CertC-MEM02ΒΆ

Immediately cast the result of a memory allocation function call into a pointer to the allocated type

Required inputs: IR

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 *.  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 

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.

Consider the following example:

#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)); /* Imminent problem */
if (p != NULL) {
  p->i = 0;                 /* Undefined behavior */
  p->d = 0.0;               /* Undefined 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)); /* Imminent 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.

Casting the result of malloc() to the appropriate pointer type enables the compiler to catch subsequent inadvertent pointer conversions. When allocating individual objects, the "appropriate pointer type" is a pointer to the type argument in the sizeof expression passed to malloc().

In this code example, malloc() allocates space for a gadget, and the cast immediately converts the returned pointer to a gadget *: 

widget *p;

/* ... */

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

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

Compliant Solution (Hand Coded)

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

widget *p;

/* ... */

p = (widget *)malloc(sizeof(widget));
Compliant Solution (Macros)

Repeating the same type in the sizeof expression and the pointer cast is easy to do but still invites errors. Packaging the repetition in a macro, such as 

#define MALLOC(type) ((type *)malloc(sizeof(type)))

further reduces the possibility of error.

widget *p;

/* ... */

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

Here, the entire allocation expression (to the right of the assignment operator) allocates storage for a widget and returns a widget *. If p were not a widget *, the compiler would complain about the assignment.

When allocating an array with N elements of type T, the appropriate type in the cast expression is still T *, but the argument to malloc() should be of the form N * sizeof(T). Again, packaging this form as a macro, such as 

#define MALLOC_ARRAY(number, type) \
    ((type *)malloc((number) * sizeof(type)))

reduces the chance of error in an allocation expression.

enum { N = 16 };
widget *p;

/* ... */

p = MALLOC_ARRAY(N, widget);    /* 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 EXP39-C. Do not access a variable through a pointer of an incompatible type). 

/* 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)))

The following is an example:

enum month { Jan, Feb, /* ... */ };
typedef enum month month;

typedef struct date date;
struct date {
  unsigned char dd;
  month mm;
  unsigned yy;
};

typedef struct string string;
struct string {
  size_t length;
  char text[];
};

date *d, *week, *fortnight;
string *name;

d = MALLOC(date);
week = MALLOC_ARRAY(7, date);
name = MALLOC_FLEX(string, 16, char);
fortnight = CALLOC(14, date);

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 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() 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).

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++.

Recommendation Severity Likelihood Remediation Cost Priority Level
MEM02-C Low Unlikely Low P3 L3
Related Guidelines
SEI CERT C++ Coding Standard VOID MEM02-CPP. Immediately cast the result of a memory allocation function call into a pointer to the allocated type
Bibliography
[ Summit 2005] Question 7.7
Question 7.7b
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/recommendations/memory-management-mem/mem02-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

malloc_with_wrong_cast

Wrong cast on result of allocator, should cast to {} instead

None

False

malloc_without_cast

Missing cast to allocated type.

None

False

Options