Data Model

As stated previously, the biggest difference between the 32-bit environment and 64-bit environment is the change in two fundamental data types.

The C data-type model used for 32-bit applications is the ILP32 model, so named because ints, longs, and pointers are 32-bit. The LP64 data model is the C data-type model for 64-bit applications. This model was agreed upon by a consortium of companies across the industry. LP64 is so named because longs and pointers grow to 64-bit quantities. The remaining C types int, short, and char are the same as in the ILP32 model.

The following sample program, foo.c, directly illustrates the effect of the LP64 data model in contrast to the ILP32 data models. The same program can be compiled as either a 32–bit program or a 64–bit program.

#include <stdio.h>
int
main(int argc, char *argv[])
{
        (void) printf("char is \t\t%lu bytes\n", sizeof (char));
        (void) printf("short is \t%lu bytes\n", sizeof (short));
        (void) printf("int is \t\t%lu bytes\n", sizeof (int));
        (void) printf("long is \t\t%lu bytes\n", sizeof (long));
        (void) printf("long long is \t\t%lu bytes\n", sizeof (long long));
        (void) printf("pointer is \t%lu bytes\n", sizeof (void *));
        return (0);
} 

The result of 32–bit compilation is:

% cc -O -o foo32 foo.c
% foo32 
char is          1 bytes
short is         2 bytes
int is           4 bytes
long is          4 bytes
long long is     8 bytes
pointer is       4 bytes   

The result of 64–bit compilation is:

% cc -xarch=generic64 -O -o foo64 foo.c 
% foo64
char is          1 bytes
short is         2 bytes
int is           4 bytes
long is          8 bytes
long long is     8 bytes 
pointer is       8 bytes

The standard relationship between C integral types still holds true.

sizeof (char) <= sizeof (short) <= sizeof (int) <= sizeof (long)

Table 4-1 lists the basic C types, and their corresponding sizes in bits in the data type models for both LP32 and LP64.

Table 4-1 Data Type Sizes in Bits

Some older 32-bit applications use int, long, and pointer types interchangeably. The size of longs and pointers grow in the LP64 data model. You need to be aware that this change alone can cause many 32-bit to 64-bit conversion problems.

In addition, declarations and casts become very important in showing what is intended. How expressions are evaluated can be affected when the types change. The effects of standard C conversion rules are influenced by the change in data-type sizes. To adequately show what is intended, you might need to declare the types of constants. Casts might also be needed in expressions to make certain that the expression is evaluated the way that you intended. Correct evaluation of expressions is particularly crucial in the case of sign extension, where explicit casting might be essential to achieve the intended effect.

Other problems arise with built-in C operators, format strings, assembly language, and compatibility and interoperability.

The rest of this chapter advises you how to overcome these problems by:

• Explaining the problems outlined above in more detail

• Describing some of the derived types and include files that are useful to make code safe for both 32-bit and 64-bit

• Describing the tools available for helping to make code 64-bit safe

• Providing general rules for making code portable between the 32-bit and 64-bit environments