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The C Standard, subclause 6.7.2.1, paragraph 18 [ISO/IEC 9899:2011], says:

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.

The following is an example of a structure that contains a flexible array member:

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

This definition means that when allocating storage, only the first member, num, is considered. Consequently, the result of accessing the member data of a variable of nonpointer type struct flexArrayStruct is undefined. DCL38-C. Use the correct syntax when declaring flexible array members describes the correct way to declare a struct with a flexible array member.

To avoid the potential for undefined behavior, structures that contain a flexible array member should always be allocated and operated on dynamically. Flexible array structures should

• Have allocated storage duration (be allocated via malloc() or another dynamic allocation function)
• Be dynamically copied using memcpy() or a similar function
• Be passed as a pointer to functions

Noncompliant Code Example (Storage Duration)

This noncompliant code example uses automatic storage for a structure containing a flexible array member:

#include <stddef.h>
 
struct flexArrayStruct {
  size_t num;
  int data[];
};
 
void func(void) {
  struct flexArrayStruct flexStruct;
  size_t array_size = 4;

  /* Initialize structure */
  flexStruct.num = array_size;

  for (size_t i = 0; i < array_size; ++i) {
    flexStruct.data[i] = 0;
  }
}

Because flexStruct does not use allocated memory, no space is reserved for the data member. Accessing the data member is undefined behavior.

Compliant Solution (Storage Duration)

This compliant solution dynamically allocates storage for struct flexArrayStruct:

#include <stdlib.h>
 
struct flexArrayStruct {
  size_t num;
  int data[];
};
 
void func(void) {
  struct flexArrayStruct *flexStruct;
  size_t array_size = 4;

  /* Dynamically allocate memory for the struct */
  flexStruct = (struct flexArrayStruct *)malloc(
    sizeof(struct flexArrayStruct) + sizeof(int) * array_size);
  if (flexStruct == NULL) {
    /* Handle error */
  }

  /* Initialize structure */
  flexStruct->num = array_size;

  for (size_t i = 0; i < array_size; ++i) {
    flexStruct->data[i] = 0;
  }
}

Noncompliant Code Example (Copying)

This noncompliant code example attempts to copy an instance of a structure containing a flexible array member (struct flexArrayStruct) by assignment:

#include <stddef.h>
 
struct flexArrayStruct {
  size_t num;
  int data[];
};
 
void func(struct flexArrayStruct *structA,
          struct flexArrayStruct *structB) {
  *flexStructB = *flexStructA;
}

When the structure is copied, the size of the flexible array member is not considered, and only the first member of the structure, num, is copied, leaving the array contents untouched.

Compliant Solution (Copying)

This compliant solution uses memcpy() to properly copy the content of structA into structB:

#include <string.h>
 
struct flexArrayStruct {
  size_t num;
  int data[];
};
 
void func(struct flexArrayStruct *structA,
          struct flexArrayStruct *structB) {
  if (structA->num > structB->num) {
    /* Insufficient space; handle error */
    return;
  }
  memcpy(structB, structA,
         sizeof(struct flexArrayStruct) + (sizeof(int)
           * structA->num));
}

Noncompliant Code Example (Function Arguments)

In this noncompliant code example, the flexible array structure is passed directly to a function that prints the array elements:

#include <stdio.h>
#include <stdlib.h>
 
struct flexArrayStruct {
  size_t num;
  int data[];
};
 
void print_array(struct flexArrayStruct structP) {
  puts("Array is: ");
  for (size_t i = 0; i < structP.num; ++i) {
    printf("%d", structP.data[i]);
  }
}

void func(void) {
  struct flexArrayStruct *structP;
  size_t array_size = 4;

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

  for (size_t i = 0; i < array_size; ++i) {
    structP->data[i] = i;
  }
  print_array(*structP);
}

Because C passes the argument by value, the size of the flexible array member is not considered when the structure is copied, and only the first member of the structure, num, is copied.

Compliant Solution (Function Arguments)

In this compliant solution, the print_array() function accepts a pointer to the structure rather than the structure itself:

#include <stdio.h>
#include <stdlib.h>
 
struct flexArrayStruct {
  size_t num;
  int data[];
};
 
void print_array(struct flexArrayStruct *structP) {
  puts("Array is: ");
  for (size_t i = 0; i < structP->num; ++i) {
    printf("%d", structP->data[i]);
  }
}
 
void func(void) {
  struct flexArrayStruct *structP;
  size_t array_size = 4;

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

  for (size_t i = 0; i < array_size; ++i) {
    structP->data[i] = i;
  }
  print_array(structP);
}

Risk Assessment

Failure to use structures with flexible array members correctly can result in undefined behavior. 

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography