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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 memory allocation functions aligned_alloc(), malloc(), calloc(), and realloc().

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

void *malloc(size_t);

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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.

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#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.

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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:

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widget *p;

/* ... */

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

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/* 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)))

/* 
 * allocatesAllocates 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)))

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MEM02-EX1: Do not immediately cast the results of malloc() for code that will be compiled using a C90-conforming compiler because it is possible for the cast to hide a more critical defect.   See DCL31-C. Declare identifiers before using them for a code example that uses malloc() without first declaring it.

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Automated Detection

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