Datasheet

Your library is now ready to use. You can add to the list of files to be used by the compiler to create your

program like this:

$ gcc -o program program.o libfoo.a

$ ./program

bill: we passed Hello World

You could also use the –l option to access the library, but because it is not in any of the standard places,

you have to tell the compiler where to find it by using the

–L option like this:

$ gcc –o program program.o –L. –lfoo

The –L. option tells the compiler to look in the current directory (.) for libraries. The –lfoo option tells

the compiler to use a library called

libfoo.a (or a shared library, libfoo.so, if one is present). To see

which functions are included in an object file, library, or executable program, you can use the

nm com-

mand. If you take a look at

program and lib.a, you see that the library contains both fred and bill,

but that

program contains only bill. When the program is created, it includes only functions from the

library that it actually needs. Including the header file, which contains declarations for all of the func-

tions in the library, doesn’t cause the entire library to be included in the final program.

If you’re familiar with Windows software development, there are a number of direct analogies here,

illustrated in the following table.

Shared Libraries

One disadvantage of static libraries is that when you run many applications at the same time and they

all use functions from the same library, you may end up with many copies of the same functions in

memory and indeed many copies in the program files themselves. This can consume a large amount of

valuable memory and disk space.

Many UNIX systems and Linux-support shared libraries can overcome this disadvantage. A complete

discussion of shared libraries and their implementation on different systems is beyond the scope of this

book, so we’ll restrict ourselves to the visible implementation under Linux.

Shared libraries are stored in the same places as static libraries, but shared libraries have a different filename

suffix. On a typical Linux system, the shared version of the standard math library is

/lib/libm.so.

When a program uses a shared library, it is linked in such a way that it doesn’t contain function code

itself, but references to shared code that will be made available at run time. When the resulting program

is loaded into memory to be executed, the function references are resolved and calls are made to the

shared library, which will be loaded into memory if needed.

Item UNIX Windows

object module

func.o FUNC.OBJ

static library

lib.a LIB.LIB

program

program PROGRAM.EXE

Chapter 1: Getting Started

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