How to use extern symbols in shared library in C - c

Question

I am trying to compile following test files to create a shared library:
answer.c
#include <stdio.h>
#include "add.h"
extern int myvar();
int answer()
{
printf("\r\n myvar:%d \r\n", myvar());
setSummand(20);
return add(22); // Will return 42 (=20+22)
}
add.c
#include <stdio.h>
int gSummand;
void setSummand(int summand)
{
printf("1Library is initialized\n");
gSummand = summand;
}
int add(int summand)
{
return gSummand + summand;
}
I want to create a shared library from the 2 files "answer.c" "add.c", I am using following commands:
gcc -c answer.c -o answer.o
gcc -c add.c -o add.o
gcc -shared add.o answer.o -o libtest.so
However third command gives following error:
answer.o:answer.c:(.text+0x9): undefined reference to `myvar'
answer.o:answer.c:(.text+0x9): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `myvar'
collect2: error: ld returned 1 exit status
Same setup links successfully if I use the following command to create static library instead of dynamic. Therefore, I don't see any error if I try following command:
ar rcs libtest.a add.o answer.o
Want to know if I am missing here something. Also want to know how to use external symbols in shared library.

Answer 1

ELF shared libraries need to be position independent. They will be mapped into the executable's address space at an address that isn't known until run time. This means no absolute address call instructions, such as might be used to call myvar().
You need to specify -fpic when you compile the source into object files, when those object files will be placed into a shared library. This tells the compiler to generate code that does not use absolute addresses, etc. so it can be position independent.
Example main.c file to use this library:
extern int answer(void);
int myvar() { return 1; }
int main(void) { return answer(); }
Example without -fpic:
[test]$ gcc -c add.c
[test]$ gcc -c answer.c
[test]$ gcc -shared add.o answer.o -o libtest.so
/usr/bin/ld: add.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: answer.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Nonrepresentable section on output
collect2: error: ld returned 1 exit status
Example with -fpic:
[test]$ gcc -fpic -c add.c
[test]$ gcc -fpic -c answer.c
[test]$ gcc -shared add.o answer.o -o libtest.so
[test]$ gcc main.c libtest.so
[test]$ LD_LIBRARY_PATH=. ./a.out
myvar:1
1Library is initialized