About structure size calculation in C language

The calculation of the size of the structure. On the internet, a lot of things are different. Alignment misalignment, offset, etc... Here is a brief summary of the following examples: Alignment Principle: The end offset of each member must be aligned to a multiple of the latter member typeThe principle of completing: Final size is a multiple of the maximum number of members take a question to do an example: "(start offset) + here byte = end offset】

1 structMyStruct2 {3     intI//(0) + 4 = 4, followed by 2, aligned4     CharC//(4) + 2 = 6, followed by a struct, directly open to see the first, int=4, so to arranges (4) + 4 = 85     structinnerstruct6     {7         intI//(8) + 4 = 12, next 4, already aligned (Learn Java note, 32-bit C language Long is 4, and Java is not the same)8         LongL//(12) + 4 = 16, next 8, already aligned9         DoubleD//(16) + 8 = 24, next 1, already alignedTen         CharC//(24) + 1 = 25, which is the last one, aligns it to a multiple of the maximum value of 8, so the completion is (24) + 8 = 32, the final result =32 One } innerstruct; A }; 

"Note" If Specify the justification byte, this designation is compared to the next one when aligned, and the smaller is aligned. At the time of completion, compared with the largest, also take the smaller one to complement. The code is as follows:

1 structA {2     Charb;3     intA;4      ShortC;5 };6 7 #pragmaPack (2)/* Specify 2-byte alignment */8 structB {9     Charb;Ten     intA; One      ShortC; A }; - #pragmaPack ()/* Cancels the specified alignment, restores the default alignment */ -  the  -  - #pragmaPack (1)/* Specify 1-byte alignment */ - structC { +     Charb; -     intA; +      ShortC; A }; at #pragmaPack ()/* Cancels the specified alignment, restores the default alignment */ -  - //calculate the occupied bytes -     ints1=sizeof(structA); -     intS2=sizeof(structB); -     ints3=sizeof(structC); in      -printf"%d\n", S1); toprintf"%d\n", S2); +printf"%d\n", S3);

Output: 12//By the next alignment, by the maximum 8//by 2 for alignment and completion of 7//by 1 to align and complement, the equivalent of directly calculate each size and then add

About structure size calculation in C language