Chapter 3: Pointers - Revision history

Arjun Suresh: /* Problems with pointers */

2014-06-16T19:58:22Z

‎Problems with pointers

2014-06-16T19:55:53Z

‎Example

2014-06-16T19:55:33Z

‎Example

2014-06-16T19:55:15Z

‎Example

2014-06-16T19:54:48Z

‎Assigning values to pointers

2014-06-16T19:41:21Z

2014-06-16T14:50:33Z

‎Problems with pointers

2014-06-16T14:49:56Z

‎Does a pointer have memory?

2014-06-16T14:49:23Z

‎Does pointer have a memory?

2014-06-16T14:49:05Z

‎Pointer Arithmetic

@@ Line 85: / Line 85: @@
 ==Problems with pointers==
-Pointers are very powerful but with power comes problems. Since, pointer allows direct access to memory address, if programmer is not careful, pointer usage can cause segmentation faults. Then pointer dereferencing is difficult to debug especially when there are more than two levels of indirection (pointer to pointers).
+Pointers are very powerful but with power comes problems. Since, pointer allows direct access to memory address, if programmer is not careful, pointer usage can cause segmentation faults. The other problem with pointer is that pointer dereferencing is difficult to debug especially when there are more than two levels of indirection (pointer to pointers).
 ==Exercise Questions==

@@ Line 74: / Line 74: @@
   #include <stdlib.h> //contains the declaration for malloc
   int main()
-−
+ {
-  int a;
+   int a;
-  int* p = &a;
+   int* p = &a;
-  *p = 6;
+   *p = 6;
-  p = malloc(10 * sizeof(int));
+   p = malloc(10 * sizeof(int));
-  *(p+5) = 7;
+   *(p+5) = 7;
-  printf("a = %d, *(p+5) = %d, p[5] = %d\n", a, *(p+5), p[5]);
+   printf("a = %d, *(p+5) = %d, p[5] = %d\n", a, *(p+5), p[5]);
-−
+ }
 </syntaxhighlight>

@@ Line 54: / Line 54: @@
   #include <stdio.h>
   int main()
-−
+ {
-  int a;
+   int a;
-  int* p;
+   int* p;
-  p = &a;
+   p = &a;
-  int** q; //q is a pointer to a pointer to int
+   int** q; //q is a pointer to a pointer to int
-  q = &p;
+   q = &p;
-  printf("a = %d, *p =%d, **q = %d\n", a, *p, **q);
+   printf("a = %d, *p =%d, **q = %d\n", a, *p, **q);
-−
+ }
 </syntaxhighlight>

@@ Line 78: / Line 78: @@
    int* p = &a;
    *p = 6;
- p = malloc(10 * sizeof(int));
+  p = malloc(10 * sizeof(int));
- *(p+5) = 7;
+  *(p+5) = 7;
- printf("a = %d, *(p+5) = %d, p[5] = %d\n", a, *(p+5), p[5]);
+  printf("a = %d, *(p+5) = %d, p[5] = %d\n", a, *(p+5), p[5]);
+}
 </syntaxhighlight>

← Older revision		Revision as of 19:54, 16 June 2014
Line 68:		Line 68:

	One way of assigning value to a pointer variable is by using & operator on an already defined variable. Thus, the pointer variable will now be holding the address of that variable. The other way is to use malloc function which returns an array of dynamic memory created on the heap. This method is usually used to create dynamic arrays.		One way of assigning value to a pointer variable is by using & operator on an already defined variable. Thus, the pointer variable will now be holding the address of that variable. The other way is to use malloc function which returns an array of dynamic memory created on the heap. This method is usually used to create dynamic arrays.
−
−	~~===Examples===~~

	===Example===		===Example===

@@ Line 66: / Line 66: @@
 ==Assigning values to pointers==
 Since pointers should hold valid memory address (OS allocates certain memory region for a process and it should use only that region and should not try to access other memory region), we should not assign random values to a pointer variable. Assigning some values to a pointer is allowed, but if that value is not in the memory address for the process, dereferencing that pointer using * operator will cause [http://gatecse.in/wiki/Chapter_1:_C_program_-_A_System_View#Segmentation_Fault segmentation fault].
 ==Problems with pointers==

@@ Line 50: / Line 50: @@
 ==Does a pointer have memory?==
 Yes, pointer is a data type and so has a memory for it. On a 32 bit system, pointer requires 4 bytes (32 bits) of memory while on a 64 bit system it requires 8 bytes. And it is the same for pointer to any data type. Since pointer has a memory, we can have a pointer to pointer as well and this can be extended to any level.
 <syntaxhighlight lang="c">
   #include <stdio.h>

@@ Line 44: / Line 44: @@
   p+q //Not allowed and throws compilation error
 </syntaxhighlight>
   There is no multiplication or division operations in pointer arithmetic as they have no meaning.