Summary of the function and usage of const, volatile and restrict

const
const(constant) Keywords can modify variables 、 Function parameter 、 Return value or function body .

Define read-only variables .
Limiting function parameters . Prevent accidental modification . Increase program security and reliability .
Such as const char *p Express ,p The value pointed to by the pointer cannot be changed , This pointer cannot be used to modify data
Such as char *const p Express ,p The address pointed to by the pointer cannot be changed
Such as const int arr[] Then the values in the array cannot be modified
const char *const p Express , The value and address pointed to cannot be changed
const double locked[4]
double *pc=rates
pc=locked This sentence is illegal . because locked It can't be modified
In the source file const int i = 1 In other documents extern const int i, If in the header file, apply static const int i = 1, Because each file needs to contain this header file , need static Ensure internality . But if you store arrays in this way , Because each file needs to create this static array , It may take up more storage space , We need to consider its advantages and disadvantages from more aspects .

volatile
volatile Qualifiers indicate that variables can be modified by external agents in addition to programs , For example, the variable saves the current clock time .
The compiler optimizes the compilation and puts the unchanged value in the register , For reading , Mark volatile It can ensure that it is not put into the register .
If it cannot be changed by the program but can be changed by the agent const volatile int i.

restrict
restrict Can only be used for pointers int* restrict ptr, So that makes ptr Is the only way to point to a data block .
It can prevent data from being operated by other pointers . If in memcpy In order to prevent overlapping areas , use restrict Modifies the incoming pointer . meanwhile ,restrict It is also conducive to compiler optimization .

author ： Hu hahaha
link ：https://www.jianshu.com/p/58f618bd9ea3
source ： Simple books
The copyright belongs to the author . Commercial reprint please contact the author for authorization , Non-commercial reprint please indicate the source .