MISRA C:2023 Rule 8.16

The alignment specification of zero should not appear in an object declaration

Since R2024a

Description

The alignment specification of zero should not appear in an object declaration¹ .

Rationale

If the operand expr of _Alignas(expr) type var evaluates to zero, the specifier has no effect on the alignment of the object var.

If it is your intent to specify the alignment of an object, specify a non-zero value for the operand of _Alignas. If you intend to disable the alignment specification based on a condition such as platform implementation details, use preprocessor directives to abstract this requirement.

For example, this code snippet illustrates how to specify an alignment on a Linux platform but disable the alignment specification on other platforms.

#ifdef __linux__  
    #define ALIGN_SPEC 16
#else
    #define ALIGN_SPEC 0  
#endif

#define ALIGNAS_PLATFORM(expr) _Alignas(expr)

void func() {
    ALIGNAS_PLATFORM(ALIGN_SPEC) int conditionalAlignVar;  
    
    //...
}

Polyspace Implementation

The coding rule checkers reports a violation on when you use _Alignas(0) in a variable declaration.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

Use of _Alignas(0) in variable declaration

#include <stdint.h>

struct {
    int32_t varNoAlign;
    _Alignas(16) int32_t varAlign;
    _Alignas(0) int32_t varNoAlignZero; //Noncompliant.
} data;

In this example, the structure data has multiple members with different alignment specifications. Polyspace reports the declaration of varNoAlignZero as noncompliant with the coding rule because of the use of _Alignas (0).

Check Information

Group: Declarations and definitions

Category: Advisory

AGC Category: Advisory

PQL Name: std.misra_c_2023.R8_16

Version History

Introduced in R2024a