Sizeof keyword

1.sizeof It's a keyword, not a function , It is 32 One of the keywords .
2.sizeof When calculating the space occupied by variables , Brackets can be omitted , Calculation type ( Mold ) Size cannot be omitted . In general , Let's not steal this lazy , Write brackets obediently .
3.sizeof The returned footprint is the size opened up for this variable , Not the space it uses . So when it comes to structures , In most cases, we have to consider the problem of byte alignment .
4.sizeof The data type returned is unsigned int; An unsigned number and a signed number operation , Most compilers convert to unsigned type operations .
5. Pay attention to the difference between array names and pointer variables . Usually , We always think that array names are similar to pointer variables , But in use sizeof It's very different when , Use... For array names sizeof Returns the size of the entire array , When operating on a pointer variable, the return is the space occupied by the pointer variable itself , stay 32 Under the condition of bit machine, it is generally 4 byte . And when the array name is used as a function parameter , Inside the function , A formal parameter is just a pointer , So no longer return the size of the array .

sizeof（int）*p What do you mean ？
answer ： For calculation int Number of bytes occupied by type , And then multiply by p.

32 A system. ：
The pointer size is determined by the current CPU The addressing bits of the operation mode determine .Intel 32 Bit processor 32 Bit operation mode , Logical addressing bits 32, The pointer is 32 position , namely 4 Bytes .Intel 32 Bit processor 16 Bit virtual machine operation mode , Logical addressing bits 16, The pointer is 16 position , namely 2 Bytes .
int *p = NULL;
sizeof( p ) What's the value of ？
answer ： Pointer address is used DWORD, sizeof( p ) yes 4 byte
sizeof(*p) Well ？
answer ： be equal to sizeof(int) , Probably yes 2 byte It could be 4 byte
(DWORD Namely Double Word, Every word by 2 The length of bytes ,DWORD Double word is 4 Bytes , Each byte is 8 position , common 32 position .)

int a[100];
sizeof (a) What's the value of ？
answer ：sizeof(a) = sizeof(int) * 100 = 4 * 100 = 400 byte
sizeof(a[99]) Well ？
answer ： Please 100 The size of an element , use int Deposit , Occupy 4 Bytes
sizeof(&a) Well ？
answer ： Means to store a The size of the address space and memory address , namely &a It's also an address value . An address is used int Storage is also 4 Bytes
sizeof(&a[0]) Well ？
answer ： Take the address of the first element , use int Deposit , Occupy 4 Bytes

int b[100];
void fun(int b[100])
{
    
	sizeof(b);
}

sizeof (b) What's the value of ？
answer ：fun In the parameter list b It's a pointer , stay 32 Bit system , The pointer is always 4 Bytes .