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Using type definitions (typedef) can often improve code readability. However, type definitions to pointer types can make it more difficult to write const-correct code because the const qualifier will be applied to the pointer type, not to the underlying declared type.

Noncompliant Code Example

The following type definition improves readability at the expense of introducing a const-correctness issue. In this example, the const qualifier applies to the typedef instead of to the underlying object type. Consequently, func does not take a pointer to a const struct obj but instead takes a const pointer to a struct obj.

struct obj {
  int i;
  float f;
};
typedef struct obj *ObjectPtr;
 
void func(const ObjectPtr o) {
  /* Can actually modify o's contents, against expectations */
}

Compliant Solution

This compliant solution makes use of type definitions but does not declare a pointer type and so cannot be used in a const-incorrect manner:

struct obj {
  int i;
  float f;
};
typedef struct obj Object;
 
void func(const Object *o) {
  /* Cannot modify o's contents */
}

Noncompliant Code Example (Windows)

The Win32 SDK headers make use of type definitions for most of the types involved in Win32 APIs, but the following noncompliant solution demonstrates a const-correctness bug:

#include <Windows.h>
/* typedef char *LPSTR; */
 
void func(const LPSTR str) {
  /* Can mutate str's contents, against expectations */
}

Compliant Solution (Windows)

This compliant solution demonstrates a common naming convention found in the Win32 APIs, using the proper const type:

#include <Windows.h>
/* typedef const char *LPCSTR; */
 
void func(LPCSTR str) {
  /* Cannot modify str's contents */
}

Noncompliant Code Example (Windows) 

Note that many structures in the Win32 API are declared with pointer type definitions but not pointer-to-const  type definitions (LPPOINT, LPSIZE, and others). In these cases, it is suggested that you create your own type definition from the base structure type.

#include <Windows.h>
/*
  typedef struct tagPOINT {
    long x, y;
  } POINT, *LPPOINT;
*/
 
void func(const LPPOINT pt) {
  /* Can modify pt's contents, against expectations */
}

Compliant Solution (Windows)

#include <Windows.h>
/*
  typedef struct tagPOINT {
    long x, y;
  } POINT, *LPPOINT;
*/
 
typedef const POINT *LPCPOINT;
void func(LPCPOINT pt) {
  /* Cannot modify pt's contents */
}

Exceptions

Function pointer types are an exception to this recommendation. 

Noncompliant Code Example

The following declaration of the signal() function is difficult to read and comprehend:

void (*signal(int, void (*)(int)))(int);

Compliant Solution

This compliant solution makes use of type definitions to specify the same type as in the noncompliant code example:

typedef void SighandlerType(int signum);
extern SighandlerType *signal(
  int signum,
  SighandlerType *handler
);

Risk Assessment

Code readability is important for discovering and eliminating vulnerabilities.

Automated Detection

Related Vulnerabilities

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

Related Guidelines