- Example program for C function (using call by reference): In this program, the address of the variables “m” and “n” are passed to the function “swap”. These values are not copied to formal parameters “a” and “b” in swap function. Because, they are just holding the address of those variables.
- Note: Replace the C: RepairSource Windows placeholder with the location of your repair source. For more information about using the DISM tool to repair Windows, reference Repair a Windows Image. At the command prompt, type the following command, and then press ENTER.
- The C language is similar to most modern programming languages in that it allows the use of functions, self contained 'modules' of code that take inputs, do a computation, and produce outputs. C functions must be TYPED (the return type and the type of all parameters specified).
As always, a function is a module of code that takes information in (referring to that information with local symbolic names called parameters), does some computation, and (usually) returns a new piece of information based on the parameter information.
Download the Microsoft Visual C Redistributable for Visual Studio 2015, 2017 and 2019. The following updates are the latest supported Visual C redistributable packages for Visual Studio 2015, 2017 and 2019. Included is a baseline version of the Universal C Runtime see MSDN for details. X86: vcredist.x86.exe. X64: vcredist.x64.exe.
Basic Function Design Pattern
For the basic syntax of a function in C, please refer to the C Function Design Pattern chapter.
Dot C files
The 'recipe' for a function (the function's code) is always stored in a '.C' file. In C there can be many functions written in a single file.
Ordering of functions in a file
The order of functions inside a file is arbitrary. It does not matter if you put function one at the top of the file and function two at the bottom, or vice versa.
Caveat: In order for one function to 'see' (use) another function, the 'prototype' of the function must be seen in the file before the usage. If a function uses another function that is textually written above it in the file, then this will automatically be true. If the function uses a function that is 'below it' in a file, then the prototype should occur at the top of the file... see prototypes below.
A Function Prototype
In C, all functions must be written to return a specific TYPE of information and to take in specific types of data (parameters). This information is communicated to the compiler via a function prototype.
Here is the syntax for the function declaration or Prototype:
A Prototype can occur at the top of a C source code file to describe what the function returns and what it takes (return type and parameter list). When this is the case (occuring at the top of the file), the function prototype should be followed by a semi-colon
The function prototype is also used at the beginning of the code for the function. Thus the prototype can occur twice in a C source code file. When the prototype occurs with the code NO semicolon is used.
The Main Function
In C, the 'main' function is treated the same as every function, it has a return type (and in some cases accepts inputs via parameters). The only difference is that the main function is 'called' by the operating system when the user runs the program. Thus the main function is always the first code executed when a program starts.
Examples of C Functions:
Return Type of a C function
Every C function must specify the type of data that is being generated. For example, the max function above returns a value of type 'double'. Inside the function, the line 'return X;' must be found, where X is a value or variable containing a value of the given type.
The return statement
When a line of code in a function that says: 'return X;' is executed, the function 'ends' and no more code in the function is executed. The value of X (or the value in the variable represented by X) becomes the result of the function.
Calling a C function (aka invoke a function)
When one piece of code invokes or calls a function, it is done by the following syntax:
The function name must match exactly the name of the function in the function prototype. The args are a list of values (or variables containing values) that are 'passed' into the function.
The number of args 'passed' into a function must exactly match the number of parameters required for the function. The type of each arg must exactly match the type of each parameter. The return variable type must exactly match the return type of the function.
The 'variable' in the example above must have a type equivalent to the return type of the function. Inside the function, somewhere will be the line 'return X'. The value of X is then copied into the 'variable'.
Parameters in C functions
A Parameter is the symbolic name for 'data' that goes into a function. There are two ways to pass parameters in C: Pass by Value, Pass by Reference.
Pass by Value
Pass by Value, means that a copy of the data is made and stored by way of the name of the parameter. Any changes to the parameter have NO affect on data in the calling function.
Pass by Reference
A reference parameter 'refers' to the original data in the calling function. Thus any changes made to the parameter are ALSO MADE TO THE ORIGINAL variable.
There are two ways to make a pass by reference parameter:
ARRAYS
Arrays are always pass by reference in C. Any change made to the parameter containing the array will change the value of the original array.
The ampersand used in the function prototype.
function ( & parameter_name )
To make a normal parameter into a pass by reference parameter, we use the '& param' notation. The ampersand (&) is the syntax to tell C that any changes made to the parameter also modify the original variable containing the data.
Using Chopsticks
Pass by Value Example:
In C, the default is to pass by value. For example:
Pass by Reference Example:
Warning: C++
I suggest that you use a C++ compiler such as g++ which allows the following pass by reference syntax (a much more modern style). The Syntax is to use the '&' in front of the parameter name in the function declaration. The calling code and usage inside the function are the same as before. For example:
Warning: Standard C - Using 'Pointers'
With standard C you have to put the & in the calling location as opposed to next to the parameter in the function declaration; further, you must use a '*' in the parameter list, and use a '*' whenever using the parameter inside the function.
The '*' is used to define a 'pointer', a discussion of which is beyond the scope of this simple example. Feel free to Google 'Pointers in C' for a long treatise on how to use them... or take my advice, and (as a beginning programmer) avoid them.
In summary, if you use a reference parameter, any changes to the parameter inside the function are reflected 'outside' of the function (i.e., in the calling function)! If you don't use the & (pass by reference), then we get the same behavior as in Matlab (i.e., the value is changed inside the called function, but maintains its original value in the calling function.
One reason to use reference parameters is to make the program more 'efficient'. Consider passing in a structure as a parameter. If the structure is very big, and we copy all of it, then we are using a lot of unnecessary memory.
Array Parameter Example (ALWAYS pass by reference)
Using Coffee Grounds In The Garden
Arrays are always passed by reference in C. They do not use the '&' notation, but are pass by reference none the less. For example:
Constant Reference
To protect from accidentally changing a reference parameter, when we really want it not to be changed (we just want to save time/memory) we can use the C keyword const. For example:
Void Functions
If a function does not return a value, then a special 'TYPE' is used to tell the computer this. The return type is 'void' (all lower case).
Void functions are mostly used in two classes of functions.
Using Condoms After Giving Birth
The first is a function that prints information for the user to read. For example (for our purposes), the printf function is treated as a void function. (In actuality, printf returns an integer which is the number of characters printed... but we almost always ignore this value.)
The second use of void functions is with 'reference' parameters (e.g., Arrays). A reference parameter is not a copy of the input data, as is so often the case. A reference parameter is an 'alias' for the same bucket in memory as the input data. Thus any change made to a reference parameter is in fact made to the original variable!
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