Master Linux Static and Shared Libraries: Build, Link, and Load

Libraries greatly improve C/C++ code reuse, but beginners often find them challenging; this article provides a concise Linux‑focused overview of static libraries, shared libraries, and runtime‑loaded libraries.

Linux Library Types

Linux supports two main library formats:

Static library (.a) : linked directly into the executable at build time.

Shared library (.so) : can be linked at build time (visible to the linker) or loaded dynamically at runtime using explicit loading functions.

Naming Convention

Libraries must start with lib but the -l linker flag omits the prefix and suffix. Example: gcc src-file.c -lm -lpthread This links libm.a and libpthread.a.

Creating a Static Library (.a)

Compile object files, e.g., cc -Wall -c ctest1.c ctest2.c (produces ctest1.o and ctest2.o).

Create the archive: ar -cvq libctest.a ctest1.o ctest2.o.

Optionally list contents: ar -t libctest.a.

Generate a symbol table (often integrated): ranlib libctest.a.

The .a format corresponds to Windows .lib files.

Creating a Shared Library (.so)

Compile objects with position‑independent code: gcc -Wall -fPIC -c *.c.

Link with -shared and set the soname: gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 *.o.

Create convenient symlinks:

mv libctest.so.1.0 /opt/lib
ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so.1
ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so

The libctest.so link is used at compile time ( -lctest), while libctest.so.1 is the runtime version.

Link an application against the shared library:

gcc -Wall -I/path/to/include-files -L/path/to/libraries prog.c -lctest -o prog

Inspecting Dependencies

Use ldd prog to list required shared libraries, e.g.:

ldd prog
libctest.so.1 => /opt/lib/libctest.so.1 (0x00002aaaaaaac000)
libc.so.6 => /lib64/tls/libc.so.6 (0x0000003aa4e00000)
/lib64/ld-linux-x86-64.so.2 (0x0000003aa4c00000)

Object File Formats

Object files differ across systems: Windows uses PE, Linux/Unix use ELF, macOS uses Mach‑O, AIX uses XCOFF. Tools such as nm, objdump, and readelf can examine symbols.

Runtime Library Search Path

The system locates shared libraries via /etc/ld.so.conf and the cache updated by ldconfig. Environment variables like LD_LIBRARY_PATH (Linux/Solaris), LIBPATH (AIX), or DYLD_LIBRARY_PATH (macOS) also affect the search.

Dynamic Loading and Unloading

For plugin‑style architectures, libraries can be loaded at runtime using dlopen, dlsym, and dlclose from dlfcn.h. Executables that use this mechanism should be linked with -rdynamic to export all symbols to the dynamic symbol table.

GNU Libtool

Many projects use libtool, a wrapper script that simplifies building portable libraries. It generates .la files that reference the underlying .a or .so and contain version information.