Static And Shared Library

Code distribution among different programs and machines.

1. Introduction

A static library is a collection of object files that are linked into the program during the linking phase of compilation, becoming part of the final executable. A shared library is a collection of object files that are loaded into the program at runtime. Multiple programs can share the same library code in memory.

2. Example

2.1 Implementation

Source code can be found at: https://github.com/chuzcjoe/static_shared_lib

Project structure

.
├── CMakeLists.txt
├── README.md
├── main.cpp
└── math
    ├── CMakeLists.txt
    ├── mathlib.cpp
    └── mathlib.h

We have a simple math library that does simple math operations.

// mathlib.h
#pragma once

class MathLib {
public:
    MathLib();
    ~MathLib();

    double add(double, double);
    double subtract(double, double);
    double multiply(double, double);
    double divide(double, double);
};

// mathlib.cpp
#include "mathlib.h"
#include <iostream>

double MathLib::add(double a, double b) {
    return a + b;
}

double MathLib::subtract(double a, double b) {
    return a - b;
}

double MathLib::multiply(double a, double b) {
    return a * b;
}

double MathLib::divide(double a, double b) {
    if (b == 0) {
        throw std::domain_error("Division by zero.");
    }
    return a / b;
}

MathLib::MathLib() {}
MathLib::~MathLib() {}

We will compile it into both static and shared libraries.

cmake_minimum_required(VERSION 3.5.0)

project(Math VERSION 0.0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# Create a shared library
add_library(math_shared SHARED mathlib.cpp)
# Create a static library
add_library(math_static STATIC mathlib.cpp)

For the most outside CMakeLists.txt, we will generate different executables that link to static/shared libraries.

cmake_minimum_required(VERSION 3.5.0)

project(main VERSION 0.0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_subdirectory(${CMAKE_SOURCE_DIR}/math)

# add an executable that links to a shared library
add_executable(main_shared main.cpp)
target_include_directories(main_shared PRIVATE ${CMAKE_SOURCE_DIR}/math)
target_link_libraries(main_shared PRIVATE math_shared)

# add an executable that links to a static library
add_executable(main_static main.cpp)
target_include_directories(main_static PRIVATE ${CMAKE_SOURCE_DIR}/math)
target_link_libraries(main_static PRIVATE math_static)

2.2 Build

Let’s build it.

mkdir build
cd build
cmake ..
make

In the build folder, we can get two executables and inside the math folder, we have generated static/shared libraries. They should have the same output.

├── main_shared
├── main_static
└── math
    ├── CMakeFiles
    ├── Makefile
    ├── cmake_install.cmake
    ├── libmath_shared.dylib
    └── libmath_static.a

> ./main_static
input x and y: 10, 5
Addition: 15
Subtraction: 5
Multiplication: 50
Division: 2

> ./main_shared 
input x and y: 10, 5
Addition: 15
Subtraction: 5
Multiplication: 50
Division: 2

2.3 Inspect

We could use otool on MacOS to check the dependencies of each executable. Or on Linux we could use ldd.

> otool -L main_static 
main_static:
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 1500.65.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1319.100.3)

> otool -L main_shared 
main_shared:
	@rpath/libmath_shared.dylib (compatibility version 0.0.0, current version 0.0.0)
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 1500.65.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1319.100.3)

Because static libraries will become part of the executable, so libmath_static.a is not a dependency. On the other hand, shared libraries are loaded during runtime, so main_shared replies on libmath_shared.dylib.

2.4 File size

Linking a static library involves incorporating the entire library into the final executable. Compared with linking shared library, executables that link to static library usually will have larger file size.

> ls -lh main_static 
-rwxr-xr-x  1 zongchengchu  staff    40K Jun 29 13:55 main_static
> ls -lh main_shared 
-rwxr-xr-x  1 zongchengchu  staff    39K Jun 29 13:55 main_shared

2.5 Delete dependencies

If we delete both libmath_static.a and libmath_shared.dylib, main_static can still run without any issues.

> ./main_static 
input x and y: 10, 5
Addition: 15
Subtraction: 5
Multiplication: 50
Division: 2

However, main_shared needs to load libmath_shared.dylib at runtime, and it looks for the library at current project as well as system paths. If this library is not found, then we will get a link error.

> ./main_shared 
dyld[55250]: Library not loaded: @rpath/libmath_shared.dylib
  Referenced from: <4F8039F2-AFFB-303C-999C-FAF867191A7C> /Users/zongchengchu/Documents/repos/static_shared_lib/build/main_shared
  Reason: tried: '/Users/zongchengchu/Documents/repos/static_shared_lib/build/math/libmath_shared.dylib' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/Users/zongchengchu/Documents/repos/static_shared_lib/build/math/libmath_shared.dylib' (no such file), '/Users/zongchengchu/Documents/repos/static_shared_lib/build/math/libmath_shared.dylib' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/Users/zongchengchu/Documents/repos/static_shared_lib/build/math/libmath_shared.dylib' (no such file), '/usr/local/lib/libmath_shared.dylib' (no such file), '/usr/lib/libmath_shared.dylib' (no such file, not in dyld cache)

2.5 More

If a static/shared library depends on another library. The relationship can be:

dependencies	valid?
shared_lib links to static_lib	✗
shared_lib links to shared_lib	✓
static_lib links to static_lib	✓
static_lib links to shared_lib	✓

Shared libraries can only be linked to shared libraries. Static libraries can be linked to both shared and static library.

3. Conclusion

static library

• Advantages:

  • Simplicity: No external dependencies are required at runtime.
  • Performance: Sometimes faster to start and run due to the absence of the need to resolve symbols or load additional libraries at runtime.

• Disadvantages:

  • File Size: Can lead to larger executable sizes because each program has its own copy of the library.
  • Updates: If the library is updated, all applications using it must be recompiled and redeployed to benefit from the updates.

shared library

• Advantages:

  • Reduced Memory Usage: Since the library is shared in memory among multiple programs, it can reduce memory usage overall.
  • Ease of Updating: Updating a shared library can provide immediate benefits to all programs that use it, without needing recompilation.

• Disadvantages:

  • Complexity: Requires careful management of versions and dependencies to ensure compatibility.
  • Startup Time: Can have slower startup times as the operating system needs to link the external libraries dynamically when the program is launched.

void GLRender::nextImage() {
LOG_INFO(“switch to next image.”);
int next_index = (image_idx + 1) % IMAGE_NUM;
sdrImage.setFilepath(files[next_index]);

glBindTexture(GL_TEXTURE_2D, 0);
glBindTexture(GL_TEXTURE_2D, _texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

sdrImage.loadJPEG();
auto data = sdrImage.data();
int width = sdrImage.getWidth();
int height = sdrImage.getHeight();

if (data) {
    // generate texture
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
    glGenerateMipmap(GL_TEXTURE_2D);
    LOG_INFO("load texture success, width: %d, height: %d\n", width, height);

} else {
    LOG_ERROR("load texture error");
}

glUseProgram(tProgram);
glUniform1i(glGetUniformLocation(tProgram, "texture_load"), 0);

}