### 13.3 Finding dynamically linked libraries ¶

When a program has been compiled using shared libraries it needs to load those libraries dynamically at run-time in order to call external functions. The command `ldd` examines an executable and displays a list of the shared libraries that it needs. These libraries are referred to as the shared library dependencies of the executable.

For example, the following commands demonstrate how to find the shared library dependencies of the Hello World program:

    $ gcc -Wall hello.c
    $ ldd a.out
    libc.so.6 => /lib/libc.so.6 (0x40020000)
    /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

The output shows that the Hello World program depends on the C library `libc` (shared library version 6) and the dynamic loader library `ld-linux` (shared library version 2).

If the program uses external libraries, such as the math library, these are also displayed. For example, the `calc` program (which uses the `sqrt` function) generates the following output:

    $ gcc -Wall calc.c -lm -o calc
    $ ldd calc
    libm.so.6 => /lib/libm.so.6 (0x40020000)
    libc.so.6 => /lib/libc.so.6 (0x40041000)
    /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

The first line shows that this program depends on the math library `libm` (shared library version 6), in addition to the C library and dynamic loader library.

The `ldd` command can also be used to examine shared libraries themselves, in order to follow a chain of shared library dependencies.