How to Build and Link a Shared Library (.so) on Linux with GCC

Background

Based on Chapter 5 of C Expert Programming , the article demonstrates how to build and link a shared library on Linux.

File Naming Conventions

Shared libraries use the .so extension and are named libname.so (optionally with version numbers). Static libraries use libname.a. The transitional libname.sa is rarely used.

Environment

Operating system: Debian GNU/Linux 2.6.21‑2‑686.

GCC version: 4.1.3.

Library Source (myfunction.c)

/* Author: Godbach */
/* E‑mail: [email protected] */
#include <stdio.h>

int my_lib_function(void)
{
    printf("Library routine called from libmyfunction.so!
");
    return 0;
}

Compiling the Shared Library

From the directory /home/program/ run:

gcc -fPIC -shared -o libmyfunction.so myfunction.c

The -fPIC option generates position‑independent code required for shared objects. The -shared flag tells GCC to produce a shared library rather than an executable. It is advisable to compile as a regular user; use sudo only when installing the library system‑wide.

Test Program (test.c)

#include <stdio.h>

int main(void)
{
    my_lib_function();
    return 0;
}

Linking and Running with an Absolute Path

Compile the test program by specifying the full path to the shared library:

gcc -o test test.c /home/program/libmyfunction.so
./test

Output:

Library routine called from libmyfunction.so!

Linking via System Library Path

To avoid an absolute path, copy the library to a standard directory such as /usr/lib/ and refresh the linker cache (optional):

sudo cp libmyfunction.so /usr/lib/
sudo ldconfig   # refresh the cache
gcc -o test test.c -lmyfunction
./test

The -lmyfunction option directs the linker to search for libmyfunction.so (or libmyfunction.a) in the library search paths, omitting the lib prefix and file extension. Ensure that the directory containing the library is listed in /etc/ld.so.conf or referenced via the LD_LIBRARY_PATH environment variable.