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Use Using type definitions (typedef) to can often improve code readability.

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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 this noncompliant code example 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 */
}

Noncompliant Code Example

In this noncompliant code example, the declaration of the signal() function is difficult to read and comprehend: declaration of the signal() function does not make use of typedef names and is consequently hard to read.

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

Compliant Solution

This compliant solution makes use of typedef names type definitions to specify exactly the same type as in the non-compliant noncompliant code example.:

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

Exceptions

Function pointer types are an exception to this recommendation. 

Risk Assessment

Code readability is important for discovering and eliminating vulnerabilities.

Automated Detection

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	pointer-typedef	Fully checked
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-DCL05
CodeSonar	Include Page CodeSonar_V CodeSonar_V	LANG.STRUCT.PIT	Pointer type inside typedef
Compass/ROSE
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C5004
LDRA tool suite	Include Page LDRA_V LDRA_V	299 S	Partially implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-DCL05-a	Declare a type of parameter as typedef to pointer to const if the pointer is not used to modify the addressed object
RuleChecker	Include Page RuleChecker_V RuleChecker_V	pointer-typedef	Fully checked

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.7, "Type definitions"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "BRS Leveraging human experience"

Related Guidelines

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Image Added Image Added Image AddedDCL04-A. Do not declare more than one variable per declaration      02. Declarations and Initialization (DCL)       DCL06-A. Use meaningful symbolic constants to represent literal values in program logic