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The C Standard, subclause 6.7.3.2.1, paragraph 18 20 [ISO/IEC 9899:20112024], 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:

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 flexArrayStructflex_array_struct 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)

...

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 flexArrayStruct) flex_array_struct)
    + sizeof(int) * array_size);
  if (flexStructflex_struct == 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.

...

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.

...

In this compliant solution, the print_array() function accepts a pointer to the structure rather than the structure itselfstructure 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]);
  }
  putchar('\n');
}
 
void func(void) {
  struct flexArrayStructflex_array_struct *structPstruct_p;
  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;
  }and initialized... */

  print_array(structPstruct_p);
}

Risk Assessment

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

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MEM33-C	Low	Unlikely	Low	P3	L3

Automated Detection

Tool

Version

Checker

Description

Astrée

Include Page

	Astrée_V
	Astrée_V

flexible-array-member-assignment
flexible-array-member-declaration

Fully checked

Axivion Bauhaus Suite

Include Page

	Axivion Bauhaus Suite_V
	Axivion Bauhaus Suite_V

CertC-MEM33

Fully implemented

CodeSonar

Include Page

	CodeSonar_V
	CodeSonar_V

LANG.STRUCT.DECL.FAM

Declaration of Flexible Array Member

Compass/ROSE

Can detect all of these

Cppcheck Premium

Include Page

	Cppcheck Premium_V
	Cppcheck Premium_V

premium-cert-mem33-c

Partially implemented

Helix QAC

Include Page

	Helix QAC_V
	Helix QAC_V

C1061, C1062, C1063, C1064

Klocwork

Include Page

	Klocwork_V
	Klocwork_V

MISRA.INCOMPLETE.STRUCT
MISRA.MEMB.FLEX_ARRAY.2012

LDRA tool suite

Include Page

	LDRA_V
	LDRA_V

649 S, 650 S

Fully implemented

Parasoft C/C++test

Include Page

	Parasoft_V
	Parasoft_V

CERT_C-MEM33-a
CERT_C-MEM33-b

Allocate structures containing a flexible array member dynamically
Do not copy instances of structures containing a flexible array member

Polyspace Bug Finder

Include Page

	Polyspace Bug Finder_V
	Polyspace Bug Finder_V

CERT C: Rule MEM33-C

Checks for misuse of structure with flexible array member (rule fully covered)

RuleChecker

Include Page

	RuleChecker_V
	RuleChecker_V

flexible-array-member-assignment
flexible-array-member-declaration

Fully checked

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Taxonomy	Taxonomy item	Relationship
CERT C Secure Coding Standard	DCL38-C. Use the correct syntax when declaring a flexible array member	Prior to 2018-01-12: CERT: Unspecified Relationship

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-401 and MEM33-CPP

There is no longer a C++ rule for MEM33-CPP. (In fact, all C++ rules from 30-50 are gone, because we changed the numbering system to be 50-99 for C++ rules.)

Bibliography

[ISO/IEC 9899:

2011

2024]

Subclause 6.7.3.2

.1

, "Structure and Union Specifiers"
[JTC1/SC22/WG14 N791]	Solving the Struct Hack Problem

...

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Versions Compared

Old Version 81

New Version Current

Key

Noncompliant Code Example (Storage Duration)

Compliant Solution (Storage Duration)

Noncompliant Code Example (Copying)

Noncompliant Code Example (Function Arguments)

Risk Assessment

Automated Detection

Related Vulnerabilities

Related Guidelines

CERT-CWE Mapping Notes

CWE-401 and MEM33-CPP

Bibliography