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64-Bit Considerations

With Macintosh computers using 64-bit PowerPC and Intel processors, developers can begin writing software to take advantage of the 64-bit architecture provided by these chips. For many developers, however, compiling their code into 64-bit programs may not offer any inherent advantages. Unless your program needs more than 4 GB of addressable memory, supporting 64-bit pointers may only reduce the performance of your application.

When you compile a program for a 64-bit architecture, the compiler doubles the size of all pointer variables. This increased pointer size makes it possible to address more than 4 GB of memory, but it also increases the memory footprint of your application. If your application does not take advantage of the expanded memory limits, it may be better left as a 32-bit program.

Regardless of whether your program is currently 32-bit or 64-bit, there are some guidelines you should follow to make your code more interoperable with other programs. Even if you don’t plan to implement 64-bit support soon, you may need to communicate with 64-bit applications. Unless you are explicit about the data you exchange, you may run into problems. The following guidelines are good to observe regardless of your 64-bit plans.

• Avoid casting pointers to anything but a pointer. Casting a pointer to a scalar value has different results for 32-bit and 64-bit programs. These differences could be enough to break your code later or cause problems when your program exchanges data with other programs.

• Be careful not to make assumptions about the size of pointers or other scalar data types. If you want to know the size of a type, always use the sizeof (or equivalent) operator.

• If you write integer values to a file, make sure your file format specifies the exact size of the value. For example, rather than assume the generic type int is 32 bits, use the more explicit types SInt32 or int32_t, which are guaranteed to be the correct size.

• If you exchange integer data with other applications across a network, make sure you specify the exact size of the integer.

There are several documents to help you create 64-bit applications. For general information about making the transition, see 64-Bit Transition Guide. For Cocoa-specific information, see 64-Bit Transition Guide for Cocoa. For Carbon-specific information, see 64-Bit Guide for Carbon Developers.

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