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

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

Code Block
struct flexArrayStructflex_array_struct { int num; int data[]; };

This definition means that when allocating storagecomputing the size of such a structure, only the first member, num, is considered. ConsequentlyUnless the appropriate size of the flexible array member has been explicitly added when allocating storage for an object of the struct, the result of accessing the member data of a variable of nonpointer type struct flex_array_struct flexArrayStruct is undefined. DCL38-C. Use the correct syntax when declaring a flexible array member 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 shouldstructures must

Have allocated dynamic storage duration (be allocated via malloc() or another dynamic allocation function)
Be dynamically copied using memcpy() or a similar function and not by assignment
When used as an argument to a function, be passed by pointer and not copied by valueBe passed as a pointer to functions

Noncompliant Code Example (Storage Duration)

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Code Block

bgColor	#FFcccc
lang	c

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

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

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

Because flexStruct does not use allocated memorythe memory for flex_struct is reserved on the stack, 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 flex_array_struct flexArrayStruct:

Code Block

bgColor	#ccccff
lang	c

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

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

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

  for (size_t i = 0; i < array_size; ++i) {
    flexStructflex_struct->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 flex_array_struct flexArrayStruct) by assignment:

Code Block

bgColor	#FFcccc
lang	c

#include <stddef.h>
 
struct flexArrayStructflex_array_struct {
  size_t num;
  int data[];
};
 
void func(struct flexArrayStructflex_array_struct *structAstruct_a,
          struct flexArrayStructflex_array_struct *structBstruct_b) {
  *flexStructBstruct_b = *flexStructAstruct_a;
}

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.

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

Code Block

bgColor	#ccccff
lang	c

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

Noncompliant Code Example (Function Arguments)

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

Code Block

bgColor	#FFcccc
lang	c

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

void func(void) {
  struct flexArrayStructflex_array_struct *structPstruct_p;
  size_t array_size = 4;

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

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

Because C passes the argument is passed 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.

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In this compliant solution, the print_array() function accepts a pointer to the structure rather than the structure itselfthe structure is passed by reference and not by value:

Code Block

bgColor	#ccccff
lang	c

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

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

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

Risk Assessment

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

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Related Vulnerabilities

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

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Bibliography

[ISO/IEC 9899:2011]	Subclause 6.7.2.1, "Structure and Union Specifiers"
[JTC1/SC22/WG14 N791]	Solving the Struct Hack Problem

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Noncompliant Code Example (Storage Duration)

Compliant Solution (Storage Duration)

Noncompliant Code Example (Copying)

Noncompliant Code Example (Function Arguments)

Risk Assessment

Related Vulnerabilities

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