Several keywords in C language

auto： Automatic local variables , Generally, when we define local variables , Namely auto local variable , But omitting it when we write has the same effect , Local variables are stored on the stack .

extern： Life global variable . For example, I a.c Can be defined in int a, b.c I want to call This a But it's not defined , At this time b.c You need to use this a Before the variable , use extern a, To tell a It is defined in other files , You can use it .

static： There are two uses , The first is to modify local variables , It is called static local variable . Static local variables are the same as assigned global variables , Save in data segment , The life cycle is the same . His connection is connectionless .

The second is to modify global variables or modify functions , It turns external links into internal connection properties , Can only be used in this document , Don't want to be called by other files . External connection attributes are ordinary functions and global variables , Connectionless attributes are local variables and static local variables .

register： It is generally used to decorate global variables that are often called . Will put it in the register , For example uboot Medium gd Global variables , Such benefits can improve efficiency . But he tried to put it in the register , Instead of necessarily putting it in the register .

volatile： Variable , Changeable . There are three cases where the value of a variable is unknown ,1. Interrupt the changed value ,2. The same variable used in multithreading ,3. Value of hardware change （ register ）, All three compilers cannot know the change of value . So it is used to tell the compiler when it encounters this variable , Don't optimize this variable . So there will be no errors .

const： Often used to define constants , Indicates that this constant cannot be modified . It mainly depends on const Which is the nearest behind , Is to decorate which , near * The value cannot be modified , Close to the variable is the address, which cannot be modified .

• const int * a： Constant pointer , The value cannot be modified , The address pointed to can be modified .
• int * const a： constant pointer , Value can be modified , The address pointed to cannot be modified .