Using 32-bit and 64-bit modes

You can use XL C/C++ to develop both 32-bit and 64-bit applications. To do so, specify -q32 (the default) or -q64, respectively, during compilation. Alternatively, you can set the OBJECT_MODE environment variable to 32 or 64.

However, porting existing applications from 32-bit to 64-bit mode can lead to a number of problems, mostly related to the differences in C/C++ long and pointer data type sizes and alignment between the two modes. The following table summarizes these differences.

Table 2. Size and alignment of data types in 32-bit and 64-bit modes
Data type 32-bit mode 64-bit mode
Size Alignment Size Alignment
long, unsigned long 4 bytes 4-byte boundaries 8 bytes 8-byte boundaries
pointer 4 bytes 4-byte boundaries 8 bytes 8-byte boundaries
size_t (system-defined unsigned long) 4 bytes 4-byte boundaries 8 bytes 8-byte boundaries
ptrdiff_t (system-defined long) 4 bytes 4-byte boundaries 8 bytes 8-byte boundaries

The following sections discuss some of the common pitfalls implied by these differences, as well as recommended programming practices to help you avoid most of these issues:

When compiling in 32-bit or 64-bit mode, you can use the -qwarn64 option to help diagnose some issues related to porting applications. In either mode, the compiler immediately issues a warning if undesirable results, such as truncation or data loss, have occurred.

For suggestions on improving performance in 64-bit mode, see Optimize operations in 64-bit mode.