The real name is the actual file on disk that contains the compiled library code. It uses the naming format:

Major version identifier: An integer that is incremented every time the library makes an incompatible change (e.g., removed functions, changed function signatures). Programs built against version 1 cannot use version 2 automatically.

Minor version identifier: A number (or two dot-separated numbers for patch level) that is incremented for compatible changes — bug fixes, new functions added (but old ones not removed). A program built against version 1.0.1 can use version 1.0.2 without recompilation.

Examples of real names:

The soname (shared object name) has only the major version number — it drops the minor version. Format:

The soname is a symbolic link (not a real file) that points to the most recent minor version within its major version. When you compile and link your program, the linker embeds the soname string inside your executable. At runtime, the dynamic linker reads this embedded soname and resolves it via the symbolic link.

Why this design is powerful:

• The soname libdemo.so.1 can be updated to point from libdemo.so.1.0.1 to libdemo.so.1.0.2 (a bugfix release)
• All existing programs that were linked against libdemo.so.1 automatically get the bugfix without recompilation
• Programs needing version 2 use libdemo.so.2 — both versions coexist safely

Soname symbolic links in practice:

The linker name is the simplest — it has no version numbers at all. Format:

When you write gcc -ldemo in your build command, the linker searches for libdemo.so. This is the linker name. It is a symbolic link that usually points to the soname (which in turn points to the real name).

Linker name examples:

How name resolution flows at compile time and runtime:

At Compile Time (gcc -ldemo):

At Runtime (./prog):

The three-layer naming structure visualized:

Example 1: Inspect the soname embedded in an executable

# After building a program that links against libdemo.so,
# you can inspect what soname is embedded inside the ELF binary

$ readelf -d prog | grep NEEDED
 0x0000000000000001 (NEEDED)   Shared library: [libdemo.so.1]
 0x0000000000000001 (NEEDED)   Shared library: [libc.so.6]

# The output shows libdemo.so.1 — this is the SONAME embedded at link time.
# Notice: NOT libdemo.so (linker name), NOT libdemo.so.1.0.2 (real name)
# The soname is what gets recorded inside the ELF binary.

Example 2: List all three names of a real system library

# Let's examine libreadline on a typical Linux system

$ ls -l /usr/lib/x86_64-linux-gnu/libreadline*
lrwxrwxrwx  libreadline.so    -> libreadline.so.8       # linker name
lrwxrwxrwx  libreadline.so.8  -> libreadline.so.8.2     # soname
-rwxr-xr-x  libreadline.so.8.2                          # real name (actual file)

# Pattern:
# libreadline.so      --> linker name (no version) → used by: gcc -lreadline
# libreadline.so.8    --> soname (major only)       → embedded in executables
# libreadline.so.8.2  --> real name (maj.min)       → actual file on disk

Example 3: Create symbolic links for all three names manually

# Suppose you have just compiled libdemo version 1.0.1

# Step 1: You have the real name file (from compilation)
ls libdemo.so.1.0.1
# -rwxr-xr-x libdemo.so.1.0.1

# Step 2: Create the soname symlink (major version only)
ln -s libdemo.so.1.0.1 libdemo.so.1
# libdemo.so.1 -> libdemo.so.1.0.1

# Step 3: Create the linker name symlink (no version)
ln -s libdemo.so.1 libdemo.so
# libdemo.so -> libdemo.so.1 -> libdemo.so.1.0.1

# Verify all three:
ls -l libdemo.so*
# lrwxrwxrwx libdemo.so      -> libdemo.so.1       (linker name)
# lrwxrwxrwx libdemo.so.1    -> libdemo.so.1.0.1   (soname)
# -rwxr-xr-x libdemo.so.1.0.1                      (real name)

Example 4: Minor version upgrade — update soname pointer

# Library was at version 1.0.1, now 1.0.2 is released (bugfix)

# New real name file exists:
ls libdemo.so.1.0.2

# Update soname to point to the newer minor version:
ln -sf libdemo.so.1.0.2 libdemo.so.1
#      ^^ -f forces overwrite of existing symlink

ls -l libdemo.so.1
# lrwxrwxrwx libdemo.so.1 -> libdemo.so.1.0.2   (now points to 1.0.2!)

# All programs that were using libdemo.so.1 automatically get 1.0.2
# No recompilation needed!
# The linker name libdemo.so still correctly points to libdemo.so.1