C Functions

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C Functions
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Functions

• May return only one value.
• May receive many formal parameters.
• Should only have one entrance and one exit.
• Parameters are always passed by value.
• Passing a pointer looks like passing by
  reference.
• Pointer is passed by values.

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main The way in.

• The main function is a function like any other.
• It defines the entry point to your program.

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Function definitions

• Functions are defined in three parts.
• Return data type.
• Name of the function (subject to variable naming
  rules.
• Parameters to be passed into the function.
• void main( void)
• void - indicates that the function does not
  return a value.
• main name of the function.
• (void) indicates that no parameters are passed
  into the function.

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Function prototypes

• A function prototype is a forward reference to a
  function.
• They provide a way to define functions prior to
  their real definition.
• int add( int, int)
• Note that formal parameters are not necessary
  only data types.

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Function Definition

• To define a function start with the function
  header.
• Define the return data type.
• Define the function name.
• Define the formal parameters.
• int add( int a, int b)
• return ab

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Function Definition

• No semicolon at the end of the function
  definition.
• Formal parameters a and b are declared.
• Returns the value of ab.
• Example
• in main function.
• int a add(2, 3)
• returns the sum of 2 and 3 (5).
• stores this value in the variable a.

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Dealing with structs

• Since parameters are passed by value structs are
  copied to the function.
• With large structures this may be time consuming
  ( and wasteful of space ).
• The alternative is to pass in the address of your
  structure. (A pointer to the structure.)

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Dealing With Pointers

• Pointers may be declared to any data type.
• datatype variablename
• To access the actual value you need to
  dereference the pointer.
• variablename 3
• Without the variablename will be the address of
  the variable with the it is the value of the
  variable.
• \\MERCURY\DATA\USERS\MHOLMES\spsdemo\demo1.c
• spsdemo\Debug\demo1.exe