Why is the size of the structure different from the total size of the member variable?

Looking at it, the structure size is larger than the combined size of the member variables. Why is it like this?

2022-09-22 12:01

1 Answers

A typical 32-bit (8-byte) processor cannot be accessed as large as two or four bytes.

struct {
    char a[3];
    short int b;
    long int c;
    char d[3];
};

You may think that memory is held as shown below (12 bytes):

+-------+-------+-------+-------+
|           |           a           |   b   |
+-------+-------+-------+-------+
|   |   b   |           c           |
+-------+-------+-------+-------+
|   |   c   |           d           |
+-------+-------+-------+-------+

The processor is easier to approach (12 bytes):

+-------+-------+-------+
|           |           a           |
+-------+-------+-------+
|       |       b       |
+-------+-------+-------+-------+
|               |               c               |
+-------+-------+-------+-------+
|           |           d           |
+-------+-------+-------+

This isn't a good way for the processor either. Because a,b,d isn't full of eight bytes So most compilers fill in the remaining parts. This is called a pad. The results of pad appear as follows.

+-------+-------+-------+-------+
|           |           a           | pad1  |
+-------+-------+-------+-------+
|       |       b       |     pad2      |
+-------+-------+-------+-------+
|               |               c               |
+-------+-------+-------+-------+
|           |           d           | pad3  |
+-------+-------+-------+-------+

2022-09-22 12:01

If you have any answers or tips

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