Concept of void pointer in C programming

Question

Is it possible to dereference a void pointer without type-casting in the C programming language   Also  is there any way of generalizing a function which can receive a pointer and store it in a void pointer and by using that void pointer  can we make a generalized function   for e g    void abc void  a  int b       if b  1        printf   d    int  a          If integer pointer is received    else if b  2        printf   c    char  a          If character pointer is received    else if b  3        printf   f    float  a          If float pointer is received     I want to make this function generic without using if-else statements - is this possible    Also if there are good internet articles which explain the concept of a void pointer  then it would be beneficial if you could provide the URLs   Also  is pointer arithmetic with void pointers possible

User · Answer

So far my understating on void pointer is as follows.

When a pointer variable is declared using keyword void – it becomes a general purpose pointer variable. Address of any variable of any data type (char, int, float etc.)can be assigned to a void pointer variable.

main()
{
    int *p;

    void *vp;

    vp=p;
}

Since other data type pointer can be assigned to void pointer, so I used it in absolut_value(code shown below) function. To make a general function.

I tried to write a simple C code which takes integer or float as a an argument and tries to make it +ve, if negative. I wrote the following code,

#include<stdio.h>

void absolute_value ( void *j) // works if used float, obviously it must work but thats not my interest here.
{
    if ( *j < 0 )
        *j = *j * (-1);

}

int main()
{
    int i = 40;
    float f = -40;
    printf("print intiger i = %d \n",i);
    printf("print float f = %f \n",f);
    absolute_value(&i);
    absolute_value(&f);
    printf("print intiger i = %d \n",i);
    printf("print float f = %f \n",f);
    return 0;
}

But I was getting error, so I came to know my understanding with void pointer is not correct :(. So now I will move towards to collect points why is that so.

The things that i need to understand more on void pointers is that.

We need to typecast the void pointer variable to dereference it. This is because a void pointer has no data type associated with it. There is no way the compiler can know (or guess?) what type of data is pointed to by the void pointer. So to take the data pointed to by a void pointer we typecast it with the correct type of the data holded inside the void pointers location.

void main()

{

    int a=10;

    float b=35.75;

    void *ptr; // Declaring a void pointer

    ptr=&a; // Assigning address of integer to void pointer.

    printf("The value of integer variable is= %d",*( (int*) ptr) );// (int*)ptr - is used for type casting. Where as *((int*)ptr) dereferences the typecasted void pointer variable.

    ptr=&b; // Assigning address of float to void pointer.

    printf("The value of float variable is= %f",*( (float*) ptr) );

}

A void pointer can be really useful if the programmer is not sure about the data type of data inputted by the end user. In such a case the programmer can use a void pointer to point to the location of the unknown data type. The program can be set in such a way to ask the user to inform the type of data and type casting can be performed according to the information inputted by the user. A code snippet is given below.

void funct(void *a, int z)
{
    if(z==1)
    printf("%d",*(int*)a); // If user inputs 1, then he means the data is an integer and type casting is done accordingly.
    else if(z==2)
    printf("%c",*(char*)a); // Typecasting for character pointer.
    else if(z==3)
    printf("%f",*(float*)a); // Typecasting for float pointer
}

Another important point you should keep in mind about void pointers is that – pointer arithmetic can not be performed in a void pointer.

void *ptr;

int a;

ptr=&a;

ptr++; // This statement is invalid and will result in an error because 'ptr' is a void pointer variable.

So now I understood what was my mistake. I am correcting the same.

References :

http://www.antoarts.com/void-pointers-in-c/

http://www.circuitstoday.com/void-pointers-in-c.

The New code is as shown below.

#include<stdio.h>
#define INT 1
#define FLOAT 2

void absolute_value ( void *j, int *n)
{
    if ( *n == INT) {
        if ( *((int*)j) < 0 )
            *((int*)j) = *((int*)j) * (-1);
    }
    if ( *n == FLOAT ) {
        if ( *((float*)j) < 0 )
            *((float*)j) = *((float*)j) * (-1);
    }
}


int main()
{
    int i = 0,n=0;
    float f = 0;
    printf("Press 1 to enter integer or 2 got float then enter the value to get absolute value\n");
    scanf("%d",&n);
    printf("\n");
    if( n == 1) {
        scanf("%d",&i);
        printf("value entered before absolute function exec = %d \n",i);
        absolute_value(&i,&n);
        printf("value entered after absolute function exec = %d \n",i);
    }
    if( n == 2) {
        scanf("%f",&f);
        printf("value entered before absolute function exec = %f \n",f);
        absolute_value(&f,&n);
        printf("value entered after absolute function exec = %f \n",f);
    }
    else
    printf("unknown entry try again\n");
    return 0;
}

Thank you,

User · Answer

Is it possible to dereference the void pointer without type-casting in C programming language      No  void indicates the absence of type  it is not something you can dereference or assign to      is there is any way of generalizing a function which can receive pointer and store it in void pointer and by using that void pointer we can make a generalized function     You cannot just dereference it in a portable way  as it may not be properly aligned  It may be an issue on some architectures like ARM  where pointer to a data type must be aligned at boundary of the size of data type  e g  pointer to 32-bit integer must be aligned at 4-byte boundary to be dereferenced    For example  reading uint16 t from void       may receive wrong value if ptr is not 2-byte aligned    uint16 t value     uint16 t  ptr     portable way of reading a little-endian value    uint16 t value     uint8 t  ptr                        uint8 t  ptr 1   lt  lt 8        Also  is pointer arithmetic with void pointers possible      Pointer arithmetic is not possible on pointers of void due to lack of concrete value underneath the pointer and hence the size   void  p       void  p2   p   1     what exactly is the size of void

User · Answer

In C  a void   can be converted to a pointer to an object of a different type without an explicit cast   void abc void  a  int b        int  test   a                  This doesn t help with writing your function in a more generic way  though   You can t dereference a void   with converting it to a different pointer type as dereferencing a pointer is obtaining the value of the pointed-to object  A naked void is not a valid type so derefencing a void   is not possible   Pointer arithmetic is about changing pointer values by multiples of the sizeof the pointed-to objects  Again  because void is not a true type  sizeof void  has no meaning so pointer arithmetic is not valid on void     Some implementations allow it  using the equivalent pointer arithmetic for char

User · Answer

Void pointers are pointers that has no data type associated with it A void pointer can hold address of any type and can be typcasted to any type  But  void pointer cannot be directly be dereferenced  int x   1  void  p1  p1    amp x  cout  lt  lt   p1  lt  lt  endl     this will give error cout  lt  lt   int     p   lt  lt  endl     this is valid

User · Answer

A void pointer is known as generic pointer  which can refer to variables of any data type

User · Answer

Here is a brief pointer on void pointers  https   www learncpp com cpp-tutorial 613-void-pointers   6 13     Void pointers Because the void pointer does not know what type of object it is pointing to  it cannot be dereferenced directly  Rather  the void pointer must first be explicitly cast to another pointer type before it is dereferenced  If a void pointer doesn t know what it s pointing to  how do we know what to cast it to  Ultimately  that is up to you to keep track of  Void pointer miscellany It is not possible to do pointer arithmetic on a void pointer  This is because pointer arithmetic requires the pointer to know what size object it is pointing to  so it can increment or decrement the pointer appropriately   Assuming the machine s memory is byte-addressable and does not require aligned accesses  the most generic and atomic  closest to the machine level representation  way of interpreting a void  is as a pointer-to-a-byte  uint8 t   Casting a void  to a uint8 t  would allow you to  for example  print out the first 1 2 4 8 however-many-you-desire bytes starting at that address  but you can t do much else  uint8 t  byte p    uint8 t  p  for  uint8 t  i   byte p  i  lt  byte p   8  i        printf  quot  x  quot   i

User · Answer

You can easily print a void printer  int p 15  void  q  q  amp p  printf   d     int  q

User · Answer

You should be aware that in C  unlike Java or C   there is absolutely no possibility to successfully  guess  the type of object a void  pointer points at  Something similar to getClass   simply doesn t exist  since this information is nowhere to be found  For that reason  the kind of  generic  you are looking for always comes with explicit metainformation  like the int b in your example or the format string in the printf family of functions

User · Answer

You cannot dereference a pointer without specifying its type because different data types will have different sizes in memory i e  an int being 4 bytes  a char being 1 byte

User · Answer

Because C is statically-typed  strongly-typed language  you must decide type of variable before compile  When you try to emulate generics in C  you ll end up attempt to rewrite C   again  so it would be better to use C   instead

User · Answer

Fundamentally  in C   types  are a way to interpret bytes in memory  For example  what the following code  struct Point     int x    int y      int main       struct Point p    p x   0    p y   0      Says  When I run main  I want to allocate 4  size of integer    4  size of integer    8  total bytes  of memory  When I write   x  as a lvalue on a value with the type label Point at compile time  retrieve data from the pointer s memory location plus four bytes  Give the return value the compile-time label  int     Inside the computer at runtime  your  Point  structure looks like this   00000000 00000000 00000000 00000000 00000000 00000000 00000000   And here s what your void  data type might look like   assuming a 32-bit computer   10001010 11111001 00010010 11000101

User · Answer

This won t work  yet void   can help a lot in defining generic pointer to functions and passing it as an argument to another function  similar to callback in Java  or define it a structure similar to oop

User · Answer

void pointer is a generic pointer   Address of any datatype of any variable can be assigned to a void pointer   int a   10  float b   3 14  void  ptr  ptr    amp a  printf   data is  d        int   ptr        int   ptr used for typecasting dereferencing as int ptr    amp b  printf   data is  f        float   ptr       float   ptr used for typecasting dereferencing as float

User · Answer

I want to make this function generic    without using ifs  is it possible    The only simple way I see is to use overloading    which is not available in C programming langage AFAIK    Did you consider the C   programming langage for your programm   Or is there any constraint that forbids its use

User · Answer

No  it is not possible  What type should the dereferenced value have

User · Answer

void abc void  a  int b      char  format        d     c     f      printf format b-1   a

[c] Concept of void pointer in C programming

I tried to write a simple C code which takes integer or float as a an argument and tries to make it +ve, if negative. I wrote the following code,

Examples related to c

Examples related to void-pointers