C StructuresLesson 41Free preview

C Struct Padding

This lesson is part of C · All levels

Summary

The compiler inserts byte padding between the struct members so that each member is at an address that aligns with their needs. As a result the size of a struct is often greater than the sum of the sizes of the entire member. Padding is important to understand when data is stored in a file, sent over a network, or used together by another programming language.

Key points

Alignment means that a type is usually located at its own size multiple, e.g. int at a 4 byte multiple.
Sizeof returns the struct size including all the padding, even the padding at the end so that the array elements stay in alignment.
Member order affects size; group the same sized type to minimize padding.
Pack can force assembly layouts, but can lower data access performance. (#pragma)
For cross-platform data, serialize field per field, don't write raw struct directly to the file or network.

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