MISRA C:2012 Rule 9.4

An element of an object shall not be initialized more than once

Description

Rule Definition

An element of an object shall not be initialized more than once¹ .

Rationale

Designated initializers allow explicitly initializing elements of objects such as arrays in any order. However, using designated initializers, one can inadvertently initialize the same element twice and therefore overwrite the first initialization.

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

Array Initialization Using Designated Initializers

void func(void) {
    int a[5] = {-2,-1,0,1,2};               /* Compliant */
    int b[5] = {[0]=-2, [1]=-1, [2]=0, [3]=1, [4]=2}; /* Compliant */
    int c[5] = {[0]=-2, [1]=-1, [1]=0, [3]=1, [4]=2}; /* Non-compliant */

}

In this example, the rule is violated when the array element c[1] is initialized twice using a designated initializer.

Structure Initialization Using Designated Initializers

struct myStruct {
    int a;
    int b;
    int c;
    int d;
};

void func(void) {
    struct myStruct struct1 = {-4,-2,2,4}; /* Compliant */
    struct myStruct struct2 = {.a=-4, .b=-2, .c=2, .d=4}; /* Compliant */
    struct myStruct struct3 = {.a=-4, .b=-2, .b=2, .d=4}; /* Non-compliant */
}

In this example, the rule is violated when struct3.b is initialized twice using a designated initializer.

Check Information

Group: Initialization

Category: Required

AGC Category: Required

Version History

Introduced in R2014b

R2024a: Positional initializer element values overwriting earlier ones reported as violation

This rule checker now reports a violation if a positional initializer element overwrites earlier initialization and is preceded by an out of sequence designated element:

struct foo my_struct = { .b = 2, .c = 3, .a = 1, /* b */ 4 }; //Noncompliant 

Here, my_struct is {1,4,3,0}, which can be unexpected.

R2023b: Initializing multiple array elements inside a union no longer a violation

Polyspace no longer flags this rule checker when you initialize different array members within the same union or struct. For example:

typedef union {
	char grade[2];
} classA;


void main()
{
	classA students = {
		.grade[0] = "A",   //Compliant
		.grade[1] = "C"    //Compliant
	};
}