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Learn pointer declarations, pointer to int, function returning pointer, pointer to function, dangling pointer, void pointer with examples.

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Pointers, Pointer to integer

A pointer is a variable whose value is the address of another variable.

A pointer to integer holds an address of an integer.

A pointer to integer is declared using the following statement.

int *a; \\a is a pointer to integer.

a is an address where an integer can be stored.

a is the address of the first byte of the integer(4 bytes).

To initialize pointer a we need to assign a to the address of an integer like the follwoing.

int b; \\b is an integer.
a = &b;

An integer pointer can be initialized only with an address of another integer.

Let's understand the application of pointer to an integer using an example program below.

Illustration example

#include "stdio.h"
void PtrToInt(int *);
int main(void)
{
  int i = 10;
  PtrToInt(&i);
  printf("%d\n", i);
}
void PtrToInt(int *a)
{
  *a += 1;
}

Output:

$ gcc prog.c
$ ./a.out
11
$

In the above program we have passed a pointer to integer as argument for function PtrToInt. Inside function PtrToInt we increment the value at address a. As we passed the address of integer i while calling the function PtrToInt, the change we make inside the function PtrToInt at address a changes the content of i in main function. So value of i when printed post the PtrToInt function call, it prints the value as 11.

A function returning a pointer to integer

A function returning a pointer to integer returns the address of an integer. You can get the output of a function using a pointer to an integer.

A function returning a pointer to integer is declared in the below statement

int *a(); \\a is a function returning a pointer to integer.

Let's understand the application of a function returning a pointer to an integer using an example program below.

Illustration example

#include "stdio.h"
int *FuncRetPtr(int *);
int main(void)
{
  int i = 10;
  int *p;
  p = FuncRetPtr(&i);
  printf("Address at p:%x\n", p);
  printf("Content at p:%d\n", *p);
}
int *FuncRetPtr(int *a)
{
  *a += 1;
  printf("Address at a:%x\n", a);
  return a;
}

Output:

$ gcc prog.c
$ ./a.out
Address at a:7ebfa210
Address at p:7ebfa210
Content at p:11
$

In the above program we passed a pointer to an integer to function FuncRetPtr. Inside function FuncRetPtr we increment the content at address a by 1 and then print the address a. As the same address a is returned from the function FuncRetPtr, pointer variable p which captures the output of function FuncRetPtr also prints the same address a. So p is same as address of integer i. So *p prints the value of i incremented by 1.

A pointer to a function

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