Where my library get installed in this makefile. How to change location of installed headers and .so.0 files

3 min read 05-10-2024

Where my library get installed in this makefile. How to change location of installed headers and .so.0 files

Understanding Library Installation Paths in Makefiles

When working with Makefiles, you might wonder where your compiled library gets installed. This is a common question, especially for beginners. In this article, we'll delve into the core concepts and provide you with the knowledge to customize the installation location of your library's headers and shared object files (.so).

The Scenario: A Makefile with Missing Installation Details

Imagine you have a simple Makefile for building a library:

CXX = g++
CXXFLAGS = -Wall -g
LDFLAGS = -L. -lmylib

mylib.so: mylib.o
	$(CXX) $(LDFLAGS) -o $@ $^

mylib.o: mylib.cpp
	$(CXX) $(CXXFLAGS) -c {{content}}lt; -o $@

install: mylib.so
	cp mylib.so /usr/local/lib
	cp mylib.h /usr/local/include

clean:
	rm -f *.o mylib.so

This Makefile compiles your library (mylib.cpp) into a shared object file (mylib.so) and includes the basic steps for installation. However, it hardcodes the installation directory to /usr/local/lib for the shared object file and /usr/local/include for the header file.

Understanding the Problem: Hardcoded Paths and Limited Flexibility

The major problem with this approach is the hardcoding of installation paths. It lacks flexibility for:

System-wide vs. User-specific installations: You might want to install the library for your own use (user-specific) or for all users on the system (system-wide).
Non-standard installation locations: Project requirements might dictate using a custom installation location.
Multiple libraries with different install paths: Managing separate installation paths for different libraries can become tedious.

The Solution: Using Variables and Makefile Rules

Instead of hardcoding paths, we can use variables and Makefile rules to make installation paths more flexible and customizable.

Define Installation Variables: Introduce variables in your Makefile to represent the target installation directories for your library.

PREFIX = /usr/local
DESTDIR = $(PREFIX)

# ... other Makefile rules ...

install: mylib.so
	install -m 0644 mylib.h $(DESTDIR)/include
	install -m 0644 mylib.so $(DESTDIR)/lib

This modification defines PREFIX and DESTDIR variables to represent the root installation directory and a potential destination directory, respectively. The install rule now utilizes these variables for flexible installation.

Use DESTDIR for System-Wide and User-Specific Installations: The DESTDIR variable allows you to specify a prefix to the installation path. This is useful when you want to install libraries in a specific directory:

User-specific installations: You can set DESTDIR to ~/local to install the library in your home directory.
System-wide installations (as root): You can set DESTDIR to /usr/local for a system-wide installation.

Managing Multiple Libraries: With DESTDIR and PREFIX, you can effectively control installation locations for different libraries. Simply adjust these variables for each library's Makefile.

Enhancing the Makefile: Additional Tips

Use $(shell ...) to Determine Installation Paths: If you need dynamic determination of installation paths based on system variables or other factors, use $(shell ...) to execute commands within the Makefile.
Leverage make install with Custom Rules: You can use make install to trigger your custom installation rules. This is a common convention, making your Makefile more aligned with standard practices.
Define Default Installation Paths: Set default values for PREFIX and DESTDIR based on your project needs. This ensures consistent installation behavior when these variables are not explicitly defined.

Conclusion: Building a Flexible and Maintainable Makefile

By using variables and DESTDIR, you can effectively manage the installation locations of your library. This not only simplifies the installation process but also provides greater flexibility for various project requirements. Remember to adjust the Makefile based on your specific needs and consider utilizing best practices for managing installation paths.

References and Resources:

GNU Make Manual: Comprehensive documentation on the capabilities of Makefiles.
Makefile Tutorial: A beginner-friendly introduction to Makefile concepts.
Using DESTDIR with make: Discusses the use of DESTDIR in detail for installing to alternate locations.