Page History

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 declares malloc() as

...

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

...

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 void MEM08-C. Use realloc() only to resize dynamically allocated arrays.)

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

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

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

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

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

/* 
 * 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)))

...

...