DCL12-C. Implement abstract data types using opaque types

Abstract data types are not restricted to object-oriented languages such as C++ and Java. They should be created and used in C language programs as well. Abstract data types are most effective when used with private (opaque) data types and information hiding.

Noncompliant Code Example

This noncompliant code example is based on the managed string library developed by CERT [Burch 2006]. In this example, the managed string type and the functions that operate on this type are defined in the string_m.h header file as follows:

struct string_mx {
  size_t size;
  size_t maxsize;
  unsigned char strtype;
  char *cstr;
};

typedef struct string_mx string_mx;

/* Function declarations */
extern errno_t strcpy_m(string_mx *s1, const string_mx *s2);
extern errno_t strcat_m(string_mx *s1, const string_mx *s2);
/* ... */

The implementation of the string_mx type is fully visible to the user of the data type after including the string_m.h file. Programmers are consequently more likely to directly manipulate the fields within the structure, violating the software engineering principles of information hiding and data encapsulation and increasing the probability of developing incorrect or nonportable code.

Compliant Solution

This compliant solution reimplements the string_mx type as a private type, hiding the implementation of the data type from the user of the managed string library. To accomplish this, the developer of the private data type creates two header files: an external string_m.h header file that is included by the user of the data type and an internal file that is included only in files that implement the managed string abstract data type.

In the external string_m.h file, the string_mx type is defined to be an instance of struct string_mx, which in turn is declared as an incomplete type:

struct string_mx;
typedef struct string_mx string_mx;

/* Function declarations */
extern errno_t strcpy_m(string_mx *s1, const string_mx *s2);
extern errno_t strcat_m(string_mx *s1, const string_mx *s2);
/* ... */

In the internal header file, struct string_mx is fully defined but not visible to a user of the data abstraction:

struct string_mx {
  size_t size;
  size_t maxsize;
  unsigned char strtype;
  char *cstr;
};

Modules that implement the abstract data type include both the external and internal definitions, whereas users of the data abstraction include only the external string_m.h file. This allows the implementation of the string_mx data type to remain private.

Risk Assessment

The use of opaque abstract data types, though not essential to secure programming, can significantly reduce the number of defects and vulnerabilities introduced in code, particularly during ongoing maintenance.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography