# V2586. MISRA. Flexible array members should not be declared.

This diagnostic rule is based on the MISRA (Motor Industry Software Reliability Association) software development guide.

This rule only applies to programs written in C. Flexible array members should not be declared. Flexible array members are often used if dynamic memory allocation is expected and if the size of the stored data is unknown.

Example:

    typedef struct
    {
      size_t len;
      int data[]; // flexible array
    } S;

    S* alloc_flexible_array(size_t n)
    {
      S *obj = malloc(sizeof(S) + (n * sizeof(int)));
      obj->len = n;
      return obj;
    }

With such flexible array members, the size of the 'data' array is determined at runtime according to the actual amount of data.

Such flexible array members are dangerous because 'sizeof' gives a wrong result.

Another problem is that a copy of such flexible array member may lead to unexpected results, even if the size is calculated correctly. Consider the corresponding example:

    typedef struct
    {
      size_t len;
      int data[];
    } S;

    S* make_copy(S *s)
    {
      S *copy = malloc(sizeof(S) + (s->len * sizeof(int)));
      *copy = *s;
      return copy;
    }

Even though the correct amount of memory is allocated, only the 'len' field gets into the copy.

Flexible array members are often declared in the wrong way:

    typedef struct
    {
      size_t len;
      int data[1];
    } S;

This is a one-element array. The compiler might consider accessing such an array past the first element as undefined behavior and optimize the code unexpectedly.

This diagnostic is classified as:

MISRA-C-2012-18.7 MISRA-C-2023-18.7