Shared Libraries with GCC on linux

 

•  Create an .so file and run that file, link the .so from the same path to the executable.
  

Program:

Program file 1 Header file  (foo.h):

/* Shared Libraries with GCC on linux
 * Check : http://velrajcoding.blogspot.in
 */
#ifndef foo_h__
#define foo_h__

extern void foo(void);

#endif  // foo_h__

Program file 2 foo.c:

 
/* Shared Libraries with GCC on linux
 * Check : http://velrajcoding.blogspot.in
 */

#include <stdio.h>


void foo(void)
{
        printf("Hello, I am a shared library");
}

Program file 3 Main file (main.h):

/* Shared Libraries with GCC on linux
 * Check : http://velrajcoding.blogspot.in
 */
#include <stdio.h>
#include "foo.h"

int main(void)
{
    puts("This is a shared library test...");
    foo();
    return 0;
}

Compile & execute:

Step 1: Compiling with Position Independent Code

[so_crete]$ ls
foo.c  foo.h  main.c
[so_crete]$
[so_crete]$ gcc -c -Wall -Werror -fpic foo.c
[so_crete]$ ls
foo.c  foo.h  foo.o  main.c
[so_crete]$

Step 2: Creating a shared library from an object file

[so_crete]$ gcc -shared -o libfoo.so foo.o
[so_crete]$ ls
foo.c  foo.h  foo.o  libfoo.so  main.c
[so_crete]$

Step 3: Linking with a shared library

[so_crete]$ gcc -Wall -o test main.c -lfoo
/bin/ld: cannot find -lfoo
collect2: error: ld returned 1 exit status
[so_crete]$

Tell GCC where to find the shared library
[so_crete]$ pwd
/users/vkutrala/vel/sample/so_crete
[so_crete]$ gcc -L /users/vkutrala/vel/sample/so_crete -Wall -o test main.c -lfoo
[so_crete]$ ls
foo.c  foo.h  foo.o  libfoo.so  main.c  test
[so_crete]$

Step 4: Making the library available at runtime

[so_crete]$ ./test
./test: error while loading shared libraries: libfoo.so: cannot open shared object file: No such file or directory
[so_crete]$

Using LD_LIBRARY_PATH

[so_crete]$ echo $LD_LIBRARY_PATH

[so_crete]$ LD_LIBRARY_PATH=/users/vkutrala/vel/sample/so_crete:$LD_LIBRARY_PATH
[so_crete]$ ./test
./test: error while loading shared libraries: libfoo.so: cannot open shared object file: No such file or directory
[so_crete]$
[so_crete]$
[so_crete]$
What happened? Our directory is in LD_LIBRARY_PATH, but we did not export it. In Linux, if you do not export the changes to an environment variable, they will not be inherited by the child processes
[so_crete]$  export LD_LIBRARY_PATH=/users/vkutrala/vel/sample/so_crete:$LD_LIBRARY_PATH
[so_crete]$ ./test
This is a shared library test...
Hello, I am a shared library
[so_crete]$

Reference:
     https://www.cprogramming.com/tutorial/shared-libraries-linux-gcc.html

 

Read About Dynamic loading lib(run time load or patch update)