Use of C language and

Linux Occasionally encountered in the source code # Number , Although seldom used , But take a note ：

①#： Put a double quotation mark on the left and right of the macro variable it refers to

 Premise ：
    printf("123""456"); //printf It can be used like this , Output 123456
 example ：
    #define test(EXP) #EXP
    printf("123"test(45)"6");// Equivalent to printf("123""45""6");

②##： In the macro definition with parameters, two substrings (token) Join together to form a new substring ( The string ).

• Substring ： The smallest syntax unit recognized by the compiler

#define test(n) a##n##c
int test(b)=123;    # Equivalent to int abc=123
printf("%d",abc);   # Output 123