C Tutorial -- Pointer
Pointer? Easy(again…)! Let’s talk about it! What is pointer?
What’s pointer?
A pointer is a variable whose value is the address of another variable, that is, the direct address of a memory location.
Declare pointers
So, just like other variables or constants, you must declare pointers before storing the addresses of other variables:
type *var-name;
The asterisk * used to declare pointers is the same as the asterisk used in multiplication. However, in this statement, the asterisk is used to specify that a variable is a pointer.
Examples:
int *ip;
double *dp;
float *fp;
char *ch;
The only difference between pointers of different data types is that the data types of the variables or constants pointed to by the pointers are different.
How to use pointers?
Talk is cheap, show the code:
#include <stdio.h>
int main ()
{
int var = 10;
int *p;
p = &var;
printf("var address: %p\n", &var);
printf("p address: %p\n", p);
printf("*p: %d\n", *p );
return 0;
}
results:
var address: 000000000022FE44
p address: 000000000022FE44
*p: 10
NULL pointer in C
During variable declaration, if there is no exact address to assign, it is a good programming practice to assign a NULL value to the pointer variable. A pointer assigned a NULL value is called a null pointer.
#include <stdio.h>
int main ()
{
int *p = NULL;
printf("p address: %p\n", p);
return 0;
}
result:
p address: 0x0
On most systems, programs are not allowed to access the memory at address 0, because the memory is reserved by the operating system. However, the memory address 0 has a particularly important meaning, it indicates that the pointer does not point to an accessible memory location. But by convention, if the pointer contains a null value, it is assumed to point to nothing.
Check for null pointers
We often use if to check for pointers:
if(!p) //if p is null
{
...
}
or
if(p) //if p is not null
{
...
}
