an implement of sizeof guaranteeing bit alignment

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#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )

The above macro simply aligns the size of n to the nearest greater-or-equal sizeof(int) boundary.

The basic algorithm for aligning value a to the nearest greater-or-equal arbitrary boundary b is to

1. Divide a by b rounding up, and then
2. Multiply the quotient by b again.

In the domain of unsigned (or just positive) values the first step is achieved by the following popular trick

q = (a + b - 1) / b
// where `/` is ordinary C-style integer division (rounding down)
// Now `q` is `a` divided by `b` rounded up

Combining this with the second step we get the following

aligned_a = (a + b - 1) / b * b

In aligned_a you get the desired aligned value.

Applying this algorithm to the problem at hand one would arrive at the following implementation of _INTSIZEOF macro

#define _INTSIZEOF(n)\
  ( (sizeof(n) + sizeof(int) - 1) / sizeof(int) * sizeof(int) )

This is already good enough.

However, if you know in advance that the alignment boundary is a power of 2, you can "optimize" the calculations by replacing the divide+multiply sequence with a simple bitwise operation

aligned_a = (a + b - 1) & ~(b - 1)

That is exactly what‘s done in the above original implementation of _INTSIZEOF macro.

This "optimization" might probably make sense with some compilers (although I would expect a modern compiler to be able to figure it out by itself). However, considering that the above _INTSIZEOF(n)macro is apparently intended to serve as a compile-time expression (it does not depend on any run-time values, barring VLA objects/types passed as n), there‘s not much point in optimizing it that way.

origin: c - an implement of sizeof guaranteeing bit alignment - Stack Overflow

material: 内存对齐#define _INTSIZEOF(n) ((sizeof(n)+sizeof(int)-1)&~(sizeof(int) - 1) )

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an implement of sizeof guaranteeing bit alignment

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原文地址：http://www.cnblogs.com/liyou-blog/p/3913081.html