Using 32-bit and 64-bit modes
You can use the XL C compiler to develop either 32-bit or 64-bit applications. To do so, specify -q32 or -q64, respectively, during compilation. Alternatively, you can set the OBJECT_MODE environment variable to 32 or 64 at compile time. If both OBJECT_MODE and -q32/-q64 are specified, -q32/-q64 takes precedence.
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 long
and pointer data type sizes and alignment between the two modes. The
following table summarizes these differences.
| Data type | 32-bit mode | 64-bit mode | ||
|---|---|---|---|---|
| Size | Alignment | Size | Alignment | |
| long, signed 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 (defined in the header file <cstddef>) | 4 bytes | 4-byte boundaries | 8 bytes | 8-byte boundaries |
| ptrdiff_t (defined in the header file <cstddef>) | 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, will occur when the program is executed.
For suggestions on improving performance in 64-bit mode, see Optimize operations in 64-bit mode.
