Let’s try to create a math library with two functions add and sub.
//
add.c
int
add(int x, int y)
{
return x+y ;
}
//sub.c
int
sub(int x, int y)
{
return x-y ;
}
//main.c
#include
<stdio.h>
int
add(int, int) ; // you can create a header file
int
sub(int, int) ; // and put these
int
main(int argc, char **argv)
{
printf("%d\n", add(5,7)) ;
printf("%d\n", sub(5,7)) ;
return 0 ;
}
Now compile it with -c option
gcc
-c add.c sub.c
so you will get the object files add.o and sub.o
Creating and Linking a Static Library
Use the ar tool
ar crv libzmath.a add.o sub.o
see the content of
ar -t libzmath.a
Now statically link it like
gcc
-o test main.c -L. -lzmath
or
gcc
-o test main.c .libzmath.a
Creating and linking the dynamically linked library (shared object)
Again compile while generating Position Independent Code
gcc -c -fpic add.c sub.c
-fpic option is for Position Independent Code
Create the shared library like the following:
gcc
-shared -o libmymath.so add.o sub.o
Remember with gcc, all the .so files must start with "lib".
So if library file is libmymath.so, then you have to link it like with the –l option like -lmymath
gcc -Wall -o test main.c -L/home/swarup/work/c -lmymath
-L option is to specify the location where my libmymath.so is present.
Now running ./test may not work, because the .so file is not in the path where linux is looking for to load it. It first searches /usr/local/lib, /usr/lib and so on. So for the ld to work, we need to provide the library path in the environment variable LD_LIBRARY_PATH. We do it like:
export LD_LIBRARY_PATH=/home/swarup/work/c:$LD_LIBRARY_PATH
Note: You can see all the standard libraries in the directory /lib/x86_64-linux-gnu. The functions like printf are part of libc.so.xxx
