C struct padding

1. What is struct padding in C?
2. How to avoid structure padding in C?
3. Why padding is needed in struct?
4. How is structure padding done?

What is struct padding in C?

Structure padding is defined as the process of adding one or more empty bytes between the different data types to align data in memory. Structure padding increases memory consumption but is reduces CPU cycles. Structure contains structure members which can be accessed by a processor in chunks of 4 bytes at a time.

How to avoid structure padding in C?

In Structure, sometimes the size of the structure is more than the size of all structures members because of structure padding. Note: But what actual size of all structure member is 13 Bytes. So here total 3 bytes are wasted. So, to avoid structure padding we can use pragma pack as well as an attribute.

Why padding is needed in struct?

The answer to that lies in how a CPU accesses memory. Typically a CPU has alignment constraints, e.g. a CPU will access one word at a time, or a CPU will require data to be 16byte aligned, etc. So to make sure that data is aligned according to the constraints of the CPU, padding is required.

How is structure padding done?

The structure padding is automatically done by the compiler to make sure all its members are byte aligned. Here 'char' is only 1 byte but after 3 byte padding, the number starts at 4 byte boundary. For 'int' and 'double', it takes up 4 and 8 bytes respectively.