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Wiki MarkupThe C99 standard \[The C Standard, 6.7.3.2, paragraph 20 [ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] introduces flexible array members into the language. While flexible array members are a useful addition, they need to be understood and used with care. 2024], 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 flexArrayStructflex_array_struct {
  int num;
  int data[];
};

This definition means that , when allocating storagewhen computing 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 type struct flexArrayStruct nonpointer type struct flex_array_struct is undefined. DCL38-C. Use the correct syntax when declaring a flexible array membersmember 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 must

• Have 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 value

Noncompliant Code Example (Storage

...

Duration)

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

#include <stddef.h>
 
struct flexArrayStruct flexStruct;
size_t array_size;
size_t i;

/* Initialize array_size */

flex_array_struct {
  size_t num;
  int data[];
};
 
void func(void) {
  struct flex_array_struct flex_struct;
  size_t array_size = 4;

  /* Initialize structure */
flexStruct  flex_struct.num = 0array_size;

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

Because the memory for flex_struct is reserved on the stack, no space is reserved for the data member. Accessing the data member is undefined behavior Wiki MarkupThe problem with this code is that the {{flexArrayStruct}} does not actually reserve space for the integer array data; it can't because the size hasn't been specified. Consequently, while initializing the {{num}} member to zero is allowed, attempting to write even one value into data (that is, {{data\[0\]}}) is likely to overwrite memory outside of the object's bounds.

Compliant Solution (Storage

...

Duration)

This compliant solution dynamically allocates storage for struct flexArrayStruct. flex_array_struct:

#include <stdlib.h>
 
struct flexArrayStruct *flexStruct;
flex_array_struct {
  size_t array_sizenum;
size_t i;

/* Initialize  int data[];
};
 
void func(void) {
  struct flex_array_struct *flex_struct;
  size_t array_size = */4;

  /* Dynamically allocate memory for the structurestruct */
flexStruct  flex_struct = (struct flexArrayStructflex_array_struct *)malloc(
    sizeof(struct flexArrayStruct) flex_array_struct)
    + sizeof(int) * array_size
);
  if (flexStructflex_struct == NULL) {
    /* Handle malloc failureerror */
  }

  /* Initialize structure */
flexStruct  flex_struct->num = 0array_size;

  for (size_t i = 0; i < array_size; i++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.:

#include <stddef.h>
 
struct flexArrayStruct *flexStructA;
struct flexArrayStruct *flexStructB;
flex_array_struct {
  size_t array_sizenum;
size_t i  int data[];
};
 
/* Initialize void func(struct flex_array_sizestruct */

/* Allocate memory for flexStructA */

/* Allocate memory for flexStructB */

/* Initialize flexStructA */

/* ... */

*flexStructB = *flexStructA;
struct_a,
          struct flex_array_struct *struct_b) {
  *struct_b = *struct_a;
}

When The problem with this noncompliant code example is that 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 flexStructA into flexStructB.of struct_a into struct_b:

#include <string.h>
 
struct flexArrayStruct *flexStructA;
struct flexArrayStruct *flexStructB;
flex_array_struct {
  size_t array_sizenum;
size_t i  int data[];
};
 
/* Initialize void func(struct flex_array_sizestruct */

/* Allocate memory for flexStructA */

/* Allocate memory for flexStructB */

/* Initialize flexStructA */

/* ... */

memcpy(
  flexStructB, 
  flexStructA, 
  (sizeof(struct flexArrayStruct) + sizeof(int) * array_size)
);

...

struct_a,
          struct flex_array_struct *struct_b) {
  if (struct_a->num > struct_b->num) {
    /* Insufficient space; handle error */
    return;
  }
  memcpy(struct_b, struct_a,
         sizeof(struct flex_array_struct) + (sizeof(int)
           * struct_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 tries to print prints the array elements.:

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

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

/* initialize array_size */

/* spaceSpace is allocated for the struct */
structP  struct_p = (struct flexArrayStructflex_array_struct *)malloc(
    sizeof(struct flexArrayStruct) flex_array_struct)
    + sizeof(int) * array_size
);
  if (structPstruct_p == NULL) {
    /* Handle mallocerror failure */
  }

structP  struct_p->num = array_size;

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

  print_array(*structPstruct_p);
}

Because C passes the argument is passed by value, the structure is copied onto the stack. 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.structure is passed by reference and not by value:

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

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

/* initialize array_size */

/* spaceSpace is allocated for the struct */
structP = (struct flexArrayStruct *)malloc(
  sizeof(struct flexArrayStruct) + sizeof(int) * array_size
);
if (structP == NULL) {
  /* Handle malloc failure */
}

structP->num = array_size;

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

print_array(structP);
and initialized... */

  print_array(struct_p);
}

Risk Assessment

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

Automated Detection

...

Flexible array structs should not be

• declared on the stack, they should be on the heap.
• copied via assignment, they should be copied using memcpy() or a similar function.
• passed as raw arguments to functions, a pointer should be passed to flexArrayStruct intead.

ROSE can detect all of these.

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.

References

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Related Guidelines

Key here (explains table format and definitions)

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

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Image Added Image Added |http://www.open-std.org/jtc1/sc22/wg14/www/docs/n791.htm]\]MEM32-C. Detect and handle memory allocation errors      08. Memory Management (MEM)      Image Modified