void pointer concepts with examples

1. Generic coding with void *
02/22/17
By: Sandip Moradiya
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2. Suppose we have to declare integer pointer,character
pointer and float pointer then we need to declare 3 pointer
variables.
Instead of declaring different types of pointer
variable it is feasible to declare single pointer variable
which can act as integer pointer,character pointer.
void pointer (generic pointer)
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3. Basic of void pointer
 In C General Purpose Pointer is called as void Pointer.
 It can store address of any type of variable
 The compiler has no idea what type of object a void Pointer
really points to ?
 Memory required for void pointer is 8 byte
 Void pointer increment by 1 byte
 It does not have any data type associated with it
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4. Declaration of Void Pointer
void *pointer_name;
Void Pointer Example :
void *ptr; // ptr is declared as Void pointer
char cnum;
int inum;
float fnum;
ptr = &cnum; // ptr has address of character data
ptr = &inum; // ptr has address of integer data
ptr = &fnum; // ptr has address of float data
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5. Dereferencing void pointer
We cannot directly dereference void pointer
like,
int intData=10;
void *ptr;
ptr=&intData;
printf(“%d”,*ptr);
// output: compiler warning/error
Type casting of void pointer is needed
printf(“%d”,*(int *)ptr);
//output: 10
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6. void pointer reference to array
char string[]={“Hello World"};
void *ptr;
ptr=&string[0];
printf(“data : %sn”, (char*)ptr);
//output: data : Hello World
int data[]={1,2,3,4,5,6};
void *ptr=&data[0];
for(int i = 0; i < 6; i++)
{
printf(“%d ”,ptr);
ptr = ptr + 4; // pointing to next element in array
}
//output: 1 2 3 4 5 6 6

7. void pointer dereference from array
char string[]={“Hello world"};
void *ptr;
ptr=&string[0];
int data[]={1,2,3,4,5,6};
void *ptr=&data[0];
for(int i = 0; i < strlen(string); i++)
printf("%c",*(char *)ptr++);
//output : Hello world
for(int i = 0; i < 6; i++)
{
printf("%d ",*(int *)ptr);
ptr=ptr+4;
}
//output : 1 2 3 4 5 6
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8. void pointer reference to structure
struct structure
{
int num;
char c;
}stData;
struct structure data;
void *ptr;
ptr = &data;
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9. void pointer dereference from structure
struct structure data;
void *ptr;
ptr = &data;
data.num=10;
data.c='a';
printf("%d",*(int *)ptr);
//output: 10
s1 = s1 + 4;
printf("%c",*(char *)ptr);
// output: a
struct structure
{
int num;
char c;
}stData;
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10. Array of void pointer
int data1 = 0, data2 = 30, data3=12;
void *ptr[3];
ptr[0]=&data1;
ptr[1]=&data2;
ptr[2]=&data3;
for(int i = 0; i < 3; i++)
printf("%dn",*(int *)p[i]);
//output: 0
30
12
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11. void pointer as parameter of function
int num(void * vPtrData)
{
int result=*(int *) vPtrData;
return result;
}
int data=10;
void *ptr=&data;
printf("%dn",num(ptr));
//output: 10
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12. Void function pointer
void *num(int pdata)
{
void *result=&pdata;
return result;
}
int data=10;
void *ptr=&data;
printf("%dn", *(int *)num(data));
//output: 110
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13. Thank you
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