How Functions Work

Functions are composed of several different pieces:

function name

• A function's name is a symbol that represents the address where the function's code starts. In assembly language, the symbol is defined by typing the function's name as a label before the function's code. This is just like labels you have used for jumping.

function parameters

• A function's parameters are the data items that are explicitly given to the function for processing. For example, in mathematics, there is a sine function. If you were to ask a computer to find the sine of 2, sine would be the function's name, and 2 would be the parameter. Some functions have many parameters, others have none.^[1]

local variables

• Local variables are data storage that a function uses while processing that is thrown away when it returns. It's kind of like a scratch pad of paper. Functions get a new piece of paper every time they are activated, and they have to throw it away when they are finished processing. Local variables of a function are not accessible to any other function within a program.

static variables

• Static variables are data storage that a function uses while processing that is not thrown away afterwards, but is reused for every time the function's code is activated. This data is not accessible to any other part of the program. Static variables are generally not used unless absolutely necessary, as they can cause problems later on.

global variables

• Global variables are data storage that a function uses for processing which are managed outside the function. For example, a simple text editor may put the entire contents of the file it is working on in a global variable so it doesn't have to be passed to every function that operates on it.^[2] Configuration values are also often stored in global variables.

return address

• The return address is an "invisible" parameter in that it isn't directly used during the function. The return address is a parameter which tells the function where to resume executing after the function is completed. This is needed because functions can be called to do processing from many different parts of your program, and the function needs to be able to get back to wherever it was called from. In most programming languages, this parameter is passed automatically when the function is called. In assembly language, the call instruction handles passing the return address for you, and ret handles using that address to return back to where you called the function from.

return value

• The return value is the main method of transferring data back to the main program. Most programming languages only allow a single return value for a function.

These pieces are present in most programming languages. How you specify each piece is different in each one, however.

The way that the variables are stored and the parameters and return values are transferred by the computer varies from language to language as well. This variance is known as a language's calling convention, because it describes how functions expect to get and receive data when they are called.^[3]

Assembly language can use any calling convention it wants to. You can even make one up yourself. However, if you want to interoperate with functions written in other languages, you have to obey their calling conventions. We will use the calling convention of the C programming language for our examples because it is the most widely used, and because it is the standard for Linux platforms.

^[1]Function parameters can also be used to hold pointers to data that the function wants to send back to the program.

^[2]This is generally considered bad practice. Imagine if a program is written this way, and in the next version they decided to allow a single instance of the program edit multiple files. Each function would then have to be modified so that the file that was being manipulated would be passed as a parameter. If you had simply passed it as a parameter to begin with, most of your functions could have survived your upgrade unchanged.

^[3]A convention is a way of doing things that is standardized, but not forcibly so. For example, it is a convention for people to shake hands when they meet. If I refuse to shake hands with you, you may think I don't like you. Following conventions is important because it makes it easier for others to understand what you are doing, and makes it easier for programs written by multiple independent authors to work together.