C - Static libraries

What are libraries and why use libraries in c ?

A library is a file containing several object files, that can be used as a single entity in a linking phase of a program. Normally the library is indexed, so it is easy to find symbols (functions, variables and so on) in them. For this reason, linking a program whose object files are ordered in libraries is faster than linking a program whose object files are separate on the disk. Also, when using a library, we have fewer files to look for and open, which even further speeds up linking.

Types of libraries in C:

There are two types of libraries in C.

Static libraries

Static libraries are just collections of object files that are linked into the program during the linking phase of compilation, and are not relevant during runtime. This last comment seems obvious, as we already know that object files are also used only during the linking phase, and are not required during runtime — only the program’s executable file is needed in order to run the program.

Shared or dynamic libraries

Shared libraries are linked into the program in two stages. First, during compile time, the linker verifies that all the symbols (again, functions, variables and the like) required by the program, are either linked into the program, or in one of its shared libraries. However, the object files from the dynamic library are not inserted into the executable file. Instead, when the program is started, a program in the system (called a dynamic loader) checks out which shared libraries were linked with the program, loads them to memory, and attaches them to the copy of the program in memory.

How static libraries work

What is the difference between them

Creating A Static “C” Library

The basic tool used to create static libraries is a program called 'ar', for 'archiver'. In order to create a static library, we can use a command like this:

ar rc libutil.a util_file.o

This command creates a static library named ‘libutil.a’ and puts copie of the object file “util_file.o” .If the library file already exists, it has the object file added to it, or replaced, if they are newer than those inside the library. The 'c' flag tells ar to create the library if it doesn't already exist. The 'r' flag tells it to replace older object file in the library, with the new object file.

After an archive is created, or modified, there is a need to index it. This index is later used by the compiler to speed up symbol-lookup inside the library, and to make sure that the order of the symbols in the library won’t matter during compilation.The command used to create or update the index is called 'ranlib', and is invoked as follows:

ranlib libutil.a

Note: when an archive file’s index generation date (stored inside the archive file) is older than the file’s last modification date (stored in the file system), a compiler trying to use this library will complain its index is out of date, and abort. There are two ways to overcome the problem:

  1. Use 'ranlib' to re-generate the index.
  2. When copying the archive file to another location, use 'cp -p', instead of only 'cp'. The '-p' flag tells 'cp' to keep all attributes of the file, including its access permissions, owner (if "cp" is invoked by a superuser) and its last modification date. This will cause the compiler to think the index inside the file is still updated. This method is useful for makefiles that need to copy the library to another directory for some reason.

How to use libraries in C

After we created our archive, we want to use it in a program. This is done by adding the library’s name to the list of object file names given to the linker, using a special flag, normally '-l'. Here is an example:

gcc main.o -L. -lutil -o prog

This will create a program using object file "main.o", and any symbols it requires from the "util" static library. Note that we omitted the "lib" prefix and the ".a" suffix when mentioning the library on the link command. The linker attaches these parts back to the name of the library to create a name of a file to look for. Note also the usage of the '-L' flag - this flag tells the linker that libraries might be found in the given directory ('.', refering to the current directory), in addition to the standard locations where the compiler looks for system libraries.

“Programming is a race between developers, trying to build bigger and better idiot-proof programs, and the universe, trying to produce bigger and better idiots. For now the Universe is winning”
- Rich Cook

I hope this article has helped you understand about C libraries and their great importance in programming in C and C++. Grettings!)

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