What does void mean in C C and C

Question

Looking to get the fundamentals on where the term  void  comes from  and why it is called void  The intention of the question is to assist someone who has no C experience  and is suddenly looking at a C-based codebase

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Void is used only in method signatures  For return types it means method will not return anything to the calling code  For parameters it means  no parameters are passed to the method  e g    void MethodThatReturnsAndTakesVoid void       Method body     In C  we can omit the void for parameters and can write the above code as   void MethodThatReturnsAndTakesVoid        Method body     Void should not be confused with null  Null means for the variable whose address is on stack  the value on the heap for that address is empty

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Basically it means  nothing  or  no type   There are 3 basic ways that void is used    Function argument  int myFunc void  -- the function takes nothing   Function return value  void myFunc int  -- the function returns nothing Generic data pointer  void  data --  data  is a pointer to data of unknown type  and cannot be dereferenced   Note  the void in a function argument is optional in C    so int myFunc   is exactly the same as int myFunc void   and it is left out completely in C   It is always required for a return value

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There are two ways to use void   void foo void     or  void  bar void      The first indicates that no argument is being passed or that no argument is being returned   The second tells the compiler that there is no type associated with the data effectively meaning that the you can t make use of the data pointed to until it is cast to a known type   For example you will see void  used a lot when you have an interface which calls a function whose parameters can t be known ahead of time   For example  in the Linux Kernel when deferring work you will setup a function to be run at a latter time by giving it a pointer to the function to be run and a pointer to the data to be passed to the function   struct  deferred work   sruct list head mylist   worker func   bar   data          somedata    deferred work    Then a kernel thread goes over a list of deferred work and when it get s to this node it effectively executes   bar somedata     Then in bar you have   void bar void  mydata        int  data   mydata         do something with data

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It means  no value   You use void to indicate that a function doesn t return a value or that it has no parameters or both It s much consistent with typical uses of word void in English   Void should not be confused with null  Null means for the variable whose address is on stack  the value on the heap for that address is empty

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If you re explaining the concept to a beginner  it might be helpful to use an analogy  The use of void in all these cases is analogous in meaning to a page in a book which has the following words   This page left intentionally blank   It is to differentiate to the compiler between something which should be flagged as an error  versus a type which is intentionally to be left blank because that is the behavior you want   It always appears in code where normally you would expect to see a type appear  such as a return type or a pointer type  This is why in C   void maps to an actual CLR type  System Void because it is a type in itself   Some programming languages never developed the concept of void  just like some human cultures never invented the concept of the number zero  Void represents the same advancement in a programming language as the concept of zero represents to human language

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It means  no value   You use void to indicate that a function doesn t return a value or that it has no parameters or both  Pretty much consistent with typical uses of word void in English

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Three usage cases for void    Function signatures  void foo int bar  does not return a value  int bar void  does not take any parameters but this is usually expressed with empty argument list  int bar    Usage of the void keyword here corresponds to its meaning in English  Generic top-type pointer void   that points to unspecified data and cannot be dereferenced  Here the meaning of void is different from other meanings of void  universal type vs  no type  In casts such as  void  new Foo this  to signify that the return value is deliberately thrown away  Here the keyword usage also matches its meaning in English    Cases 1 and 2 were already covered by  Gerald but case 3 has not been addressed yet

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Void means no value required in return type from a function in all of three language

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Void is an incomplete type which  by definition  can t be an lvalue  That means it can t get assigned a value   So it also can t hold any value

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void mean that you won t be returning any value form the function or method

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It indicates the absence of a return value in a function    Some languages have two sorts of subroutines  procedures and functions  Procedures are just a sequence of operations  whereas a function is a sequence of operations that return a result   In C and its derivatives  the difference between the two is not explicit  Everything is basically a function  the void keyword indicates that it s not an  actual  function  since it doesn t return a value

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Void is the equivalent of Visual Basic s Sub

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Think of void as the  empty structure    Let me explain   Every function takes a sequence of parameters  where each parameter has a type    In fact  we could package up the parameters into a structure  with the structure slots corresponding to the parameters   This makes every function have exactly one argument    Similarly  functions produce a result  which has a type   It could be a boolean  or it could be float  or it could be a structure  containing an arbitrary set of other typed values   If we want a languge that has multiple return values  it is easy to just insist they be packaged into a structure   In fact  we could always insist that a function returned a structure   Now every function takes exactly one argument  and produces exactly one value   Now  what happens when I need a function that produces  no  value  Well  consider what I get when I form a struct with 3 slots  it holds 3 values   When I have 2 slots  it holds two values   When it has one slot  one value   And when it has zero slots  it holds    uh  zero values  or  no  value    So  I can think of a function returning void as returning a struct containing no values   You can even decide that  void  is just a synonym for the type represented by the empty structure  rather than a keyword in the language  maybe its just a predefined type     Similarly  I can think of a function requiring no values as accepting an empty structure  e g    void    I can even implement my programming language this way   Passing a void value takes up zero bytes  so passing void values is just a special case of passing other values of arbitrary size   This makes it easy for the compiler to treat the  void  result or argument    You probably want a langauge feature that can throw a function result away  in C  if you call the non-void result function foo in the following statement        foo       the compiler knows that foo produces a result and simply ignores it  If void is a value  this works perfectly and now  procedures   which are just an adjective for a function with void result  are just trivial special cases of general functions   Void  is a bit funnier    I don t think the C designers thought of void in the above way  they just created a keyword    That keyword was available when somebody needed a point to an arbitrary type  thus void  as the idiom in C  It actually works pretty well if you interpret void as an empty structure  A void  pointer is the address of a place where that empty structure has been put     Casts from void  to T  for other types T  also work out with this perspective  Pointer casts are a complete cheat that work on most common architectures to take advantage of the fact that if a compound type T has an element with subtype S placed physically at the beginning of T in its storage layout  then casting S  to T  and vice versa using the same physical machine address tends to work out  since most machine pointers have a single representation    Replacing the type S by the type void gives exactly the same effect  and thus casting to from void  works out   The PARLANSE programming language implements the above ideas pretty closely  We goofed in its design  and didn t pay close attention to  void  as a return type and thus have langauge keywords for procedure    Its mostly just a simple syntax change but its one of things you don t get around to once you get a large body working code in a language

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In c  you d use the void keyword to indicate that a method does not return a value   public void DoSomeWork       some work

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I have always taken it to mean absent  Here are four cases in the C language that matches to this use of absent   R f void  - Function parameters are absent void f P  - Return value is absent void  p - Type of what is pointed to is absent  void  p - Usage of value is absent   Other C descendants use it for other things  The D programming language uses it for cases where an initializer is absent   T t   void  - initializing value is absent

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