MISRA.LITERAL.NULL.PTR.CONST.2012

The macro NULL is the only permitted form of integer null pointer constant

MISRA C 2012 Rule 11.9: The macro NULL shall be the only permitted form of integer null pointer constant

Category: Required

Analysis: Decidable, Single Translation Unit

Applies to: C90, C99

Amplification

Ignoring whitespace and any surrounding parentheses, any such integer constant expression shall represent the entire expansion of NULL.

Note: a null pointer constant of the form (void *)0 is permitted, whether or not it was expanded from NULL.

Rationale

Using NULL rather than 0 makes it clear that a null pointer constant was intended.

Example

In the following example, the initialization of p2 is compliant because the integer constant expression 0 does not appear in one of the contexts prohibited by this rule.

int32_t *p1 = 0;                          /* Non-compliant */
int32_t *p2 = ( void * ) 0;               /* Compliant     */

In the following example, the comparison between p2 and (void *)0 is compliant because the integer constant expression 0 appears as the operand of a cast and not in one of the contexts prohibited by this rule.

#define MY_NULL_1 0 
#define MY_NULL_2 ( void * ) 0

if ( p1 == MY_NULL_1 )       /* Non-compliant */ 
{ 
} 
if ( p2 == MY_NULL_2 )       /* Compliant     */ 
{
}

The following example is compliant because use of the macro NULL provided by the implementation is always permitted, even if it expands to an integer constant expression with value 0.

/* Could also be stdio.h, stdlib.h and others */ 
#include <stddef.h>

extern void f ( uint8_t *p );

/* Compliant for any conforming definition of NULL, such as:
 *    0 
 *    ( void * ) 0 
 *    ( ( ( 0 ) ) ) 
 *    ( ( ( 1 - 1 ) ) ) 
 */ 
f ( NULL );

See also

Rule 11.4