Pointer arithmetic allows pointer variables to be manipulated using arithmetic operations. Some key rules include: 1. A pointer variable can be assigned the address of an ordinary variable using the & operator. Casting is possible between different pointer types. 2. A pointer variable can be assigned the value of another pointer variable if they are of the same type. 3. A pointer variable can be assigned the null value NULL. When manipulating pointers, the type of the pointer variable determines the amount of increment or decrement from arithmetic operations like ++. Pointers can only be subtracted if they point to elements within the same array.

1. Pointer Arithmetic
V. Kamakoti

2. Rule - 1
• A pointer variable can be assigned the
address of an ordinary variable.
• int v, *pv;
– pv = &v;
– char c;
– pv = &c; //shall give a warning incompatible types

3. Rule - 1
• Casting is possible
• unsigned *pv;
– char c[5]; c[0] = ‘0’;c[1]=‘1’;c[2]=‘2’;c[3]=‘3’;
– pv = (unsigned *) c;
– printf(“%d %d %d %d %un”,
c[3],c[2],c[1],c[0],*pv);
– In x86 m/c Shall print - 51,50,48,49,858927408
– Little Endian storage of Integers
• Least significant byte in the least address - x86
representation
• Big Endian - Most significant byte in least address - Sun
Sparc
• In Sparc m/c print out shall be 51,50,48,49, 808530483

4. Rule 1
•
•
•
•
Character casting gives error in gcc
unsigned *pv; char *c;
c = (char *) pv; //Gives error
Reason: Not every ASCII character is
usable inside C

5. Rule - 2
• A pointer variable can be assigned the
value of another pointer variable
provided they are of same type
– int *pw, *pv;
– pv = pw;
– float *pk;
– pv = pk; //Gives a warning

6. Rule - 3
• A pointer variable can be assigned a
null value
– int *pw;
– pw = NULL;
• NULL is a symbolic constant;

7. Rule - 4
• An integer quantity can be added to or
subtracted from a pointer variable
– int *pw;
– pw++; pw+3;
– The amount of increment depends on the
type of the pointer variable

8. Rule - 5
• One pointer variable can be subtracted from
another provided. It makes sense only if both
points to elements in the same array
–
–
–
–
–
int *pw, *pv, A[100];
pv = &A[51];
pw = &A[75];
printf(“%d”,pw - pv); Ans: 24
printf(“%d”,pv - pw); Ans: -24

9. Rule - 5
• One pointer variable can be subtracted from
another provided. It makes sense only if both
points to elements in the same array
–
–
–
–
–
int *pw, *pv, A[100], B[100];
pv = &A[51];
pw = &B[75];
printf(“%d”,pw - pv); Ans: Not clear
printf(“%d”,pv - pw); Ans: Not clear

10. Rule - 6
• Two pointer variables can be compared if
they point to objects of the same type
– int *pw, *pv;
– pw < pv; pw >= pv; pw <= pv; pw == pv; pw != pv;
pw == NULL
– char c[4]; if (pw < c) … ; gives a Warning
– pw < (int *) c - passes
– (char *) pw < c - also passes

11. Do Not Rules
• Pointer variable cannot be multiplied by a constant
• Pointer variables cannot be added
• Ordinary variables cannot be assigned an arbitrary
address.
• int x;
• &x = … //Gives error
• int *pv,*pw;
• pv + pw or 2*pw …. // Invalid operands for +, * etc.
• pv = pv - pw // Wrong - as pv - pw is an integer and
not a integer pointer
• Pv = (int *) (pv - pw) passes compilation.