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Learn pointer declarations, pointer to int, function returning pointer, pointer to function, dangling pointer, void pointer with examples.
int *a; \\a is a pointer to integer.
Let's understand the application of pointer to an integer using an example program below.
()
#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 $
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.
#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 $
int (*a)(int); \\a is a pointer to a function that takes an integer argument and returns an integer.
Let's understand the pointer to a function using a real example below. Here f is a pointer to a function funct. The function can take an integer as argument and return an integer.
#include "stdio.h"
int funct(int );
int main(void)
{
int b = 10;
int (*f)(int a);
f = funct;
int res;
res = (*f)(b);
printf("Result:%d\n", res);
}
int funct(int a)
{
a += 1;
return a;
}
Let's look at the following example program to demonstrate the use of dangling pointer. In this program memory is allocated first and then freed using ptr Once ptr is freed it becomes a dangling pointer. We need to initializeptr to NULL to remove this dangling pointer.
#include "stdlib.h"
int main()
{
int *ptr = (int *)malloc(sizeof(int));
// After below free call, ptr becomes a
// dangling pointer
free(ptr);
// No more a dangling pointer
ptr = NULL;
}
Rule of pointer is that a pointer should be assigned an address of the same type as mentioned in pointer declaration. But an exception to this is a pointer to void.
void *ptr;
#include "stdio.h"
int main(){
void *ptr;
int aa = 2;
float dec = 1.5;
float *fptr = &dec;
ptr = &aa;
printf("%d\n",*(int *)ptr);
ptr = fptr;
printf("%f\n",*(float *)ptr);
return 0;
}
$ gcc prog.c $ ./a.out 2 1.500000 $
A pointer that does not contain a valid memory address or is uninitialized, called NULL pointer. In C word NULL is a constant and has value of 0. The following expressions illustrate the correct and incorrect use of the NULL pointer constant.
int *p = NULL; // Correct
char *p = 0; // Correct
int a = NULL; // Incorrect
int a = 0; // a is zero of int type
if( p == 0 ); // Correct
if( p == NULL ); // Correct
if( p ); // Correct
if( a == 0 ); // Correct
char *ptr='\0'; // Correct
char *ptr=(char *)0; // Correct
if( a == NULL ); // Incorrect
if( NULL ); // Correct
if( NULL == 0 ); // Correct
NULL = 0; // Incorrect
NULL pointer is used to check the return value from memory allocation using malloc. If the return value is NULL that means memory allocation failed. This is illustrated in the below program.
#include "stdio.h"
#include "stdlib.h"
int main(){
int *ptr;
ptr = (int*)malloc(sizeof(int));
if(ptr == NULL)
{
printf("Memory allocation failed\n");
}else
{
printf("Memory allocation successful\n");
}
return 0;
}
$ gcc prog.c $ ./a.out Memory allocation successful $
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