# Using ld directly is not trivial.

$ cc -o a.o -c a.c
$ cc -o b.o -c b.c
$ ls
a.c  a.o  b.c  b.o
$ ld
ld: no input files
$ ld a.o b.o
ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000
ld: a.o: in function `main':
a.c:(.text+0x19): undefined reference to `printf'
$ ld -lc a.o b.o
ld: warning: cannot find entry symbol _start; defaulting to 0000000000401020
$ echo $?
0
$ ./a.out
-bash: ./a.out: No such file or directory
$ ls
a.c  a.o  a.out*  b.c  b.o

# The compiler can do it for you though.
$ cc a.c b.c
$ ./a.out
13

# Building a library.
$ cc -shared -o libmy.so libmy.c
$ ls
a.c  b.c  main.c  libmy.c  libmy.so*  README

# Using the library.
$ cc -lmy main.c
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lmy
collect2: error: ld returned 1 exit status

$ cc -lmy -L. main.c
$ ./a.out
./a.out: error while loading shared libraries: libmy.so: cannot open shared object file: No such file or directory

# The library name is in the executable but with no additional information on
# where to find it (so only system directories are searched).
$ ldd a.out
	linux-vdso.so.1 (0x00007fff61188000)
	libmy.so => not found
	libc.so.6 => /lib64/libc.so.6 (0x00007fc39ea3b000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fc39ecc9000)

# You can provide information through environment variables (that is dynamic
# linker).
$ LD_LIBRARY_PATH=. ./a.out
called myfn

$ cc -lmy -L. -Xlinker -R . main.c
$ ./a.out
called myfn

$ readelf -d a.out

Dynamic section at offset 0x2dd8 contains 28 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libmy.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
# this followin one is from those "-Xlinker -R." options
 0x000000000000001d (RUNPATH)            Library runpath: [.]
 0x000000000000000c (INIT)               0x1000
 0x000000000000000d (FINI)               0x1220
 0x0000000000000019 (INIT_ARRAY)         0x3dc8
 0x000000000000001b (INIT_ARRAYSZ)       8 (bytes)
 0x000000000000001a (FINI_ARRAY)         0x3dd0
 0x000000000000001c (FINI_ARRAYSZ)       8 (bytes)
 0x000000006ffffef5 (GNU_HASH)           0x2e8
 0x0000000000000005 (STRTAB)             0x3d0
 0x0000000000000006 (SYMTAB)             0x310
 0x000000000000000a (STRSZ)              168 (bytes)
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000015 (DEBUG)              0x0
 0x0000000000000003 (PLTGOT)             0x4000
 0x0000000000000002 (PLTRELSZ)           48 (bytes)
 0x0000000000000014 (PLTREL)             RELA
 0x0000000000000017 (JMPREL)             0x578
 0x0000000000000007 (RELA)               0x4b8
 0x0000000000000008 (RELASZ)             192 (bytes)
 0x0000000000000009 (RELAENT)            24 (bytes)
 0x000000006ffffffb (FLAGS_1)            Flags: PIE
 0x000000006ffffffe (VERNEED)            0x488
 0x000000006fffffff (VERNEEDNUM)         1
 0x000000006ffffff0 (VERSYM)             0x478
 0x000000006ffffff9 (RELACOUNT)          3
 0x0000000000000000 (NULL)               0x0

$ cd /
$ ~/unix-linux-prog-in-c-src/linker/a.out
/afs/ms/u/p/pechanec/unix-linux-prog-in-c-src/linker/a.out: error while loading shared libraries: libmy.so: cannot open shared object file: No such file or directory