MISRA C:2012 Rule 11.2

Conversions shall not be performed between a pointer to an incomplete type and any other type

Description

Rule Definition

Conversions shall not be performed between a pointer to an incomplete type and any other type.

Rationale

An incomplete type is a type that does not contain sufficient information to determine its size. For example, the statement struct s; describes an incomplete type because the fields of s are not defined. The size of a variable of type s cannot be determined.

Conversions to or from a pointer to an incomplete type result in undefined behavior. Typically, a pointer to an incomplete type is used to hide the full representation of an object. This encapsulation is broken if another pointer is implicitly or explicitly cast to such a pointer.

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

expand all

Casts from incomplete type

#include <stdio.h> 
struct s *sp;
struct t *tp;
short  *ip;
struct ct *ctp1;
struct ct *ctp2;


void foo(void) {

    ip = (short *) sp;            /* Non-compliant */
    sp = (struct s *) 1234;       /* Non-compliant */
    tp = (struct t *) sp;         /* Non-compliant */
    ctp1 = (struct ct *) ctp2;    /* Compliant */

    /* You can convert a null pointer constant to 
     * a pointer to an incomplete type */
    sp = NULL;                    /* Compliant - exception  */

    /* A pointer to an incomplete type may be converted into void */
    struct s *f(void);
    (void) f();                   /* Compliant - exception  */

}

In this example, types s, t and ct are incomplete. The rule is violated when:

The variable sp with an incomplete type is cast to a basic type.
The variable sp with an incomplete type is cast to a different incomplete type t.

The rule is not violated when:

The variable ctp2 with an incomplete type is cast to the same incomplete type.
The NULL pointer is cast to the variable sp with an incomplete type.
The return value of f with incomplete type is cast to void.

Check Information

Group: Pointer Type Conversions

Category: Required

AGC Category: Required

Version History

Introduced in R2014b