MISRA.MEMB.FLEX_ARRAY.2012

MISRA C 2012 Rule 18.7: Flexible array members shall not be declared

C99 [Undefined 59]

Category: Required

Analysis: Decidable, Single Translation Unit

Applies to: C90, C99

Rationale

Flexible array members are most likely to be used in conjunction with dynamic memory allocation which is banned by Dir 4.12 and Rule 21.3.

The presence of flexible array members modifies the behaviour of the sizeof operator in ways that might not be expected by a programmer. The assignment of a structure that contains a flexible array member to another structure of the same type may not behave in the expected manner as it copies only those elements up to but not including the start of the flexible array member.

Example

#include <stdlib.h>

struct s 
{
  uint16_t len; 
  uint32_t data[ ];          /* Non-compliant - flexible array member */ 
} str;

struct s *copy ( struct s *s1 ) 
{
  struct s *s2;

  /* Omit malloc ( ) return check for brevity */ 
  s2 = malloc ( sizeof ( struct s ) + ( s1->len * sizeof ( uint32_t ) ) );

  *s2 = *s1;                 /* Only copies s1->len                   */

  return s2; 
}

See also

Dir 4.12, Rule 21.3