Many common or standard functions, whose definitions have already been written, are ready to be used in any program. They are organized into a group of functions (think of them as several books) and are collectively called a standard library. There are many functions organized into several libraries For example, within most programming languages many math functions exist and have been coded (and placed into libraries). These functions were written by programmers and tested to ensure that they work properly. In most cases, the functions were reviewed by several people to double and triple check to ensure that they did what was expected. We have the advantage of using these functions with confidence that they will work properly in our programs, thus saving us time and money.
The main program must establish the existence of functions used in that program. Depending on the programming language, there is a formal way to:
- define a function
- declare a function (a prototype is a declaration to a compiler)
- call a function
When we create functions in our program, we usually see them in the following order in our source code listing:
- declare the function (prototype)
- call the function
- define the function
When we use functions created by others that have been organized into a library, we include a header file in our program which contains the prototypes for the functions. Just like functions that we create, we see them in the following order in our source code listing:
- declaring the function (prototype provided in the include file)
- call the function (with parameter passing of values)
- define the function (it is either defined in the header file or the linker program provides the actual object code from a Standard Library object area)
In most cases, the user can look at the prototype and understand exactly how the communications (parameter passing) into and out of the function will occur when the function is called. Let’s look at the math example of absolute value.
Not wanting to have a long function name the designers named it: abs instead of “absolute”. This might seem to violate the identifier naming rule of using meaningful names, however, when identifier names are established for standard libraries they are often shortened to a name that is easily understood by all who would be using them. If I had two integer variables named apple and banana; and I wanted to store the absolute value of banana into apple; then a line of code to call this function would be:
apple = abs(banana);
Let’s say it in English, pass the function absolute the value stored in variable banana and assign the returning value from the function to the variable apple. Thus, if you know the prototype you can usually properly call the function and use its returning value (if it has one) without ever seeing the definition of the code (i.e. the source code that tells the function how to get the answer; that is written by someone else; and either included in the header file or compiled and placed into an object library; and linked during the linking step of the Integrated Development Environment (IDE).
- A function within a standard library which stands for absolute value.
- The reliance that Standard Library functions work properly.
- standard library
- A set of specific task functions that have been added to the programming language for universal use.