ANSI C Type qualifier

1.const Type qualifier

​ With const Keyword declared object , Its value cannot be assigned or incremented 、 Decrement to modify .

Especially when using pointers, you need to pay attention to several points ：

const float * pf;//pf Point to one float Type of const value 

The above creation means pf The value on the address cannot be changed , and pf It can be changed .

float * const pt;//pt It's a const The pointer 

The above represents pt Must point to the same address , But the value it points to can be changed .

const float * const ptr;

The above code indicates ptr Can't point anywhere , The value it points to cannot be changed .

float const * pfc;// And const float * pfc identical 

The meaning of the above code is the same as that of the first sentence .

const int array[] And const int * array Both of them mean that they cannot be changed array Point to data .

2.volatile Type qualifier

​ volatile The qualifier represents a proxy ( Not the program where the variable is located ) You can change the value of this variable . It is usually used for hardware addresses and sharing data in threads run by other programs or colleagues .

Grammar and const equally ：

volatile int loc1;//loc1 It's a volatile position 
volatile int * ploc;//ploc Is a pointer to a variable position 

Use volatile The reason for the keyword is that the compiler will temporarily store the value of a variable in a register , Read it from the register only when necessary ( Instead of reading it from the location of the original memory ), To save time , This is called cache . But if there is a numerical change between two agents, the compiler cannot recognize , To do this, you need to use volatile Keyword to ensure whether the value of the variable changes .

3.restrict Type qualifier

​ restrict Keyword allows the compiler to optimize a part of the code to better support computation . It can only be used for pointers , Indicates that the pointer is the only and initial way to access the data object .

If not used restrict keyword , The compiler must assume the worst case ( That is, other identifiers may have changed the data between the two uses of the pointer ). If used restrict Keyword can choose shortcut optimization calculation .

​ restrict Qualifiers can also be used for pointers in function parameters . Indicates that the compiler can assume that its identifier in the function body will not modify the data pointed to by the pointer , And the compiler can try to optimize it , Make it useless for other purposes .

4._Atomic Type qualifier

​ This qualifier is mainly in C11 Version implementation , Its main function is concurrent programming, which divides program execution into multiple threads that can be executed at the same time .