Libraries in C

What are libraries and why use libraries in c ?

Types of libraries in C:

Static libraries

Shared or dynamic libraries

How static libraries work

What is the difference between them

Creating A Static “C” Library

gcc -c *.c

This will create * .o files that will serve us to create the library, for that we need to use the following command:

ar -rc liball.a *.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 liball.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last modification date (stored in the file system), a compiler trying to use this library will complain its index is out of date, and abort.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

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.


First setp compiple with gcc and obtain files *.o:

using the command to create lirabrie with files *.o : ar rc libutil.a util_file.o

this created the file libHolbertonSchool.a, after this we can use ‘ranlib’ to generate the index , And our dynamic library would already be created

Creating A Dynamic “C” Library

gcc -c -fPIC *.c

Next, we are going to put together those objects files into one library. To do this as a Dynamic Library we also use gcc but with the -shared option. The -o is to specify the name of the file you want it to have.

gcc -shared -o *.o

This way you must have your library created. To verify that you did it and have the right functions as dynamic symbols you can use:

nm -D

Great! at this point, you have your Dynamic Library created!

How to use it


Then, you can compile it by typing the following:

gcc -L . 0-main.c -l all -o example

Note that the name we gave to the library in this example was ‘all’. Here we use the -L option to tell the program where to find the library, in this case . that refers to the current directory. The -l option is to tell the compiler to look for the library.


Create a Dynamic library from the following * .c files

Compile files *.c with gcc [ gcc *.c -c -fpic ]:

This created files *.o after this use [ gcc *.o -shared -o ]:

This will produce the file and that is all , enjoy your dynamic library :)

To see your librarie use the next command :

[ nm -D — defined-only ]

Only those who risk going too far can discover how far they can go — T.S. Eliot

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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