LDD(1) Linux Programmer's Manual LDD(1)

ldd - print shared object dependencies

ldd [option]... file...

ldd prints the shared objects (shared libraries) required by each program or shared object specified on the command line. An example of its use and output is the following:

$  ldd /bin/ls
        linux-vdso.so.1 (0x00007ffcc3563000)
        libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f87e5459000)
        libcap.so.2 => /lib64/libcap.so.2 (0x00007f87e5254000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f87e4e92000)
        libpcre.so.1 => /lib64/libpcre.so.1 (0x00007f87e4c22000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007f87e4a1e000)
        /lib64/ld-linux-x86-64.so.2 (0x00005574bf12e000)
        libattr.so.1 => /lib64/libattr.so.1 (0x00007f87e4817000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f87e45fa000)

In the usual case, ldd invokes the standard dynamic linker (see ld.so(8)) with the LD_TRACE_LOADED_OBJECTS environment variable set to 1. This causes the dynamic linker to inspect the program's dynamic dependencies, and find (according to the rules described in ld.so(8)) and load the objects that satisfy those dependencies. For each dependency, ldd displays the location of the matching object and the (hexadecimal) address at which it is loaded. (The linux-vdso and ld-linux shared dependencies are special; see vdso(7) and ld.so(8).)

Be aware that in some circumstances (e.g., where the program specifies an ELF interpreter other than ld-linux.so), some versions of ldd may attempt to obtain the dependency information by attempting to directly execute the program, which may lead to the execution of whatever code is defined in the program's ELF interpreter, and perhaps to execution of the program itself. (In glibc versions before 2.27, the upstream ldd implementation did this for example, although most distributions provided a modified version that did not.)
Thus, you should never employ ldd on an untrusted executable, since this may result in the execution of arbitrary code. A safer alternative when dealing with untrusted executables is:

$  objdump -p /path/to/program | grep NEEDED

Note, however, that this alternative shows only the direct dependencies of the executable, while ldd shows the entire dependency tree of the executable.

Print the version number of ldd.
-v, --verbose
Print all information, including, for example, symbol versioning information.
-u, --unused
Print unused direct dependencies. (Since glibc 2.3.4.)
-d, --data-relocs
Perform relocations and report any missing objects (ELF only).
-r, --function-relocs
Perform relocations for both data objects and functions, and report any missing objects or functions (ELF only).
Usage information.

ldd does not work on a.out shared libraries.
ldd does not work with some extremely old a.out programs which were built before ldd support was added to the compiler releases. If you use ldd on one of these programs, the program will attempt to run with argc = 0 and the results will be unpredictable.

pldd(1), sprof(1), ld.so(8), ldconfig(8)