What does void mean and how to use it

Question

Today when I was reading others  code  I saw something like void  func void  i    what does this void  mean here for the function name and for the variable type  respectively    In addition  when do we need to use this kind of pointer and how to use it

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The function takes a pointer to an arbitrary type and returns one such

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A void pointer is known as generic pointer  I would like to explain with a sample pthread scenario    The thread function will have the prototype as  void    start routine  void     The pthread API designers considered the argument and return values of thread function  If those thing are made generic  we can type cast to void  while sending as argument  similarly the return value can be retrieved from void  But i never used return values from thread function     void  PrintHello void  threadid       long tid            Arg sent in main is retrieved          tid    long threadid     printf  Hello World  It s me  thread   ld  n   tid      pthread exit NULL      int main  int argc  char  argv         pthread t threads NUM THREADS      int rc     long t     for t 0  t lt NUM THREADS  t                 t will be type cast to void  and send as argument        rc   pthread create  amp threads t   NULL  PrintHello   void   t            if  rc            printf  ERROR  return code from pthread create   is  d n   rc            exit -1                          Last thing that main   should do       pthread exit NULL

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Using a void   means that the function can take a pointer that doesn t need to be a specific type  For example  in socket functions  you have    send void   pData  int nLength    this means you can call it in many ways  for example  char   data    blah   send data  strlen data     POINT p  p x   1  p y   2  send  amp p  sizeof POINT

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You can have a look at this article about pointers http   www cplusplus com doc tutorial pointers  and read the chapter   void pointers   This also works for C language      The void type of pointer is a special type of pointer  In C    void   represents the absence of type  so void pointers are pointers that   point to a value that has no type  and thus also an undetermined   length and undetermined dereference properties        This allows void pointers to point to any data type  from an integer   value or a float to a string of characters  But in exchange they have   a great limitation  the data pointed by them cannot be directly   dereferenced  which is logical  since we have no type to dereference   to   and for that reason we will always have to cast the address in   the void pointer to some other pointer type that points to a concrete   data type before dereferencing it

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C is remarkable in this regard  One can say void is nothingness void  is everything  can be everything    It s just this tiny   which makes the difference    Rene has pointed it out  A void   is a Pointer to some location  What there is how to  interpret  is left to the user    It s the only way to have opaque types in C  Very prominent examples can be found e g in glib or general data structure libraries  It s treated very detailed in  C Interfaces and implementations     I suggest you read the complete chapter and try to understand the concept of a pointer to   get it

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A pointer to void is a  generic  pointer type   A void   can be converted to any other pointer type without an explicit cast   You cannot dereference a void   or do pointer arithmetic with it  you must convert it to a pointer to a complete data type first   void   is often used in places where you need to be able to work with different pointer types in the same code   One commonly cited example is the library function qsort   void qsort void  base  size t nmemb  size t size              int   compar  const void    const void        base is the address of an array  nmemb is the number of elements in the array  size is the size of each element  and compar is a pointer to a function that compares two elements of the array   It gets called like so   int iArr 10   double dArr 30   long lArr 50       qsort iArr  sizeof iArr sizeof iArr 0   sizeof iArr 0   compareInt   qsort dArr  sizeof dArr sizeof dArr 0   sizeof dArr 0   compareDouble   qsort lArr  sizeof lArr sizeof lArr 0   sizeof lArr 0   compareLong     The array expressions iArr  dArr  and lArr are implicitly converted from array types to pointer types in the function call  and each is implicitly converted from  pointer to int double long  to  pointer to void    The comparison functions would look something like   int compareInt const void  lhs  const void  rhs      const int  x   lhs      convert void   to int   by assignment   const int  y   rhs     if   x  gt   y  return 1    if   x     y  return 0    return -1      By accepting void    qsort can work with arrays of any type     The disadvantage of using void   is that you throw type safety out the window and into oncoming traffic   There s nothing to protect you from using the wrong comparison routine   qsort dArr  sizeof dArr sizeof dArr 0   sizeof dArr 0   compareInt     compareInt is expecting its arguments to be pointing to ints  but is actually working with doubles   There s no way to catch this problem at compile time  you ll just wind up with a missorted array

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it means pointer you can use this link to get more info about pointer http   www cprogramming com tutorial c lesson6 html

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void    is a  pointer to memory with no assumptions what type is there stored   You can use  for example  if you want to pass an argument to function and this argument can be of several types and in function you will handle each type

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a void  is a pointer  but the type of what it points to is unspecified  When you pass a void pointer to a function you will need to know what its type was in order to cast it back to that correct type later in the function to use it  You will see examples in pthreads that use functions with exactly the prototype in your example that are used as the thread function  You can then use the void  argument as a pointer to a generic datatype of your choosing and then cast it back to that type to use within your thread function  You need to be careful when using void pointers though as unless you case back to a pointer of its true type you can end up with all sorts of problems

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C11 standard  n1570    6 2 2 3 al1 p55 says       A pointer to void may be converted to or from a pointer to any object   type  A pointer to any object type may be converted to a pointer to   void and back again  the result shall compare equal to the original   pointer    You can use this generic pointer to store a pointer to any object type  but you can t use usual arithmetic operations with it and you can t deference it

[c] What does void* mean and how to use it?

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