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An object of type void * is a generic data pointer. It can point to any data object. For any incomplete or object type T, C permits implicit conversion from T * to void * or from void * to T *. The standard C library uses void * to declare parameters and return types of functions designed to work for objects of different types. Such is the case with the standard   C Standard memory allocation functions aligned_alloc(), malloc(), calloc(), and realloc() use void * to declare parameters and return types of functions designed to work for objects of different types.

For example, the C Library library declares malloc() as

void *malloc(size_t);

Calling malloc(s) allocates memory for an object whose size is s and returns either a null pointer or a pointer to the allocated memory. A program can implicitly convert the pointer that malloc() returns into a different pointer type.

Because objects returned by the C Standard memory allocation functions are implicitly converted into any object type, we recommend casting the results of these functions into a pointer of the allocated type because it increases the chances that the compiler will catch and diagnose a mismatch between the intended type of the object and the actual type of the object.

Noncompliant Code Example

The argument to malloc() can be any value of (unsigned) type size_t. If the program uses the allocated storage to represent an object (possibly an array) whose size is greater than the requested size, the behavior is undefined. The implicit pointer conversion lets this slip by without complaint from the compiler.

...

#include <stdlib.h>

typedef struct gadget gadget;
struct gadget {
  int i;
  double d;
};

typedef struct widget widget;
struct widget {
  char c[10];
  int i;
  double d;
};

widget *p;

/* ... */

p = malloc(sizeof(gadget)); /* imminentImminent problem */
if (p != NULL) {
  p->i = 0;                 /* undefinedUndefined behavior */
  p->d = 0.0;               /* undefinedUndefined behavior */
}

An implementation may add padding to a gadget or widget so that sizeof(gadget) equals sizeof(widget), but this is highly unlikely. More likely, sizeof(gadget) is less than sizeof(widget). In that case,

p = malloc(sizeof(gadget)); /* imminentImminent problem */

quietly assigns p to point to storage too small for a widget. The subsequent assignments to p->i and p->d will most likely produce memory overruns.

...

widget *p;

/* ... */

p = (gadget *)malloc(sizeof(gadget)); /* invalidInvalid assignment */

This lets the compiler detect the invalid assignment because it attempts to convert a gadget * into a widget *.

Compliant Solution (Hand

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Coded)

This compliant solution repeats the same type in the sizeof expression and the pointer cast:

...

widget *p;

/* ... */

p = MALLOC(widget);     /* OK */
if (p != NULL) {
  p->i = 0;           /* OK */
  p->d = 0.0;         /* OK */
}

...

A small collection of macros can provide secure implementations for common uses for the standard memory allocation functions. The omission of a REALLOC() macro is intentional . (See MEM08see EXP39-C. Use realloc() only to resize dynamically allocated arrays.)Do not access a variable through a pointer of an incompatible type).

/* allocatesAllocates a single object using malloc(). */
#define MALLOC(type) ((type *)malloc(sizeof(type)))

/* allocatesAllocates an array of objects using malloc(). */
#define MALLOC_ARRAY(number, type) \
    ((type *)malloc((number) * sizeof(type)))

/* allocates
 * Allocates a single object with a flexible
 * array member using malloc(). 
 */
#define MALLOC_FLEX(stype, number, etype) \
    ((stype *)malloc(sizeof(stype) \
    + (number) * sizeof(etype)))

/* allocatesAllocates an array of objects using calloc(). */
#define CALLOC(number, type) \
    ((type *)calloc(number, sizeof(type)))

/* reallocatesReallocates an array of objects using realloc(). */
#define REALLOC_ARRAY(pointer, number, type) \
    ((type *)realloc(pointer, (number) * sizeof(type)))

/* reallocates
 * Reallocates a single object with a flexible
 * array member using realloc(). 
 */
#define REALLOC_FLEX(pointer, stype, number, etype) \
    ((stype *)realloc(pointer, sizeof(stype) \
    + (number) * sizeof(etype)))

...

If one or more of the operands to the multiplication operations used in many of these macro definitions can be influenced by untrusted data, these operands should be checked for overflow before the macro is invoked . (See see INT32-C. Ensure that operations on signed integers do not result in overflow).)

The use of type-generic function-like macros is an allowed exception (PRE00-C-EX4) to PRE00-C. Prefer inline or static functions to function-like macros.

Exceptions

MEM02-C-EX1: Do not immediately cast the results of malloc() when compiling as C90 or earlier because for code that will be compiled using a C90-conforming compiler because it is possible for the cast to hide a bug in platforms where pointers are larger than ints.  

For example, if stdlib.h is not properly included, compilers will implicitly declare malloc() as int malloc(int).  If the size of int on the platform is 32 bits, but the size of a pointer is 64 bits, the resulting pointer could be truncated due to the implicit declaration of malloc() returning a 32-bit integer.

/* #include <stdlib.h> is missing */
 
int main() {
  int i;
  for (i = 0; i < 100; ++i) {
    char *ptr = (char*)malloc(0x10000000); /* int malloc() assumed */
    *ptr = 'a';
  }
  return 0;
}

When compiled with Microsoft Visual Studio, the above code will eventually cause an access violation when dereferencing ptr in the loop.  Note that this code also does not comply with more critical defect (see DCL31-C. Declare identifiers before using them by using an implicit declaration of malloc( for a code example that uses malloc() without first declaring it).

Risk Assessment

Failing to cast the result of a memory allocation function call into a pointer to the allocated type can result in inadvertent pointer conversions. Code that follows this recommendation will compile and execute equally well in C++.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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