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.

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 take into account the full 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 = malloc(sizeof(struct flexArrayStruct) + sizeof(int) * (ARRAY_SIZE - 1));
if (!structP) {
     /* 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

GNU C uses zero-length arrays for flexible array members.

struct flexArrayStruct {
     int num;
     int data[0];
};

/* ... */
/* Space is allocated for the struct */
struct flexArrayStruct *structP = malloc(sizeof(struct flexArrayStruct) + sizeof(int) * ARRAY_SIZE);
if (!structP) {
     /* 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;
}

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 = malloc(sizeof(struct flexArrayStruct) + sizeof(int) * ARRAY_SIZE);
if (!structP) {
  /* 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. You cannot have an array of structure if the structure contains flexible array members.
3. Structures that contain a flexible array member cannot be used as a member in the middle of another structure.
4. You cannot apply the sizeof operator to a flexible array.

Risk Assessment

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

Related Vulnerabilities

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

References

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