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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.

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

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

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Code Block

bgColor	#ccccff
lang	c

#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 structure.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;
  }
}

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Code Block

bgColor	#ccccff
lang	c

#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; Handlehandle error */
    return;
  }
  memcpy(structB, structA,
         sizeof(struct flexArrayStruct) + (sizeof(int)
           * structA->num));
}

...

Code Block

bgColor	#FFcccc
lang	c

#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);
}

...

Code Block

bgColor	#ccccff
lang	c

#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);
}

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Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MEM33-C	lowLow	unlikelyUnlikely	lowLow	P3	L3

Automated Detection

Tool	Version	Checker	Description
Compass/ROSE			Can detect all of these

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[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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