What is byte alignment padding?

structure A short int is 2 byte aligned. If the the short int element is immediately allocated after the char element, it will start at an odd address boundary. The compiler will insert a padding byte after the char to ensure short int will have an address multiple of 2 (i.e. 2 byte aligned).

What is aligned memory?

Alignment refers to the arrangement of data in memory, and specifically deals with the issue of accessing data as proper units of information from main memory. Example: A 32bit memory that is byte addressable. Each row denotes a location with a fixed size of eight bits (1byte) labeled zero through seven.

What is a byte boundary?

Each byte is 8 bits, so to align on a 16 byte boundary, you need to align to each set of two bytes. Similarly, memory aligned on a 32 bit (4 byte) boundary would have a memory address that’s a multiple of four, because you group four bytes together to form a 32 bit word.

Why do we align data?

Alignment helps the CPU fetch data from memory in an efficient manner: less cache miss/flush, less bus transactions etc. Some memory types (e.g. RDRAM, DRAM etc.) need to be accessed in a structured manner (aligned “words” and in “burst transactions” i.e. many words at one time) in order to yield efficient results.

How many padding bytes are needed to align a structure?

The following formulas provide the number of padding bytes required to align the start of a data structure (where mod is the modulo operator): For example, the padding to add to offset 0x59d for a 4-byte aligned structure is 3. The structure will then start at 0x5a0, which is a multiple of 4.

When to insert a padding byte in structb _ T?

The compiler will insert a padding byte after the char to ensure short int will have an address multiple of 2 (i.e. 2 byte aligned). The total size of structa_t will be sizeof (char) + 1 (padding) + sizeof (short), 1 + 1 + 2 = 4 bytes. The first member of structb_t is short int followed by char.

How does structure padding work in a C program?

Explanation: “Char” data type takes only 1 byte after 3 byte padding (Char, Int and Double) then the number will starts at 4 bytes boundary and rest “Int” and “Double” will takes 4 and 8 bytes respectively. Below are the different eamples of Structure Padding in C.

How to minimize padding in structure member alignment?

By now, it may be clear that padding is unavoidable. There is a way to minimize padding. The programmer should declare the structure members in their increasing/decreasing order of size. An example is structd_t given in our code, whose size is 16 bytes in lieu of 24 bytes of structc_t. What is structure packing?