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Flexible array members are a special type of array in which the last element of a structure with more than one named member has an incomplete array type; that is, the size of the array is not specified explicitly within the structure. This "struct hack" was widely used in practice and supported by a variety of compilers. Consequently, a variety of different syntaxes have been used for declaring flexible array members. For C-compliant implementations, use the syntax guaranteed valid by the C Standard [ISO/IEC 9899:2011]Standard.

Flexible array members are defined in section 6.7.2.1 of the C Standard [ISO/IEC 9899:2011] 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.

...

struct flexArrayStruct {
  int num;
  int data[1];
};

/* ... */

size_t array_size;
size_t i;

/* initializeInitialize array_size */

/* spaceSpace is allocated for the struct */
struct flexArrayStruct *structP
  = (struct flexArrayStruct *)
     malloc(sizeof(struct flexArrayStruct)
          + sizeof(int) * (array_size - 1));
if (structP == NULL) {
  /* Handle malloc failure */
}
structP->num = 0;

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

...

This compliant solution uses the flexible array member to achieve a dynamically sized structure.:

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

/* ... */

size_t array_size;
size_t i;

/* Initialize array_size */

/* Space is allocated for the struct */
struct flexArrayStruct *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;
}

This compliant solution allows the structure to be treated as if it had declared the member data[] to be data[array_size] in a manner that conforms to the C Standard.   Note that Microsoft Visual Studio implements support for flexible array members, but some versions (such as MSVC 11) warn that this is a non-standard extension.

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