Question

#ifndef EIGHT_BIT
#define THIRTYTWO_BIT // default 32 bit
#endif

#ifdef THIRTYTWO_BIT
#define WORD unsigned long
#define WORDLENGTH 4

#if defined(WIN32) && !defined(__GNUC__)
#define WORD64  unsigned __int64
#else
#define WORD64  unsigned long long
#endif

// THIRTYTWO_BIT
#endif


#ifdef EIGHT_BIT

#define WORD unsigned short
#define WORDLENGTH 4

// EIGHT_BIT
#endif

Answer 1

The first thing to note about this code is that none of it will actually be compiled into C. Every line that isn’t whitespace or a comment starts with a pound sign (#), meaning they are preprocessor directives. A preprocessor directive alters the code before it even makes it to the compiler. For more information of preprocessor directives, see this article.

Now that we know that much, let’s look through the code:

---

#ifndef EIGHT_BIT
#define THIRTYTWO_BIT // default 32 bit
#endif

If the macro EIGHT_BIT is not defined, define another macro called THIRTYTWO_BIT. This is most likely referring to the number of bits in a word on a processor. This code intends to be cross-platform, meaning that it can run on a number of processors. The snippet you posted pertains to managing different word widths.

---

#ifdef THIRTYTWO_BIT
#define WORD unsigned long
#define WORDLENGTH 4

If the macro THIRTYTWO_BIT is defined, then define a WORD to be an unsigned long, which has a WORDLENGTH of 4 (presumably bytes). Note that this statement isn’t necessarily true, as the C standard only guarantees that a long will be as least as long as an int.

---

#if defined(WIN32) && !defined(__GNUC__)
#define WORD64  unsigned __int64
#else
#define WORD64  unsigned long long
#endif

If this is a 32-bit Windows platform and the GNU C compiler is not available, then use the Microsoft-specific datatype for 64-bit words (unsigned __int64). Otherwise, use the GNU C datatype (unsigned long long).

---

// THIRTYTWO_BIT
#endif

Every #if and #ifdef directive must be matched by a corresponding #endif to delineate where the conditional section ends. This line ends the #ifdef THIRTYTWO_BIT declaration made previously.

---

#ifdef EIGHT_BIT

#define WORD unsigned short
#define WORDLENGTH 4

// EIGHT_BIT
#endif

If the target processor has a word width of 8 bits, then define a WORD to be an unsigned short, and define the WORDLENGTH to be 4 (again, presumably in bytes).