C static libraries

Why use libraries?

Software is made up of functions. Functions are kind of like mini-programs that can be “called” by another program to perform some type of action such as a computation or modification of input.

How they work

Static libraries, also called “archives”, are just collections of object files that contain functions. All the functions within the library are organized and indexed with a symbol and address, kind of like a table of contents, which makes it easier to find what you’re looking for.

Static libraries are joined to the main module of a program during the linking stage of compilation before creating the executable file. After a successful link of a static library to the main module of a program, the executable file will contain both the main program and the library.

How to create them

Static libraries are created using some type of archiving software, such as ar. ar takes one or more object files (that end in .o), zips them up, and generates an archive file (ends in .a) — This is our “static library”.

Before using ar, we must first have some object files to give to it. Perhaps we’ve written some functions in C that we want to include in our library. We can use the -c option with the GNU compiler (gcc) to stop the compiling process after the assembling stage, translating our files from .c to .o.

$ gcc -c sum.c // produces a sum.o object file

Now that we have the object file(s), we can archive them and make a static library using ar.

$ ar -rc libforme.a sum.o

The command above will create a static library called “libforme.a”. Inside of it will be our sum() function that has been translated to an object file via the gcc command earlier.

Note: the -rc options create the archive without a warning and replaces any pre-existing object files in the library with the same name.

Some archivers automatically organize and index the library, but in case indexing didn’t occur, we can use a command called ranlib to generate and store an index in the archive.

The index lists each symbol defined by a member of an archive that is a relocatable object file. To see a list of the symbols from object files we can use a command called nm.

$ nm libforme.a// sample outputsum.o:000000000000002e T sum

So now that we’ve created object files, zipped them in up an library and indexed it, we are ready to use our library.

How to use them

Let’s say we want to compile our C program from the beginning of this post; the one that uses sum() to store the value of two numbers:

int main(void)
{
    int x = 5;
    int y = 8;
    int result;    result = sum(x, y);
    return (0);
}

When we try to compile the program we might see an error like this:

gcc my_program.c// oops... ///tmp/ccGLAk66.o: In function `main':my_program.c:(.text+0x26): undefined reference to `sum'collect2: error: ld returned 1 exit status

The compiler doesn’t know what sum is. We need to tell it to look in our library:

gcc my_program.c -L. -lforme -o my_program