Appendix: Advanced C Data Types

Pointers

To construct linked data structures

To reference dynamically allocated data structures

To implement call by reference

To access data elements and iterate through them

When passing arrays as arguments

For referring to functions

As an alias for another value

To represent character strings

For direct access to system memory

Structures

To group together data elements typically used as a whole

To return multiple data elements from a function

To construct linked data structures

To map the organization of data on

hardware devices
network links
storage media

(This use is non-portable)

To implement abstract data types

To program in an object-oriented fashion

Dynamic Memory Allocation

Ensure memory allocated with malloc is always freed

Pointers do NOT have to initialized with malloced memory

When objects are shared memory is often managed with a use count and a garbage collector

alloca is sometimes used to allocate dynamic data on the stack (non-portable)

Structures with a variable-sized array element are sometimes allocated in a single block

typedef Declarations

A typedef declaration creates a new name for an already existing type.

typedef unsigned char cc_t;

Read typedefs as variable declarations

typedef char ut_line_t[UT_LINESIZE];

Used for:

Creating portable names for hardware quantities:
typedef ___signed char      int8_t;
typedef unsigned char    u_int8_t;
typedef short             int16_t;
typedef unsigned short  u_int16_t;
typedef int               int32_t;
typedef unsigned int    u_int32_t;
typedef long              int64_t;
typedef unsigned long   u_int64_t;
Creating abstract names for known hardware representatrions:
typedef u_int32_t in_addr_t;
typedef u_int16_t in_port_t;
Emulating C++/Java class definitions:
typedef struct path path;
struct path {
/* [...] */
}

Further Reading

Hans-Juergen Boehm. Garbage collection in an uncooperative environment. Software: Practice & Experience, 18(9):807–820, September 1988.

Maurice Bruynooghe. The memory management of Prolog implementations. In Keith L. Clark and Sten-Åke Tärnlund, editors, Logic Programming, pages 83–98. Academic Press, London, 1982.

D. R. Chase, W. Wegman, and F. K. Zadeck. Analysis of pointers and structures. ACM SIGPLAN Notices, 25(6):296–319, June 1990.

Thomas W. Christopher. Reference count garbage collection. Software: Practice & Experience, 14(6):503–507, June 1984.

David Detlefs, Al Dosser, and Benjamin Zorn. Memory allocation costs in large C and C++ programs. Software: Practice & Experience, 24(6):527–542, June 1994.

Christopher W. Fraser and David R. Hanson. Exploiting machine-specific pointer operations in abstract machines. Software: Practice & Experience, 12:367–373, 1982.

C. A. R. Hoare. Recursive data structures. Technical Report AIM-223 STAN-CS-73-400, Stanford University, Computer Science Department, October 1973.

Brian W. Kernighan and Dennis M. Ritchie. The C Programming Language. Prentice-Hall, Englewood Cliffs, NJ, second edition, 1988.

Kai Li and Paul Hudak. A new list compaction method. Software: Practice & Experience, 16(2):145–163, February 1986.

Ravi Sethi and Tom Stone. Programming Languages: Concepts and Constructs. Addison-Wesley, second edition, 1996.

Diomidis Spinellis. Code Reading: The Open Source Perspective, pages 61–93. Effective Software Development Series. Addison-Wesley, Boston, MA, 2003.

Norihisa Suzuki. Analysis of pointer ``rotation''. Communications of the ACM, 25(5):330–335, May 1982.

Exercises and Discussion Topics

If you are familiar with C++ or Java explain how it is possible to minimize (in C++) or avoid (in Java) the use of pointers.

How does a C pointer differ from a memory address? How does that affect your understanding of code? Which tools take advantage of the difference?

How can a program using structures to specify a memory layout ensure that they are correctly allocated?

How would you translate a program using unions into Java?