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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 C99-compliant conforming C implementations, use the syntax guaranteed to be valid by C99 [ISO/IEC 9899:1999]the C Standard.

Flexible array members are defined in Section the C Standard, 6.7.3.2.1, paragraph 16 of the C99 standard , paragraph 20 [ISO/IEC 9899:2024], 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.

Structures with a flexible array members member can be used to produce code with defined behavior. 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 a flexible array membersmember.
3. Structures that contain a flexible array member cannot be used as a member in the middle of another structure.

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1. The structure must contain at least one named member in addition to the flexible array member.

Noncompliant Code Example

Prior to the Before the introduction of flexible array members in the C99 standardC 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 example attempts to allocated allocate a flexible array-like 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.

#include <stdlib.h>
 
struct flexArrayStruct {
  int num;
  int data[1];
};

/* ... */

void func(size_t array_size;
size_t i;

/* initialize 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 = 0array_size;

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

This example has undefined behavior when accessing any element The problem with using this approach is that the behavior is undefined when accessing other than the first element of the data array. (See Section the C Standard, 6.5.6, Paragraph 8 of the C99 standard7.) 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 standard C syntax. Microsoft Visual Studio 2005 does not implement the C99 syntax.

Compliant Solution

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

#include <stdlib.h>
 
struct flexArrayStruct{
  int num;
  int data[];
};

/* ... */

void func(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 = 0array_size;

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

This compliant solution allows the structure to be treated as if it had declared the its member data[] was declared to be data[array_size] in a manner that conforms to the C99 standardC Standard.

Risk Assessment

Failing to use the correct syntax when declaring a flexible array member can result in undefined behavior, although the incorrect syntax will work on most implementations.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

This rule supplements MEM33-C. Allocate and copy structures containing a flexible array member dynamically

Bibliography

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Bibliography

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Image Added Image Added Image Added ;login:, July 2001, Volume 26, Number 4Image Removed      02. Declarations and Initialization (DCL)      DCL38-C. Use the correct syntax when declaring flexible array members