MISRA C:2023 Rule 18.9

An object with temporary lifetime shall not undergo array-to-pointer conversion

Since R2024a

Description

Rule Definition

An object with temporary lifetime shall not undergo array-to-pointer conversion.

Rationale

A temporary object can undergo array-to-pointer conversion if an array is member of a temporary object. For example, the structure my_struct contains an array. It is possible to create a pointer pointing to the member bytes that has a temporary lifetime:

typedef struct my_struct {
	char bytes[10];
} MS;

MS get_my_struct();
p = get_my_struct().bytes;

Calling get_my_struct().bytes creates a temporary structure. The address of the first element of the bytes array of the temporary structure is stored in p. The lifetime of the object pointed to by p ends immediately after this statement. Using this pointer to modify or access the array can result in undefined behavior.

The lifetime of a temporary array ends:

When the full expression that generates the temporary array ends, as defined in the C11 Standard.
After the next sequence point, as defined in the C90 and C99 Standards. A sequence point is a point in the execution of a program where all previous evaluations are complete and no subsequent evaluation has started yet.

To avoid undefined behavior, store the temporary object containing the array into a named object.

Polyspace Implementation

This rule checker reports a violation If these conditions are met:

Your code creates a temporary object, such as a structure or a union, that contains an array.
Your code attempts to read or modify the temporary array.

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

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Accessing Temporary Arrays

In this example, the function getStruct() returns an array_wrapper structure. The function foo() creates temporary structures containing arrays by calling getStruct(). Reading or writing into this temporary array is a violation of this rule.

 #include <stdint.h>


typedef struct array_wrapper {
    int a[10];
} AW;

AW getStruct();

void access_array(int *p);

void foo() {
    int* p;
    p = getStruct().a + 1;                 // Noncompliant
    *(&(getStruct().a[1])) = 1;            // Noncompliant
    *(getStruct().a + 1) = 1;              // Noncompliant
    access_array(getStruct().a + 1);       // Noncompliant
}

Check Information

Group: Pointers and Arrays

Category: Required

AGC Category: Required

Version History

Introduced in R2024a