1. Minor Version Upgrade — Step by Step

A minor version upgrade is a compatible change — bug fixes, performance improvements, or new functions. Programs using the old version continue to work with the new version without recompiling.

Code Example 1 — Full Minor Version Upgrade

/* ===== BEFORE UPGRADE: libdemo.so.1.0.1 ===== */

/* mod1.c — version 1.0.1 */
#include <stdio.h>
#include <string.h>

int add_numbers(int a, int b) {
    return a + b;
}

/* Bug: this had an off-by-one in certain conditions */
int count_chars(const char *s, char c) {
    int count = 0;
    while (*s) {
        if (*s == c) count++;
        s++;
    }
    return count;  /* was: count - 1; (the bug) */
}

void hello(void) {
    printf("libdemo v1.0.1\n");
}

/* ===== AFTER UPGRADE: libdemo.so.1.0.2 ===== */

/* mod1.c — version 1.0.2: bug fixed in count_chars */
#include <stdio.h>
#include <string.h>

int add_numbers(int a, int b) {
    return a + b;          /* unchanged */
}

/* Bug fixed! Now returns correct count */
int count_chars(const char *s, char c) {
    int count = 0;
    while (*s) {
        if (*s == c) count++;
        s++;
    }
    return count;  /* fixed */
}

/* New function added — compatible */
int subtract_numbers(int a, int b) {
    return a - b;
}

void hello(void) {
    printf("libdemo v1.0.2 (bug fixed!)\n");
}

# === BUILD THE NEW VERSION ===

gcc -g -c -fPIC -Wall mod1.c mod2.c

# soname stays libdemo.so.1 — same major version, compatible change
gcc -g -shared -Wl,-soname,libdemo.so.1 \
    -o libdemo.so.1.0.2 \
    mod1.o mod2.o

# === INSTALL ===
sudo cp libdemo.so.1.0.2 /usr/local/lib/

# Check before ldconfig
ls -l /usr/local/lib/libdemo*
# lrwxrwxrwx libdemo.so.1 -> libdemo.so.1.0.1  ← still pointing to old
# -rwxr-xr-x libdemo.so.1.0.1
# -rwxr-xr-x libdemo.so.1.0.2

# === RUN LDCONFIG ===
sudo ldconfig -v | grep libdemo
# libdemo.so.1 -> libdemo.so.1.0.2   (changed) ← NOW points to new!

# Check after ldconfig
ls -l /usr/local/lib/libdemo*
# lrwxrwxrwx libdemo.so.1 -> libdemo.so.1.0.2  ← updated!
# -rwxr-xr-x libdemo.so.1.0.1                  ← old file still exists
# -rwxr-xr-x libdemo.so.1.0.2                  ← new file

# === WHAT HAPPENS TO RUNNING PROGRAMS ===
# Programs already running have their own mmap of libdemo.so.1.0.1
# That file (inode) stays valid until all processes close it
# Running programs: still use 1.0.1 (unaffected by symlink change)
# New programs: load libdemo.so.1 -> libdemo.so.1.0.2 -> get the fix!

2. Major Version Upgrade — Step by Step

A major version upgrade introduces incompatible changes. Old programs must NOT use the new version. Both versions must coexist on the system simultaneously.

Code Example 2 — Full Major Version Upgrade

# === CURRENT STATE: libdemo v1 is installed ===
ls -l /usr/lib/libdemo*
# lrwxrwxrwx libdemo.so   -> libdemo.so.1
# lrwxrwxrwx libdemo.so.1 -> libdemo.so.1.0.2
# -rwxr-xr-x libdemo.so.1.0.2

# === BUILD v2 (INCOMPATIBLE — new soname) ===
# Assume mod1_v2.c and mod2_v2.c have breaking changes

gcc -g -c -fPIC -Wall mod1_v2.c mod2_v2.c

# NOTE: soname is libdemo.so.2 ← DIFFERENT from before
gcc -g -shared -Wl,-soname,libdemo.so.2 \
    -o libdemo.so.2.0.0 \
    mod1_v2.o mod2_v2.o

# Verify the embedded soname
readelf -d libdemo.so.2.0.0 | grep SONAME
# (SONAME) Library soname: [libdemo.so.2]

# === INSTALL (alongside v1 — do NOT remove v1!) ===
sudo cp libdemo.so.2.0.0 /usr/lib/

# === RUN LDCONFIG ===
sudo ldconfig -v | grep libdemo
#   libdemo.so.1 -> libdemo.so.1.0.2    ← v1 unchanged
#   libdemo.so.2 -> libdemo.so.2.0.0    ← v2 newly created

# Verify all files
ls -l /usr/lib/libdemo*
# lrwxrwxrwx libdemo.so   -> libdemo.so.1          ← still v1 linker name
# lrwxrwxrwx libdemo.so.1 -> libdemo.so.1.0.2
# -rwxr-xr-x libdemo.so.1.0.2
# lrwxrwxrwx libdemo.so.2 -> libdemo.so.2.0.0      ← new soname created!
# -rwxr-xr-x libdemo.so.2.0.0

# === UPDATE LINKER NAME TO POINT TO v2 ===
# (so that new compilations use v2)
sudo ln -sf libdemo.so.2 /usr/lib/libdemo.so

ls -l /usr/lib/libdemo.so
# lrwxrwxrwx libdemo.so -> libdemo.so.2   ← now points to v2!

# === VERIFY OLD AND NEW PROGRAMS ===

# Old program (compiled against v1):
ldd old_program | grep libdemo
# libdemo.so.1 => /usr/lib/libdemo.so.1.0.2

# It still works! libdemo.so.1 exists and points to v1 code.

# Compile new program (uses v2 API):
gcc -o new_program new_main.c -ldemo -L/usr/lib
# -ldemo finds /usr/lib/libdemo.so -> libdemo.so.2

ldd new_program | grep libdemo
# libdemo.so.2 => /usr/lib/libdemo.so.2.0.0

# New program uses v2!
./new_program

Code Example 3 — Installing a New 2.x Minor Version

Once you have v2 installed, later minor releases (2.0.1, 2.1.0) follow the same minor upgrade process as before — ldconfig keeps the linker name up to date automatically.

# Current state: libdemo.so.2.0.0 is installed
ls -l /usr/lib/libdemo*
# lrwxrwxrwx libdemo.so   -> libdemo.so.2
# lrwxrwxrwx libdemo.so.2 -> libdemo.so.2.0.0
# -rwxr-xr-x libdemo.so.2.0.0

# === BUILD 2.0.1 — compatible fix within v2 ===
gcc -g -c -fPIC -Wall mod1_v2.c mod2_v2.c

# soname is still libdemo.so.2 — still v2 major
gcc -g -shared -Wl,-soname,libdemo.so.2 \
    -o libdemo.so.2.0.1 \
    mod1_v2.o mod2_v2.o

# Install
sudo mv libdemo.so.2.0.1 /usr/lib/

# Run ldconfig
sudo ldconfig -v | grep libdemo
#   libdemo.so.2 -> libdemo.so.2.0.1   (changed) ← soname updated!

# Verify
ls -l /usr/lib/libdemo*
# lrwxrwxrwx libdemo.so   -> libdemo.so.2
# lrwxrwxrwx libdemo.so.2 -> libdemo.so.2.0.1    ← updated by ldconfig
# -rwxr-xr-x libdemo.so.2.0.0
# -rwxr-xr-x libdemo.so.2.0.1

# Notice: libdemo.so -> libdemo.so.2 -> libdemo.so.2.0.1
# The linker name chain is automatically "up to date" because:
# libdemo.so points to the soname, and ldconfig updated the soname.
# So the linker name is also effectively updated — no manual step needed!

4. Why Running Programs Survive — The inode Mechanism

Understanding why live upgrades work requires understanding how Linux handles file deletion and inode reference counts.

# === Demonstration: old process survives upgrade ===

# Terminal 1: Start a long-running process using the old library
./long_running_server &
SERVER_PID=$!
echo "Server PID: $SERVER_PID"

# Check which library version it loaded
cat /proc/$SERVER_PID/maps | grep libdemo
# 7f8abc000000-7f8abc001000 r-xp ... /usr/lib/libdemo.so.1.0.1

# Terminal 2: Install new version
sudo cp libdemo.so.1.0.2 /usr/lib/
sudo ldconfig -v | grep libdemo
# libdemo.so.1 -> libdemo.so.1.0.2  (changed)

# Check if server is still running — it is!
kill -0 $SERVER_PID && echo "Server still alive!"

# Server still references the OLD inode
cat /proc/$SERVER_PID/maps | grep libdemo
# 7f8abc000000-7f8abc001000 r-xp ... /usr/lib/libdemo.so.1.0.1
# ← STILL using 1.0.1 even though symlink now points to 1.0.2

# Start a NEW server instance
./long_running_server &
NEW_PID=$!

# New server gets the new version
cat /proc/$NEW_PID/maps | grep libdemo
# ... /usr/lib/libdemo.so.1.0.2  ← gets new version!

5. Minor vs Major Upgrade — Summary

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