In C++, we can build a library either statically or dynamically. In this post, I want to summarize how to build a static and a shared/dynamic library1.

# Build a static and dynamic library

Suppose we have the following source and header files.

foo.h:

#ifndef _FOO_H
#define _FOO_H
void foo();

#endif


foo.cpp:

#include "foo.h"
#include <iostream>

void foo(){
std::cout << "This is foo" << std::endl;
}


main.cpp:

 #include "foo.h"

int main()
{
foo();
return 0;
}


Here is how to build a dynamic and static library:

g++ -fPIC -c foo.cpp

# build a shared library
g++ -shared -fPIC -o libfoo.so foo.o

# build a static library
ar rvs libfoo.a foo.o

# build main.o object file
g++ -c main.cpp


# Use a static and dynamic library

To link the static library and main.o to produce an executable, we use:

g++ main.o libfoo.a -o main


The order of object file and library file matters. The following won’t work for static library:

g++ libfoo.a main.o -o main


It will result in the following error:

main.o: In function main': main.cpp:(.text+0x5): undefined reference to foo()' collect2: error: ld returned 1 exit status

It is because the linker process the files from left to right. If we put static library before the object file, there is no unresolved symbols yet, all symbol in the static lib will be dropped. When the object file is processed, the symbol foo() can be resolved since it is the last object that is processed.

According to here, if you pass --start-group to linker via -Wl,--start-group, the linker will search all libraries for unresolved symbol. So the following works:

g++ -Wl,--start-group libfoo.a main.o -o main_static


If we link the dynamic library with main.o, things are different. The following two commands both work:

g++ libfoo.so main.o -o main
g++ main.o libfoo.so -o main


In this case, the order of library and the object file does not matter. According to answer here, if we pass --as-needed option to linker via -Wl,--as-needed, then the library file must follow the object file that depend on it.

g++ -Wl,--as-needed libfoo.so main.o -o main_shared  # will fail with undefined reference error


# References

1. For the pros and cons of using static VS dynamic library, see this post↩︎