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The C99 standard \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] introduces flexible array members into the language. While flexible array members are a useful addition they should be properly understood and used with care. 

Flexible array members are defined in Section 6.7.2.1, paragraph 16 of the C99 standard as follows,

As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In most situations, the flexible array member is ignored. In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply. However, when a . (or ->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, it behaves as if it had one element but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.

The following is an example of a structure that contains a flexible array member,

struct flexArrayStruct {
  int num;
  int data[];
};

This definition means that, when allocating storage space, only the first member, num, is considered. Consequently, the result of accessing the member data of a variable of type struct flexArrayStruct is undefined. To avoid the potential for undefined behavior, structures that contain a flexible array member should always be accessed with a pointer as shown in the following code example.

The following is an example of a structure that contains a flexible array member,

struct flexArrayStruct {
  int num;
  int data[];
};

This definition means that, when allocating storage space, only the first member, num, is considered. Consequently, the result of accessing the member data of a variable of type struct flexArrayStruct is undefined. To avoid the potential for undefined behavior, structures that contain a flexible array member should always be accessed with a pointer as shown in the following code example.

struct flexArrayStruct *structP;
size_t array_size;
size_t i;

/* Initialize array_size */

/* Space is allocated for the struct */
structP = (struct flexArrayStruct *)
   malloc(sizeof(struct flexArrayStruct) + sizeof(int) * array_size);
if (

struct flexArrayStruct *structP;
size_t array_size;
size_t i;

/* Initialize array_size */

/* Space is allocated for the struct */
structP = (struct flexArrayStruct *)
   malloc(sizeof(struct flexArrayStruct) + sizeof(int) * array_size);
if (structP == NULL) {
  /* Handle malloc failure */
}

structP->num = 0;

/*
 * Access data[] as if it had been allocated
 * as data[array_size]
 */
for (i = 0; i < array_size; i++) {
  structP->data[i] = 1;
}

Structures with flexible array members can be used to produce code with defined behavior.  However, some restrictions apply:

...

 = 0; i < array_size; i++) {
  structP->data[i] = 1;
}

Noncompliant Code Example (Use Flexible Array Members)

...