Padding and packing

[Himanshu40]

Must read for bmp.h https://www.geeksforgeeks.org/structure-member-alignment-padding-and-data-packing/ @sabyasachi07 @chandrakant100 @HembramBeta777 @Ank1taS @Tushar7-coder

[Ank1taS]

__attribute__((__packed__)) in bmp.h [line 34 and 58] is used to avoid padded bytes among the attributes of structure.

Padding aligns structure members with default address boundaries by the processor whereas packing prevents the processor from doing padding. Padding makes the size of the struct bigger by applying some rules of memory alignment whereas packing provides the actual size of the struct to which each member is rounded off.
You can refer to this for more.

• Rule: Every member of the struct should be at an address divisible by its size.

EXAMPLE 1

struct structure_a {
    long l;
    int i;
    char c;
};

Member	Address	Is address multiple of size
long	0 -7	yes (8 * 0)
int	8 - 11	yes (4 * 2)
char	12	yes (1 * 12)

• with padding size of structure_a = 16, [8 + 4 + 1, then round to multiple of 8 (long's size)]
• with packing size of structure_a = 13

EXAMPLE 2

struct structure_b {
    int i;
    long l;
    char c;
};

Member	Address	Is address multiple of size
int	0-3	yes (4 * 0)
pad	4-7
long	8-15	yes (8 * 1)
char	16	yes (1 * 16)

• with padding size of structure_b = 24, [long need padding by 4 before it, then round to multiple of 8 (long's size)]
• with packing size of structure_b = 13

[Himanshu40]

Reference for #8. @chandrakant100 @sabyasachi07 @HembramBeta777 @Tushar7-coder

[chandrakant100]

done