Create shared library for C program in Linux

Consider you c program has two file, which are functions.c and main.c

Consider your current working directory is /home/sujin/shared/

functions.c

int add(int x, int y)
{
    return x + y;
}
 
int sub(int x, int y)
{
        return x - y;
}
 
int mul(int x, int y)
{
        return x * y;
}
 
int div(int x, int y)
{
        return x / y;
}

main.c

#include <stdio.h>
int add(int x, int y);
int sub(int x, int y);
int mul(int x, int y);
int div(int x, int y);
 
int main(int argc, char *argv[])
{
    printf("%d\n", add(10, 15));
        printf("%d\n", sub(50, 15));
        printf("%d\n", mul(10, 3));
        printf("%d\n", div(100, 4));
        return 0;
}

1. Compile library source file(functions.c) into position independent code

gcc -c -fpic functions.c -Wall

2. Create shared library from the object file generated

gcc -shared -o libfun.so functions.o 

3. Compile the main program with shared library

gcc -o output main.c -lfun

/usr/bin/ld: cannot find -lfun

collect2: ld returned 1 exit status

4. Compile with giving path of shared library

gcc -L/home/sujin/shared/ -Wall -o output main.c -lfun

5. Run the program

./output 

./output: error while loading shared libraries: libfun.so: cannot open shared object file: No such file or directory

6. Check current LD_LIBRARY_PATH. 

echo $LD_LIBRARY_PATH

7. If LD_LIBRARY_PATH is not current working directory or empty then set the LD_LIBRARY_PATHS as current working directory

LD_LIBRARY_PATH=/home/sujin/shared:$LD_LIBRARY_PATH

8. Run the program

./output 

./output: error while loading shared libraries: libfun.so: cannot open shared object file: No such file or directory

9. Export the LD_LIBRARY_PATH. In linux we need to export the changes in the environment variable

export LD_LIBRARY_PATH=/home/sujin/shared/:$LD_LIBRARY_PATH

10. Run the program

./output

25

35

30

25