The Linux Programming Interface

Fundamentals of Shared Libraries 843

Figure 41-2: Execution of a program that loads a shared library

41.5 Useful Tools for Working with Shared Libraries

In this section, we briefly describe a few tools that are useful for analyzing shared

libraries, executable files, and compiled object (.o) files.

The ldd command

The ldd(1) (list dynamic dependencies) command displays the shared libraries that

a program (or a shared library) requires to run. Here’s an example:

$ ldd prog

libdemo.so.1 => /usr/lib/libdemo.so.1 (0x40019000)

libc.so.6 => /lib/tls/libc.so.6 (0x4017b000)

/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

ld-linux.so

Header

Program header

Shared object dependencies:

/lib/ld-linux.so.2

mod1.o code libbar.so

mod2.o code

mod3.o code

libfoo.so

prog.o code

prog libbar.so

$ LD_LIBRARY_PATH=. ./prog

1

Process created; dynamic

loader (ld-linux.so) and

prog loaded into memory

environ: LD_LIBRARY_PATH=.

prog.o code

2

Dynamic linker

examines shared

object dependencies

3 libbar.so

found in “.”.

libfoo.so

4

libfoo.so

loaded into

virtual memory

process virtual

memory

Current directory

file system

Program header

Shared object dependencies:

/lib/ld-linux.so.2

libbar.so

symbolic link

to “libfoo.so”