MISRA.ASSIGN.OVERLAP

An object shall not be assigned to an overlapping object.

MISRA C 2012 Rule 19.1: An object shall not be assigned or copied to an overlapping object

C90 [Undefined 34, 55], C99 [Undefined 51, 94]

Category: Mandatory

Analysis: Undecidable, System

Applies to: C90, C99

Rationale

The behaviour is undefined when two objects are created which have some overlap in memory and one is assigned or copied to the other.

Exception

Example

This example also violates Rule 19.2 because it uses unions.

void fn ( void ) 
{
  union 
  {
    int16_t i;
    int32_t j; 
  } a = { 0 }, b = { 1 };

  a.j = a.i;             /* Non-compliant            */ 
  a = b;                 /* Compliant - exception 1 */ 
}

#include <string.h>

int16_t a[ 20 ];

void f ( void ) 
{
  memcpy ( &a[ 5 ], &a[ 4 ], 2u * sizeof ( a[ 0 ] ) ); /* Non-compliant */ 
}

void g ( void ) 
{
  int16_t *p = &a[ 0 ]; 
  int16_t *q = &a[ 0 ];

  *p = *q;              /* Compliant - exception 1 */ 
}

See also

Rule 19.2

MISRA-C 2004 Rule 18.2 (required): An object shall not be assigned to an overlapping object.

Object is assigned to an overlapping object.

[Undefined 34, 55]

The behaviour is undefined when two objects are created which have some overlap in memory and one is copied to the other.

MISRA-C++ 2008 Rule 0-2-1 (required): An object shall not be assigned to an overlapping object.

This rule is also covered by MISRA.UNION.

[Undefined 5.17(8)]

Rationale

Assigning between objects that have an overlap in their physical storage leads to undefined behaviour.

Example

struct s
{
   int16_t m1 [ 32 ];
};
struct t
{
   int32_t m2;
   struct s m3;
};
void fn ( )
{
   union // Breaks Rule 9—5—1
   {
      struct s u1;
      struct t u2;
   } a;
   a.u2.m3 = a.u1; // Non-compliant
}