Apple Darwin Manuel d'utilisateur télécharger pdf (Page 24)

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If you want the mask value to contain zeros in the upper 32 bits on a 64-bit architecture, the usual

fixed-width mask will work as expected, because it will be extended in an unsigned fashion to a 64-bit

quantity.

●

If you want the mask value to contain ones in the upper bits, however, you should write the mask as the

bitwise inverse of its inverse, as shown in Listing 3-5.

Listing 3-5 Using an inverted mask for sign extension

function_name(long value)

{

long mask = ~0x3; // 0xfffffffc or 0xfffffffffffffffc

return (value & mask);

}

In the code, note that the upper bits in the mask are filled with ones in the 64-bit case.

Tools Tips

Here are some tips to help you use the compiler more effectively in transitioning your code to 64-bit:

●

If data is being inadvertently truncated, to help you find the source, try turning on additional compiler

warnings.

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In 64-bit-capable versions of GCC (4.0 and later), the size of a long double will be 128 bits instead of 64

bits. This change is not limited to code compiled as a 64-bit executable, but it is a toolchain change you

should be aware of.

You can find detailed tips and information about 64-bit tools changes in Compiling 64-Bit Code (page 31).

Alignment Pragmas

Occasionally, developers use alignment pragmas to change the way that data structures are laid out in memory.

They usually do this for backward compatibility. In many cases, Apple added pragmas to maintain data structure

compatibility between 68K-based and PowerPC-based code running on the same machine under Mac OS 9

and earlier. OS X retained these alignment overrides to maintain binary compatibility with existing Carbon

data structures between Mac OS 9 and OS X.

Making Code 64-Bit Clean

Tools Tips

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Apple Darwin Manuel d'utilisateur Page 24