Example 6 another pointer pitfall

1

• Example 6 (another pointer pitfall)
• Illegal indirection
• short int i, j
• float p
• j 2
• p i
• p 124

0200 0201 0202 0203 0204 0205 0206 0207
i
j
p
2

• p now holds the address of i (p is 0200)
• Since p is a float pointer, the value of 124 is
  converted to a single precision float format of 4
  byte, and then is stored in memory locations
  0200, 0201, 0202, and 0203
• This implies memory locations 0202 and 0203 get
  overwritten
• As a result, variables i does not have the
  intended value of 124 (actually now it is 17144),
  and variable j no longer has the value 2

3

• NEVER use a pointer of one type to point to a
  variable of a different type
• The result could be unpleasant (e.g., system
  crash or wrong solution)
• If you dont believe, you might want to try...
• gcc w8ce1.c
• w8ce1.c In function main'
• w8ce1.c9 warning assignment from incompatible
  pointer type
• a.out
• Bus Error (core dumped)

4

• Example 7 (operations of pointers)
• include
• main()
• int x, y
• int ip
• ip x
• x 2
• ip ip 26
• / x is 28 now /
• y ip 7
• / y is 35 now /
• ip 4
• / x is 32 now /
• printf("x, y d, d\n", x, y)

5

• Example 8 (display pointers)
• include
• main()
• int x
• int ip
• ipx
• printf("ip p\n", ip)
• - Output (on a Sun SPARC machine)
• ip efffe89c

6

• Example 9 (display address of pointers)
• include
• main()
• int x
• int ip
• ipx
• printf(address of ip p\n", ip)
• - Output (on a Sun SPARC machine)
• ip efffe898
• Before we go ahead further, we need to review
  the increment and decrement operators

7

• The Increment and Decrement operators , --
• k or k means k k1
• k-- or --k means k k-1
• Example
• int k1
• k
• Result k is now equal to 2
• Example
• int k1
• k
• Result k is now equal to 2

8

• Example
• int k4
• k--
• Result k is now equal to 3
• Example
• int k4
• --k
• Result k is now equal to 3

9

• Difference between k and k
• Example
• k1
• i10(k) / After the operation, i is 10 /
• jk / now j is 2 /
• k is incremented after the expression is
  evaluated
• i.e.
• i10(k) means
• i10k kk1
• Example
• sj si / same as
• sj si jj1 /

10

• Example
• k1
• i10(k) / After the operation, i is 20 /
• jk / now j is 2 /
• k is incremented before the expression is
  evaluated
• i.e.
• i10(k) means
• kk1 i10k
• Example
• sj si / same as
• j j1 sj si /

11

• Example
• si c / same as
• si c i i 1 /

12

• Precedence
• -- -
• Do thongs from right to left
• However,
• a t
• / same as
• a t
• t t 1 /

13

• Example 10 (, - -, and pointers)
• include
• main()
• int x
• int ip
• x12
• ipx
• ip / x is 13 now /
• (ip) / x is 14 now /
• printf("x d\n", x)
• Consider the following
• a ip / This means

14

• Example 11
• include
• main()
• int x, val, p
• p x
• printf("(1) p p\n", p)
• / e.g., p aaaa0000 /
• val 256
• p val
• printf("(2) x, p d, p\n", x, p)
• / x, p 256, aaaa0004 /

15

• Output of Example 11 (on my Red Hat, Linux
  version 2.4.7-10)
• (1) p 0xbffff914
• (2) x, p 256, 0xbffff918
• (3) val, p 256, 0xbffff914

16

• Example 12 (pointers and types)
• include
• main()
• char c
• short int s
• int i
• float x
• double y
• char cp
• int ip
• short int sp
• float xp
• double yp
• cp c sp s ip i
• xp x yp y
• printf("Before cp, sp, ip, xp, yp p, p, p,
  p, p\n", cp, sp, ip, xp, yp)

17

• Output of Example 12
• Before cp, sp, ip, xp, yp efffe89f, efffe89c,
  efffe898, efffe894, efffe888
• After cp, sp, ip, xp, yp efffe8a0, efffe89e,
  efffe89c, efffe898, efffe890
• Should fully understand the above things
• Typical computers memory is byte-addressable,
  i.e. each cell contains a byte
• char 1 byte
• short int 2 bytes
• int, float 4 bytes
• double 8 bytes

18

• 7-2 Arrays
• 7-2-1 One Dimensional Arrays
• Example 1
• int aa10
• Declares aa to be an array of integers
• The size of aa is 10
• Array subscripts start at 0, e.g.
• aa0, aa1, , aa9.
• The notation ak refers to the k-th element of
  the array
• A subscript can be any integer expressions, e.g.
  int variables, int constants

19
aa0 aa1 aa2 aa3 aa4 aa5 aa6 a
a7 aa8 aa9
20

• Assignment
• aa4 657
• aa5 232
• Initialization
• int bb4 24, 54, 70, 17
• As the argument of a function
• Example 2 (C code)

21
/ SYEN 4399/5399 Example of arrays. May 28,
2003 / include define SIZE 100 int
read_data_and_get_size(float aa, int max_size)
/ or (float , int) / main() int k,
array_size float array_dataSIZE
array_size read_data_and_get_size(array_data,
SIZE) printf("array_size d\n",
array_size) for(k0 kprintf("data_arrayd f\n", k,
array_datak) int read_data_and_get_size(flo
at aa, int max_size) / note a complete code
must care about max_size / int i0
while(scanf("f", aai)!EOF) i return
i
22
- Input 416 43 643 12 678 45 548 44 2390 (press
control D) - Output array_size
9 data_array0 416.000000 data_array1
43.000000 data_array2 643.000000 data_array3
12.000000 data_array4 678.000000 data_array
5 45.000000 data_array6
548.000000 data_array7 44.000000 data_array8
2390.000000
23

• 7-3 Pointers and Function Arguments
• 7-3-1 Arguments - Call by Value
• Fortran or Pascal programmers might feel
  uncomfortable about this aspect of C
• Fortran call-by-reference
• Pascal can use var parameters
• This means that the code inside a function can
  access the original argument, not a local copy
• C The functions just get a local copy of the
  passed-in arguments
• Example 1

24
/ SYEN 4399/5399 Example of function Xian Liu,
May 28, 2003 / include int
power(int, int) main() int n, m n6
mpower(2, n) printf("m, n d, d\n", m,
n) int power(int base, int n) / this
function raises to n-th power / int p for
(p1 n0 --n) ppbase return p m,
n 64, 6
25

• 7-3-2 Array as Arguments
• The above story is different for arrays
• When the name of an array is used as an
  argument, the function gets the address of the
  beginning of this array
• This implies that the code inside the function
  can alter any element of the array
• We do not specify the dimension of the 1-d array
  when it is a parameter of a function

26

• Example 2
• float ff(int size, float list)
• int i
• float sum0.0
• for(i0 i
• sum sum listi
• return(sum/size)
• / If we insert the following code, then the
  original array may be changed
• list4454
• list646045765
• /