Dynamic link library nesting example

foo2.c

int foo2 = 20;
void foo2_func() {
    }

foo1.c

extern int foo2;
int foo1 = 10;
int dummy = 20;

void foo1_func()
{
    
    int a = foo2;
    int b = dummy;
    int c = foo1;

    foo2_func();
}

main.c

#include <stdlib.h>
extern int foo1;
extern int foo1;
int a[2048];
int dummy = 10;

void main()
{
    
    char *m = malloc(1024);
    foo1 = 5;

    foo1_func();
    while(1)sleep(1000);
}

function j result ：

[email protected]:~/Documents/test/libso$ gcc -shared -fPIC -o libf2.so foo2.c
[email protected]:~/Documents/test/libso$ gcc -shared -fPIC -o libf1.so foo1.c -L. -lf2
foo1.c: In function ‘foo1_func’:
foo1.c:11:5: warning: implicit declaration of function ‘foo2_func’; did you mean ‘foo1_func’? [-Wimplicit-function-declaration]
     foo2_func();
     ^~~~~~~~~
     foo1_func
[email protected]:~/Documents/test/libso$ gcc -o hello main.c -L. -lf1
main.c: In function ‘main’:
main.c:12:5: warning: implicit declaration of function ‘foo1_func’ [-Wimplicit-function-declaration]
     foo1_func();
     ^~~~~~~~~
main.c:13:13: warning: implicit declaration of function ‘sleep’ [-Wimplicit-function-declaration]
     while(1)sleep(1000);
             ^~~~~
/usr/bin/ld: warning: libf2.so, needed by ./libf1.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld: ./libf1.so: undefined reference to `foo2' /usr/bin/ld: ./libf1.so: undefined reference to `foo2_func'
collect2: error: ld returned 1 exit status
[email protected]:~/Documents/test/libso$ export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:.
[email protected]:~/Documents/test/libso$ gcc -o hello main.c -L. -lf1
main.c: In function ‘main’:
main.c:12:5: warning: implicit declaration of function ‘foo1_func’ [-Wimplicit-function-declaration]
     foo1_func();
     ^~~~~~~~~
main.c:13:13: warning: implicit declaration of function ‘sleep’ [-Wimplicit-function-declaration]
     while(1)sleep(1000);
             ^~~~~
[email protected]:~/Documents/test/libso$ ./hello 
^C

1. When the dynamic library is made , It can also be combined with other dynamic libraries .
2. -I（ Notice that it's capital i） to gcc Add the path of the customized header file ,-Imyinclude.
3. -L to gcc Add additional search library paths .
4. -l （ A lowercase letter L） Parameters are used to specify the library that the program will link to ,-l The parameter is followed by the library name , Math library file name is libm.so, It's easy to see , Put the header of the library file name lib And tail .so It's the name of the library ,lm.