64-bit platform running Debian Linux, when compiled with GCC 4.9.4 using the flags
-O2 -ftree-loop-vectorize -fvect-cost-model.
If a developer wishes to violate this rule and use undefined behavior, they must not only ensure that the hardware guarantees the behavior of the object code, but they must also ensure that their compiler, along with its optimizer, also respect these guarantees.
EXP36-C-EX2: If a pointer is known to be correctly aligned to the target type, then a cast to that type is permitted. There are several cases where a pointer is known to be correctly aligned to the target type. The pointer could point to an object declared with a suitable alignment specifier. It could point to an object returned by
realloc(), as per the C standard, section 7.22.3, paragraph 1 [ISO/IEC 9899:2011].
This compliant solution uses the alignment specifier, which is new to C11, to declare the
c with the same alignment as that of an object of type
int. As a result, the two pointers reference equally aligned pointer types:
Accessing a pointer or an object that is not properly aligned can cause a program to crash or give erroneous information, or it can cause slow pointer accesses (if the architecture allows misaligned accesses).
|Axivion Bauhaus Suite|
Can detect violations of this rule. However, it does not flag explicit casts to
MISRA C 2004 Rule 11.4
MISRA C 2012 Rule 11.1
MISRA C 2012 Rule 11.2
MISRA C 2012 Rule 11.5
MISRA C 2012 Rule 11.7
Can detect some violations of this rule when the
|LDRA tool suite|
94 S, 606 S
A cast should not be performed between a pointer to object type and a different pointer to object type
|Polyspace Bug Finder|
Checks for wrong allocated object size for cast (rule fully covered)
|0326, 3305||Fully implemented|
Key here (explains table format and definitions)
|CERT C||VOID EXP56-CPP. Do not cast pointers into more strictly aligned pointer types||Prior to 2018-01-12: CERT: Unspecified Relationship|
|ISO/IEC TR 24772:2013||Pointer Casting and Pointer Type Changes [HFC]||Prior to 2018-01-12: CERT: Unspecified Relationship|
|ISO/IEC TS 17961||Converting pointer values to more strictly aligned pointer types [alignconv]||Prior to 2018-01-12: CERT: Unspecified Relationship|
|MISRA C:2012||Rule 11.1 (required)||Prior to 2018-01-12: CERT: Unspecified Relationship|
|MISRA C:2012||Rule 11.2 (required)||Prior to 2018-01-12: CERT: Unspecified Relationship|
|MISRA C:2012||Rule 11.5 (advisory)||Prior to 2018-01-12: CERT: Unspecified Relationship|
|MISRA C:2012||Rule 11.7 (required)||Prior to 2018-01-12: CERT: Unspecified Relationship|
|[ISO/IEC 9899:2011]||188.8.131.52, "Pointers"|
|[Walfridsson 2003]||Aliasing, Pointer Casts and GCC 3.3|