There are two kinds of library: static libraries and shared
libraries. When you link to a static library, the code for the entire
library is merged with the object code for your program. If you link to
many static libraries, your executable will be enormous.
Shared libraries were developed in the late 1980s to reduce the code
size of programs on operating systems like GNU. When you link to a
shared library, the library's code is not merged with your program's
object code. Instead, stub code is inserted into your object
code. The stub code is very small and merely calls the functions in the
shared library -- the operating system does the rest. An executable
created with a shared library can therefore be far smaller than one
created with a static library. Shared libraries can also reduce the
amount of memory used.
Although shared libraries seem to have every advantage over static
libraries, static libraries are still useful. For example, sometimes
you will wish to distribute an executable to people whose computers do
not have the libraries that yours does. In that case, you might link to
a static version of the libraries. This will incorporate the library
functions that you need into your executable, so that it will run on
systems that don't have those libraries. (It is also sometimes easier
to debug a program that is linked to static libraries than one linked to
shared libraries. See Introduction to GDB, for more information.)
The file name for a library always starts with lib and ends with
either .a (if it is static) or .so (if it is shared). For
example, libm.a is the static version of the C math library, and
libm.so is the shared version. As explained above, you must use
the -l option with the name of a library, minus its lib
prefix and .a or .so suffix, to link that library to your
program (except the library glibc, which is always linked). For
example, the following shell command creates an executable program
called math from the source code file math.c and the
library libm.so.
gcc -o math math.c -lm
The shared version of the library is always linked by default. If you
want to link the static version of the library, you must use the GCC
option --static. The following example links libm.a
instead of libm.so.
gcc -o math math.c -lm --static
Type info gcc at your shell prompt for more information about GCC options.
