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Flexible array members are a special type of array where 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 C99C-compliant implementations, use the syntax guaranteed valid by C99 by the C standard [ISO/IEC 9899:19992011].

Flexible array members are defined in Section 6.7.2.1 , paragraph 16 of the C99 C 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. HoweverHowev er, 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.

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Noncompliant Code Example

Prior to the Before the introduction of flexible array members in the C99 C standard, structures with a one-element array as the final member were used to achieve similar functionality. This noncompliant code example illustrates how struct flexArrayStruct is declared in this case.

This noncompliant code attempts to allocated allocate a flexible array member with a one-element array as the final member. When the structure is instantiated, the size computed for malloc() is modified to account for the actual size of the dynamic array.

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

/* ... */

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 - 1));
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;
}

The problem with using this approach is that the behavior is undefined when accessing other than the first element of data. (See Section 6.5.6, Paragraph 8 para. 82 of the C99 standardC standard [ISO/IEC 9899:2011].) Consequently, the compiler can generate code that does not return the expected value when accessing the second element of data.

This approach may be the only alternative for compilers that do not yet implement the C99 C syntax. Microsoft Visual Studio 2005 does not implement the C99 C syntax.

Compliant Solution

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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 C99 C standard.

Risk Assessment

Failing to use the correct syntax can result in undefined behavior, although the incorrect syntax will work on most implementations.

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

ISO/IEC 9899:19992011 Section 6.7.2.1, "Structure and union specifiers"

Bibliography

[McCluskey 2001] ;login:, July 2001, Volume vol. 26, Number  no. 4.

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