void pointer and pointer arithmetic

Key points

• its like normal pointer variable but data type is void (eg: void *voidptr) 
• since its type is void it can able to point any data type( range from int, char,float to user defined data type struct, union)
• but when dereferencing then it should be casted to respective type
• *((int *) voidptr)
• *((float *) voidptr)
•   ((char*) voidptr)    or     *((char *) voidptr) [see example program]
• ((struct name *)voidptr)->structurevariable
• pointer arithmetic will not applicable to void pointer in standard c but in gcc its allowed
• so using single pointer variable can access multiple data type

int nValue;

float fValue;

struct Something

{

    int nValue;

    float fValue;

};

Something sValue;

void *pVoid;

pVoid = &nValue; // valid

pVoid = &fValue; // valid

pVoid = &sValue; // valid

example 1:

root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# cat void2.c 
#include<stdio.h>
 
struct Something
{
char *name;
int a;
};

int main()
{
 
int nValue =10;
float fValue =20.2;
char *name1 = "god";
char name2[] = "help";
struct Something sValue;
sValue.name="selva";
sValue.a=20;
 
void *pVoid;


pVoid = &nValue; 
printf("nValue=%d\n",*((int *)pVoid));

pVoid = &fValue; 
printf("nValue=%f\n",*((float *)pVoid));

pVoid = name1; 

// note than not giving the address since its already pointer *name1
printf("name1=%s\n",((char *)(pVoid)));

pVoid = name2; 

// note than not giving the address since its already array name2[]

printf("name2=%s\n",((char *)(pVoid)));



pVoid = (sValue.name); // valid
printf("name=%s\n",((char *)(pVoid)));

pVoid = &(sValue); 
printf("struct something sValue->a =%d\n",((struct Something *)(pVoid))->a);

return 0;
}
root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# gcc void2.c 
root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# ./a.out 
nValue=10
nValue=20.200001
name1=god
name2=help
name=selva
struct something sValue->a =20
root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# 

root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# cat voidptr.c 
#include<stdio.h> 
enum Type
{
    INT,
    FLOAT,
    STRING,
};

void Print(void *pValue, enum Type eType)
{
    int i;float f;char *c;
    switch (eType)
    {
        case INT:
     i = *((int *)pValue);
     printf("\n%d\n",i);
            break;
        case FLOAT:
     f = *((float *)pValue);
     printf("\n%f\n",f);
            break;
        case STRING:
     c = (char *)pValue;
     printf("\n%s\n",c);
            break;
    }
}
 
int main()
{
    int nValue = 5;
    float fValue = 7.5;
    char *szValue = "Mollie";
 
    Print(&nValue, INT);
    Print(&fValue, FLOAT);
    Print(szValue, STRING); // note here no & symbol useed for char pointer
    return 0;
}
root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# gcc voidptr.c 
root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# ./a.out 

5

7.500000

Mollie
root@selvakumar-Lenovo-B460e:/C_pgms/voidptr# 

Reference:


http://www.learncpp.com/cpp-tutorial/613-void-pointers/
http://www.c4learn.com/c-programming/c-dereferencing-void-pointers/