MEM33-C. Use the correct syntax for flexible array members

Flexible array members are a special type of array where 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. A variety of different syntaxes have been used for declaring flexible array members. For C99-compliant implementations, use the syntax guaranteed valid by C99 [[ISO/IEC 9899-1999]].

Non-Compliant Code Example

In this non-compliant code, an array of size 1 is declared, but when the structure itself is instantiated, the size computed for malloc() is modified to account for the actual size of the dynamic array. This is the syntax used by ISO C89.

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

/* ... */
/* 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 = SOME_NUMBER;

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

The problem with this code is that the only member that is guaranteed to be valid, by strict C99 definition, is structP->data[0]. Consequently, for all i > 0, the results of the assignment are undefined.

Implementation Details

The non-compliant example may be the only alternative for compilers that do not yet implement the C99 syntax.  Microsoft Visual Studio 2005 does not implement the C99 syntax.

Compliant Solution

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

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

/* ... */
/* 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 = SOME_NUMBER;

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

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 standard.

However, some restrictions apply:

1. The incomplete array type must be the last element within the structure.
2. There cannot be an array of structures that contain flexible array members.
3. Structures that contain a flexible array member cannot be used as a member in the middle of another structure.
4. The sizeof operator cannot be applied to a flexible array.

Risk Assessment

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

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 6.7.2.1, "Structure and union specifiers"
[[McCluskey 01]] ;login:, July 2001, Volume 26, Number 4

MEM32-C. Detect and handle memory allocation errors      08. Memory Management (MEM)