How many bytes at maximum can be saved by reordering the fields of this struct?

We can save 4 bytes or one third of the total size by simply reordering the members from large to small. That might not sound like much, but if you have an array with a few thousand of these structs, the savings quickly add up. You can validate the struct size yourself with the sizeof operator.

Can compiler reorder struct members?

Since the rules are fixed in the language, the compiler is able to figure out how the members were reordered, and react accordingly. As mentioned above, it will always be possible to prevent reordering in the cases where you want complete control.

How do you prevent struct 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.

Can we rearrange the members of structure to reduce padding?

struct widget{ int m2; char m1; char m3;}; also reduces the number of padding bytes to two and sizeof(widget) to eight. A Standard C compiler won’t rearrange the members of a structure automatically to reduce the structure’s size.

How do you avoid structure padding in C using pragma?

A structure padding in C increases the performance of the processor at the penalty of memory. If you want you can avoid the structure padding in C using the pragma pack (#pragma pack(1) ) or attribute ( __attribute__((__packed__)) ).

Why is structure padding done in C?

In order to align the data in memory, one or more empty bytes (addresses) are inserted (or left empty) between memory addresses which are allocated for other structure members while memory allocation. This concept is called structure padding.

Why does the compiler sometimes insert padding between fields?

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).

Does order matter in a struct?

The order of fields in a struct does matter – the compiler is not allowed to reorder fields, so the size of the struct may change as the result of adding some padding.

Why do struct types in C sometimes contain padding?

What is structure padding structure alignment reason for structure padding?

Padding aligns structure members to “natural” address boundaries – say, int members would have offsets, which are mod(4) == 0 on 32-bit platform. Padding is on by default.

Why padding is done?

The primary use of padding with classical ciphers is to prevent the cryptanalyst from using that predictability to find known plaintext that aids in breaking the encryption. Random length padding also prevents an attacker from knowing the exact length of the plaintext message.

How does padding work in C?

Structure Padding in C: 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.

How much padding is required between int and char in C?

Since char can be on any byte boundary no padding required in between short int and char, on total they occupy 3 bytes. The next member is int. If the int is allocated immediately, it will start at an odd byte boundary.

Is it possible to prevent reordering of structs?

As mentioned above, it will always be possible to prevent reordering in the cases where you want complete control. Also, requirement 2 ensures that type-safe code will never break. The reason I think such a rule could make sense is because I find it more natural to group struct members by their contents than by their types.

Why do compiler rules need to be fixed in the language?

Since the rules are fixed in the language, the compiler is able to figure out how the members were reordered, and react accordingly. As mentioned above, it will always be possible to prevent reordering in the cases where you want complete control. Also, requirement 2 ensures that type-safe code will never break.

What is the alignment requirement for C++ data type?

Every data type in C/C++ will have alignment requirement (infact it is mandated by processor architecture, not by language). A processor will have processing word length as that of data bus size. On a 32 bit machine, the processing word size will be 4 bytes.