ELF shared library order

elf_shared_lib_order.sh

$ cat prog.c
#include <stdio.h>

void foo();
void bar();

int main(void) {
  foo();
  bar();
  return 0;
}
$ cat foo.c
#include <stdio.h>

void xyz() {
  printf("foo-xyz\n");
}

void foo() {
  printf("foo\n");
  xyz();
}
$ cat bar.c
#include <stdio.h>

void xyz() {
  printf("bar-xyz\n");
}

void bar() {
  printf("bar\n");
  xyz();
}

#
# foo before bar.
#

$ gcc -g -c -fPIC -Wall -c foo.c
$ gcc -g -c -fPIC -Wall -c bar.c
$ gcc -g -shared -o libfoo.so foo.o
$ gcc -g -shared -o libbar.so bar.o
$ gcc -g -o prog prog.c libfoo.so libbar.so
$ objdump -p prog | grep NEEDED
  NEEDED               libfoo.so
  NEEDED               libbar.so
  NEEDED               libc.so.6
$ ldd prog
        linux-vdso.so.1 =>  (0x00007ffcc27ac000)
        libfoo.so => not found
        libbar.so => not found
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f5b651ab000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f5b65575000)
$ LD_LIBRARY_PATH=. ./prog
foo
foo-xyz
bar
foo-xyz

#
# bar before foo
#

$ gcc -g -o prog prog.c libbar.so libfoo.so
$ objdump -p prog | grep NEEDED
  NEEDED               libbar.so
  NEEDED               libfoo.so
  NEEDED               libc.so.6
$ ldd prog
        linux-vdso.so.1 =>  (0x00007ffe888e9000)
        libbar.so => not found
        libfoo.so => not found
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fbf103ad000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fbf10777000)
$ LD_LIBRARY_PATH=. ./prog
foo
bar-xyz
bar
bar-xyz