Shared library from archive (.a) file in C - c

Question

I was given a task to create archive file .a from objects file and create a shared library file from archive .a file. I have tried experiment having following file:
foo.h
#ifndef _foo_h__
#define _foo_h__
extern void foo(void);
extern void bar(void);
#endif //_foo_h__
foo.c
#include<stdio.h>
void foo(void)
{
puts("Hello, I'm a shared library");
}
bar.c
#include<stdio.h>
void bar(void)
{
puts("This is bar function call.");
}
main.c
#include <stdio.h>
#include"foo.h"
int main(void)
{
puts("This is a shared library test...");
foo();
bar();
return 0;
}
Makefile
CFLAGS = -Wall -Werror
LDFLAGS = -L/home/betatest/Public/implicit-rule-archive -Wl,-rpath,'$$ORIGIN'
all : run
run : main.o libfoo.so
$(CC) $(LDFLAGS) -o $# $^
libfoo.so : CFLAGS += -fPIC # Build objects for .so with -fPIC.
libfoo.so : libfoo.a
$(CC) -shared -o $# $^
libfoo.a : foo.o bar.o
ar cvq libfoo.a foo.o bar.o
# ar cr libfoo.a foo.o bar.o
# Compile any .o from .c. Also make dependencies automatically.
%.o : %.c
$(CC) -c $(CFLAGS) -o $# $<
#Include dependencies on subsequent builds.
.PHONY : all clean
clean :
-rm -f *.o *.d run libfoo.*
This simple test program seems to run fine but while compiling using make it producing error as:
cc -c -Wall -Werror -o main.o main.c
cc -c -Wall -Werror -fPIC -o foo.o foo.c
cc -c -Wall -Werror -fPIC -o bar.o bar.c
ar cvq libfoo.a foo.o bar.o
a - foo.o
a - bar.o
cc -shared -o libfoo.so libfoo.a
cc -L/home/betatest/Public/implicit-rule-archive -Wl,-rpath,'$ORIGIN' -o run main.o libfoo.so
main.o: In function `main':
main.c:(.text+0xf): undefined reference to `foo'
main.c:(.text+0x14): undefined reference to `bar'
collect2: ld returned 1 exit status
Makefile-test:7: recipe for target 'run' failed
make: *** [run] Error 1
Somebody please point out that where I am getting wrong?? Thanks a lot.

Answer 1

If you wish to build a shared library from a static library you have to tell the linker to use all function/symbols included in static library (.a). Otherwise nothing will be included in the shared library (.so).
You have to use --whole-archive/--no-whole-archive pair in linking.
CFLAGS = -Wall -Werror
LDFLAGS = -L/home/betatest/Public/implicit-rule-archive -Wl,-rpath,'$$ORIGIN'
all : run
run : main.o libfoo.so
$(CC) $(LDFLAGS) -o $# $^
libfoo.so : CFLAGS += -fPIC # Build objects for .so with -fPIC.
libfoo.so : libfoo.a
$(CC) -shared -o $# -Wl,--whole-archive $^ -Wl,--no-whole-archive
libfoo.a : foo.o bar.o
ar cvq libfoo.a foo.o bar.o
# Compile any .o from .c. Also make dependencies automatically.
%.o : %.c
$(CC) -c $(CFLAGS) -o $# $<
#Include dependencies on subsequent builds.
.PHONY : all clean
clean :
-rm -f *.o *.d run libfoo.*
You can check the exported functions using nm command:
$ nm -D libfoo.so | grep ' T '
0000000000000702 T bar
0000000000000714 T _fini
00000000000006f0 T foo
0000000000000590 T _init
When no --whole-archive/--no-whole-archive pair is used you get:
$ nm -D libfoo.so | grep ' T '
0000000000000714 T _fini
0000000000000590 T _init

Answer 2

Your mistake is in the question: do not make shared library from archive, make it from objects, i.e. change Makefile like this
libfoo.so : foo.o bar.o
$(CC) -shared -o $# foo.o bar.o
you can also use %.o to mean all objects
also you need CFLAGS = -fPIC globally, to affect compilation command, not only the link step, that is you .o object files should be position-independent.
also here extern is not needed, all C functions are implicitly extern
extern void foo(void);
I would also not recommend using rpath and better
export LD_LIBRARY_PATH=.
to load .so from current directory (or export path to .so's directory)