Extern keyword

1 Functional specifications

extern It can be placed in front of variables or functions , Used to identify variables or functions Definition In other documents , Prompt the compiler to look for definitions in other modules when encountering this variable and function . Such as adding extern, Declare only functions or variables in the header file , And in the .c File for the definition of functions .

2 Add before function or variable extern

2.1 introduces

A common way is to add extern, And then in .C First protect the header file in the file , Then define .
main.c file

  1 #include <stdio.h>
  2 #include "exter.h"
  3 #include "exter2.h"
  4 int main()
  5 {
    
  6     int num1 = 10;
  7     int num2 = 20;
  8     int num3 = add(num1, num2);
  9     printf("sum = %d\n", num3);		//sum = 30
 10 
 11     num = 200;
 12     printf("now num = %d\n", num);	//now num = 200
 13 
 14     multi(num,num2);
 15 
 16 }

exter.h file

  1 #ifndef EXTER_H
  2 #define EXTER_H
  3 
  4 #include <stdio.h>
  5 
  6 extern int num;
  7 extern int add(int a, int b);
  8 
  9 #endif

exter.c file

  1 #include "exter.h"
  2 
  3 int num = 120;
  4 
  5 int add(int a, int b)
  6 {
    
  7     printf("into exter.c and num = %d\n", num);
  8 
  9     int c = a + b;
 10     return c;
 11 }

exter2.h

  1 #ifndef EXTER2_H
  2 #define EXTER2_H
  3 
  4 extern int multi(int a, int b);
  5 
  6 #endif

exter2.c

  1 #include "exter2.h"
  2 #include "exter.h"
  3 
  4 int multi(int a, int b)
  5 {
    
  6     printf("exter2 now num = %d\n",num);
  7     printf("a * b = %d\n",a * b);	//a * b = 4000
  8 }

Such as some above .h Through the file extern Keywords decorate functions or variables , Then the modified functions and variables are in main.c Use... In the document . Here's the variable num stay exter.h Declaration in the document , stay exter.c Assignment in file , stay main.c Revision in China , stay exter2.c Use in . Modified variable num The value of has changed , In the later use, the new value is used .

2.2 The build process

adopt gcc Compile check the compilation process and you will find , Pass in a file include Mode contains another file , In the preprocessing stage, the contents of the included header file will be included in the current file , So add extern It will be included once .gcc You can refer to gcc The compiling process and linking principle of the program
Here's how it works ： gcc -E -o main.i main.c Later obtained main.i The form in the document .

837 # 2 "main.c" 2
838 # 1 "exter.h" 1
839 
840 
841 
842 
843 
844 extern int num;
845 extern int add(int a, int b);
846 # 3 "main.c" 2
847 # 1 "exter2.h" 1
848 
849 
850 
851 extern int multi(int a, int b);
852 # 4 "main.c" 2
853 
854 int main()
855 {
    
856  int num1 = 10;
857  int num2 = 20;
858  int num3 = add(num1, num2);
859  printf("sum = %d\n", num3);
860 
861  num = 200;
862  printf("now num = %d\n", num);
863 
864  multi(num,num2);
865 
866 }

The above is pretreated main.i file , It can be seen that it is equivalent to main The function is declared before num Variables and add function

1.2 Add before function or variable extern C

Pass the same gcc Display the results of the compilation process *.o It can be seen from the document that extern C With or without extern C The difference between .
Get the assembly file ：gcc -c -o main.o main.c
View assembly file ：nm main.o

[[email protected] extern]$ gcc -c -o main.o main.c
[[email protected] extern]$ nm main.o
                 U add
0000000000000000 T main
                 U multi
                 U num
                 U printf