gcc linker resolving symbols

I have been struggling a bit to understand how the gcc linker works and how things are different when linking a shared library with another shared library and when linking a binary with a shared library. I am using Ubuntu 16.04.1 LTS, gcc 5.4.0 and ld 2.26.1.

Following are two sets of a sequence of commands executed on some C source files.

Sequence 1:-

ammal@ubuntu:~/linktest6$ cat a.c 
#include <stdio.h>

int a_func() {
    printf("Running a_func()n");

    return 0;
}

ammal@ubuntu:~/linktest6$ cat b.c 
#include <stdio.h>

int a_func();

int b_func() {
    printf("Running b_func()n");
    a_func();

    return 0;
}

ammal@ubuntu:~/linktest6$ cat d.c 
#include <stdio.h>

int b_func();

int d_func() {
    printf("Running d_func()n");
    b_func();

    return 0;
}

ammal@ubuntu:~/linktest6$ cat myprog.c 
#include <stdio.h>

int d_func();

int main() {
    printf("Running myprog_func()n");
    d_func();

    return 0;
}

ammal@ubuntu:~/linktest6$ gcc -shared -fPIC -o liba.so a.c 

ammal@ubuntu:~/linktest6$ ldd -r liba.so 
    linux-vdso.so.1 =>  (0x00007ffc6fded000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f1898da3000)
    /lib64/ld-linux-x86-64.so.2 (0x0000555e2853c000)

ammal@ubuntu:~/linktest6$ gcc -shared -fPIC -o libb.so b.c 

ammal@ubuntu:~/linktest6$ ldd -r libb.so 
    linux-vdso.so.1 =>  (0x00007ffd82127000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f0cc6253000)
    /lib64/ld-linux-x86-64.so.2 (0x00005586e6a36000)
undefined symbol: a_func    (./libb.so)

ammal@ubuntu:~/linktest6$ gcc -shared -fPIC -o libd.so d.c 

ammal@ubuntu:~/linktest6$ ldd -r libd.so 
    linux-vdso.so.1 =>  (0x00007ffc3addb000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f408db59000)
    /lib64/ld-linux-x86-64.so.2 (0x0000558720efc000)
undefined symbol: b_func    (./libd.so)

ammal@ubuntu:~/linktest6$ gcc -fPIC -o myprog myprog.c -L. -ld -lb -la

ammal@ubuntu:~/linktest6$ ldd -r myprog
    linux-vdso.so.1 =>  (0x00007ffe807aa000)
    libd.so => not found
    libb.so => not found
    liba.so => not found
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f344acce000)
    /lib64/ld-linux-x86-64.so.2 (0x0000563a89eb4000)
undefined symbol: d_func    (./myprog)

Sequence 2:-

ammal@ubuntu:~/linktest6$ cat a.c 
#include <stdio.h>

int a_func() {
    printf("Running a_func()n");

    return 0;
}

ammal@ubuntu:~/linktest6$ cat b.c 
#include <stdio.h>

int a_func();

int b_func() {
    printf("Running b_func()n");
    a_func();

    return 0;
}

ammal@ubuntu:~/linktest6$ cat d.c 
#include <stdio.h>

int b_func();

int d_func() {
    printf("Running d_func()n");
    b_func();

    return 0;
}

ammal@ubuntu:~/linktest6$ cat myprog.c 
#include <stdio.h>

int d_func();

int main() {
    printf("Running myprog_func()n");
    d_func();

    return 0;
}

ammal@ubuntu:~/linktest6$ gcc -shared -fPIC -o liba.so a.c 

ammal@ubuntu:~/linktest6$ ldd -r liba.so 
    linux-vdso.so.1 =>  (0x00007ffe8c9ee000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f88b91c2000)
    /lib64/ld-linux-x86-64.so.2 (0x0000555cf72ef000)

ammal@ubuntu:~/linktest6$ gcc -shared -fPIC -o libb.so b.c 

ammal@ubuntu:~/linktest6$ ldd -r libb.so 
    linux-vdso.so.1 =>  (0x00007fffbffc4000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f69ea310000)
    /lib64/ld-linux-x86-64.so.2 (0x000055996b3e5000)
undefined symbol: a_func    (./libb.so)

ammal@ubuntu:~/linktest6$ gcc -shared -fPIC -o libd.so d.c -L. -lb -Wl,-rpath=/home/ammal/linktest6

ammal@ubuntu:~/linktest6$ ldd -r libd.so 
    linux-vdso.so.1 =>  (0x00007ffe21bb6000)
    libb.so => /home/ammal/linktest6/libb.so (0x00007fca1cdb6000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fca1c9d5000)
    /lib64/ld-linux-x86-64.so.2 (0x000055f517b11000)
undefined symbol: a_func    (/home/ammal/linktest6/libb.so)

ammal@ubuntu:~/linktest6$ gcc -fPIC -o myprog myprog.c -L. -ld -lb -la
./libb.so: undefined reference to `a_func'
collect2: error: ld returned 1 exit status

My question is, why I am able to successfully compile the binary in the first case and not the second.

---

In Sequence 2, libd.so is linked to a shared object (namely to libb.so). gcc needs a path to resolve it. Try the following. $ gcc -fPIC -o myprog myprog.c -L. -ld -lb -la -Wl,-rpath=/home/ammal/linktest6

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