Volatile qualifier

Let's start with two questions

C/C++ Should I use in multithreaded programming volatile？

Some people think that volatile There is a great risk in solving the multi thread contention problem , Unless the environmental system used is unreliable, it will be added for insurance volatile, Or to realize the very low-level interface from the perspective of extreme efficiency . This requires the writer to be very clear about the direction of program logic , Otherwise you'll make a mistake . Isn't that amazing , I believe many people may not have written this keyword , But it's probably no stranger to experienced programmers .

C++11 The standard clearly points out that atomic operation or mutex lock should be used to solve the problem of multi-threaded data competition .

C and C++ Medium volatile It is not used to solve the problem of multi thread contention , It is used to modify some variables that change due to uncontrollable factors of the program , For example, accessing variables of underlying hardware devices , To remind the compiler not to optimize the access to this variable without authorization .

volatile Definition

keyword volatile Is an instruction to the compiler , Point out that such an object should not be optimized . stay C,C++,Java Inside , Once controversial , Later, with the memory model and multithreading , High concurrency scenarios , Bring him back to life .

volatile It's a type modifier （type modifier）. It is designed to modify variables accessed and modified by different threads . without volatile, Basically, it will lead to such a result ： Or you can't write multithreaded programs , Or the compiler loses a lot of optimization opportunities . Of course, single thread can only limit compiler optimization .

Application scenarios

Using and const Qualifiers are used in the same way volatile qualifiers .volatile A qualifier is an additional modifier to a type . If you're a rookie , You think of it as const Modifier that would be good .

Embedded aspect , use volatile The modified variable means that the variable may be changed by hardware , Such as register operation .

For example, programs that directly process hardware often have such data members , Their values are controlled by processes other than those controlled by the program itself . for example , The program can contain variables updated by the system clock . When the value of an object can be changed in a way other than compiler control or detection , The object should be lived as volatile.