Extra bytes when declaring a member of a struct as uint32_t

Question

I have a problem when using the uint32_t type from the stdint.h library. If I run the following code (on Ubuntu linux 11.10 x86_64, g++ version 4.6.1):

#include          


        

Answer 1

Structs are padded to be an integer multiple of 4 bytes¹ so that they are word-aligned. http://en.wikipedia.org/wiki/Data_structure_alignment#Data_structure_padding

See also:

• Why isn't sizeof for a struct equal to the sum of sizeof of each member?
• Structure padding and packing
• Use of struct padding

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¹ As @Mooing Duck commented, this isn't always true:

It's not always a multiple of 4 bytes, it varies (slightly) depending on the members. On the other hand, 99% of the time it's a multiple of 4 bytes.