The Linux Programming Interface

FUNDAMENTALS OF

SHARED LIBRARIES

Shared libraries are a technique for placing library functions into a single unit that

can be shared by multiple processes at run time. This technique can save both disk

space and RAM. This chapter covers the fundamentals of shared libraries. The next

chapter covers a number of advanced features of shared libraries.

41.1 Object Libraries

One way of building a program is simply to compile each of its source files to pro-

duce corresponding object files, and then link all of these object files together to

produce the executable program, like so:

$ cc -g -c prog.c mod1.c mod2.c mod3.c

$ cc -g -o prog_nolib prog.o mod1.o mod2.o mod3.o

Linking is actually performed by the separate linker program, ld. When we link

a program using the cc (or gcc) command, the compiler invokes ld behind the

scenes. On Linux, the linker should always be invoked indirectly via gcc, since

gcc ensures that ld is invoked with the correct options and links the program

against the correct library files.

In many cases, however, we may have source files that are used by several programs. As

a first step toward saving ourselves some work, we could compile these source files