Where do we find functions?

1. We've already begun writing our own functions, in the same file as main(), to simplify our code and to make it easier to read.

2. Soon, we'll write our own functions in other, multiple files, and call them from our main file.

3. Collections of related functions are termed libraries of functions.
   The most prominent example, that we've already seen, is C's standard library - a collection of frequently required functions that must be provided by a standards' conforming C compiler.
   In our programming, so far, we've already called library functions such as:
   printf(),  atoi(),  and  exit().

4. Similarly, there are many task-specific 3rd-party libraries. They are not required to come with your C compiler, but may be downloaded or purchased - from Lecture 1 -

   Application domain	(a sample of) 3rd-party libraries
   operating system services (files, directories, processes, inter-process communication)	OS-specific libraries, e.g. glibc, System32, Cocoa
   web-based programming	libcgi, libxml, libcurl
   data structures and algorithms	the generic data structures library (GDSL)
   GUI and graphics development	OpenGL, GTK, Qt, wxWidgets, UIKit, Win32, Tcl/Tk
   image processing (GIFs, JPGs, etc)	GD, libjpeg, libpng
   networking	Berkeley sockets, AT&T's TLI
   security, cryptography	openssl, libmp
   scientific computing	NAG, Blas3, GNU scientific library (gsl)
   concurrency, parallel and GPU programming	OpenMP, CUDA, OpenCL, openLinda (thread support is defined in C11, but not in C99)