What is the Dynamic Linker?

How the Dynamic Linker Finds Libraries

The dynamic linker searches for shared libraries in this order:

ldconfig — The Library Cache Manager

Methods to Make a Shared Library Available

Coding Example 1 — Library Search Path Demonstration

We’ll create a shared library and demonstrate different ways to make it available, including the effect of ldconfig and LD_LIBRARY_PATH.

/* sysinfo.h - system info library */
#ifndef SYSINFO_H
#define SYSINFO_H

typedef struct {
    int  cpu_count;
    long total_ram_mb;
    char hostname[64];
} SystemInfo;

void sysinfo_get(SystemInfo *info);
void sysinfo_print(const SystemInfo *info);

#endif

/* sysinfo.c - implementation */
#include "sysinfo.h"
#include <stdio.h>
#include <string.h>
#include <unistd.h>

/* Must be compiled with -fPIC for shared library use! */

void sysinfo_get(SystemInfo *info) {
    if (!info) return;

    /* Get number of CPUs */
    info->cpu_count = (int)sysconf(_SC_NPROCESSORS_ONLN);

    /* Get total RAM */
    long pages = sysconf(_SC_PHYS_PAGES);
    long page_size = sysconf(_SC_PAGE_SIZE);
    info->total_ram_mb = (pages * page_size) / (1024 * 1024);

    /* Get hostname */
    gethostname(info->hostname, sizeof(info->hostname));
}

void sysinfo_print(const SystemInfo *info) {
    if (!info) return;
    printf("=== System Information ===\n");
    printf("Hostname  : %s\n", info->hostname);
    printf("CPU Cores : %d\n", info->cpu_count);
    printf("Total RAM : %ld MB\n", info->total_ram_mb);
}

/* app_sysinfo.c - application using sysinfo shared library */
#include <stdio.h>
#include "sysinfo.h"

int main(void) {
    SystemInfo si;
    sysinfo_get(&si);
    sysinfo_print(&si);

    printf("\nThis app uses libsysinfo.so.1 shared library\n");
    return 0;
}

## ============================================
## BUILD AND INSTALL THE SHARED LIBRARY
## ============================================

## Step 1: Build the shared library
$ gcc -g -fPIC -c sysinfo.c -o sysinfo.o
$ gcc -g -shared -Wl,-soname,libsysinfo.so.1 \
    -o libsysinfo.so.1.0.0 sysinfo.o
$ ln -sf libsysinfo.so.1.0.0 libsysinfo.so.1
$ ln -sf libsysinfo.so.1     libsysinfo.so

## Step 2: Compile the application
$ gcc -g -o app_sysinfo app_sysinfo.c -L. -lsysinfo

## Step 3: Try running — will FAIL without telling linker where .so is
$ ./app_sysinfo
## Error: ./app_sysinfo: error while loading shared libraries:
##        libsysinfo.so.1: cannot open shared object file

## ---- SOLUTION A: LD_LIBRARY_PATH (for testing) ----
$ LD_LIBRARY_PATH=. ./app_sysinfo   # Works!

## ---- SOLUTION B: RPATH (embed path in binary) ----
## Recompile with rpath set to current directory
$ gcc -g -o app_sysinfo_rpath app_sysinfo.c \
    -L. -lsysinfo \
    -Wl,-rpath,'$ORIGIN'    # $ORIGIN = directory of the binary itself
$ ./app_sysinfo_rpath       # Works without LD_LIBRARY_PATH!

## Verify the rpath is embedded:
$ readelf -d app_sysinfo_rpath | grep -E "RPATH|RUNPATH"

## ---- SOLUTION C: Install to system (production) ----
## Copy to /usr/local/lib
$ sudo cp libsysinfo.so.1.0.0 /usr/local/lib/

## Create symlink
$ sudo ln -sf libsysinfo.so.1.0.0 /usr/local/lib/libsysinfo.so.1

## Update the ld.so.cache
$ sudo ldconfig

## Verify ldconfig sees it:
$ ldconfig -p | grep sysinfo
## libsysinfo.so.1 (libc6,x86-64) => /usr/local/lib/libsysinfo.so.1.0.0

## Now run without LD_LIBRARY_PATH:
$ ./app_sysinfo   # Works!

## ---- SOLUTION D: Add custom dir to ld.so.conf ----
$ echo "/home/ravi/mylibs" | sudo tee /etc/ld.so.conf.d/mylibs.conf
$ sudo ldconfig

The ldd Tool — Inspect Library Dependencies

Coding Example 2 — Using ldd and Debugging Library Issues

## ============================================
## USING ldd TO INSPECT LIBRARY DEPENDENCIES
## ============================================

## Show all shared libraries needed by a system program:
$ ldd /usr/bin/python3
## Sample output:
##     linux-vdso.so.1 (0x00007ffe...)
##     libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
##     libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2
##     libutil.so.1 => /lib/x86_64-linux-gnu/libutil.so.1
##     libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6
##     libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6
##     /lib64/ld-linux-x86-64.so.2 (0x00007f...)

## Check our own app:
$ ldd app_sysinfo
##     libsysinfo.so.1 => not found     (← this is the problem!)
##     libc.so.6 => /lib/...

## "not found" means dynamic linker cannot locate libsysinfo.so.1

## After installing (Solution C above):
$ ldd app_sysinfo
##     libsysinfo.so.1 => /usr/local/lib/libsysinfo.so.1 (0x7f...)

## ============================================
## COMMON TROUBLESHOOTING COMMANDS
## ============================================

## 1. List all libraries in the ld.so.cache:
$ ldconfig -p | head -20

## 2. Search for a specific library in the cache:
$ ldconfig -p | grep libz
## libz.so.1 (libc6,x86-64) => /lib/x86_64-linux-gnu/libz.so.1

## 3. Run with verbose library loading (debug):
$ LD_DEBUG=libs ./app_sysinfo 2>&1 | head -30
## Shows every library the dynamic linker tries to open

## 4. Show library search details:
$ LD_DEBUG=libs,files ./app_sysinfo 2>&1

## 5. Check if a .so has missing dependencies:
$ ldd -r libsysinfo.so.1.0.0
## "undefined symbol" lines mean unresolved references

## ============================================
## BUILDING FOR EMBEDDED: Static vs Dynamic
## ============================================

## Compare binary sizes:
$ gcc -o app_dynamic app_sysinfo.c -L. -lsysinfo
$ gcc -static -o app_static app_sysinfo.c sysinfo.c
$ ls -lh app_dynamic app_static

## For embedded Linux (e.g., Buildroot, Yocto):
## Shared libraries save space when multiple apps share them
## Static linking is useful for initramfs or single-binary deployments

## Check if a binary is dynamically or statically linked:
$ file app_dynamic
## ELF 64-bit LSB pie executable, ..., dynamically linked

$ file app_static
## ELF 64-bit LSB executable, ..., statically linked

Interview Questions — Section 41.5