# Run-time libraries#

To illustrate that even a simple program depends on external run-time libraries, here is the output from the ldd utility that reports such dependencies:

\$ ldd tabulate.exe
ntdll.dll => /cygdrive/c/WINDOWS/SYSTEM32/ntdll.dll (0x7ff88f2b0000)
KERNEL32.DLL => /cygdrive/c/WINDOWS/System32/KERNEL32.DLL (0x7ff88e450000)
KERNELBASE.dll => /cygdrive/c/WINDOWS/System32/KERNELBASE.dll (0x7ff88b9e0000)
cygwin1.dll => /usr/bin/cygwin1.dll (0x180040000)
cyggfortran-5.dll => /usr/bin/cyggfortran-5.dll (0x3efd20000)
cyggcc_s-seh-1.dll => /usr/bin/cyggcc_s-seh-1.dll (0x3f7000000)


Other compilers or other versions of the same compiler will probably require different dynamic libraries. As long as you run the program on the same computer — or, more accurately, within the same environment — there should be no problem. However, when such a library cannot be found, you will get (hopefully) an error message and the program stops immediately.

Therefore it is good to know what libraries are required. On Linux and Linux-like environments, the ldd utility is a great help. On Windows, you may want to use the dependency walker (the latest version, which works very nicely with Windows 10, is found here: https://github.com/lucasg/Dependencies)

Another thing you should know is where the program tries to find these libraries. That is a vast subject in its own right and full of complications and history. Here we merely scratch the surface:

On Linux:

• The environment variable LD_LIBRARY_PATH is used. It consists of a list of directories to be searched, each directory separated via colons (:) from the others. For instance: /usr/lib:/usr/local/lib — typical system directories.

• At the link step you can also use an option to set RPATH, a list of directories that is put into the executable file itself.

• Then there are several system directories that are searched.

On Windows:

• The directory containing the executable program may also contain dynamic libraries.

• The environment variable “PATH” is used. Again a list of directories to be searched, but now the separating character is the semicolon (;).

• A set of system directories is searched.

Unfortunately, the details can change from one version of the operating system to the next. The above is merely an indication — use tools like “ldd” or “dependency walker” to find out what libraries are loaded and where they are found.

If you want to share your program with colleagues or clients or simply users all over the world, you will have to take care that, besides the program, you also distribute the libraries it depends on. For more information: see below.