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CSA2090Systems ProgrammingIntroduction to C
Lecture 5 Pointers and Strings

• Dr. Christopher Staff
• Department of Computer Science AI
• University of Malta

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Aims and Objectives

• More about pointers
• Traversing arrays using pointers
• Pointer arithmetic
• Strings

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Pointer types

• Pointers are not just memory addresses
• They are also variables, so they have types
• int p, p is a pointer-to-int
• char c, c is a pointer-to-char
• struct person fred, fred is a pointer-to-struct-p
  erson

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Pointer sizes

• Pointers are always 4 bytes
• depending on the system architecture
• they must be large enough to address the largest
  supported memory address
• But the size of the region that they point to
  depends on the size of the type of the data they
  point to

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Pointer sizes
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Pointer sizes

• If fred is struct person , how many bytes in
  memory does fred point to?
• struct person
• int age
• int height
• char surname20
• fred

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Pointer arithmetic

• So C knows how many bytes are occupied by the
  data pointed at by a pointer
• Useful especially when arrays are traversed using
  pointers
• More later

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Pointers and Arrays

• ptr ptr 1
• If ptr is a pointer-to-char then ptr will
  increase by just one byte
• But if ptr is a pointer-to-int, then ptr will
  increase by 4 bytes

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Pointers and Arrays

• int numbers10, i
• int iptr numbers0
• numbers01
• numbers12
• numbers23
• for (i0 ilt3 i)
• printf(iptr is d\n, iptr)
• iptriptr1

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Pointers and Structures

• struct person fred
• fred.age
• struct person fred
• (fred).age
• (fred).age becomes easy to make mistakes with,
  especially when member is a pointer
• fred-gtage

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Pointers and Functions

• Lets say that we want to create a variable in a
  calling function, and pass it to a called
  function so that we can store some result in it
• See func.c, func_ptr1.c, func_ptr2.c

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Strings

• In C a string is just an array of characters,
  with a NULL or zero byte terminating the string
• String handling functions in string.h expect to
  find trailing \0

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Strings

• char str110
• char str2 hello // \0 added by C
• char str3 // uninitialised
• str1 goodbye // illegal!!!
• strcpy(str1, goodbye)
• But C doesnt do array bounds checking!

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Pointers and Strings

• What will happen?
• strcpy(str3, horror)

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Pointers and Strings

• What will happen?
• strcpy(str3, horror)
• Because str3 was not initialised, it will contain
  garbage data
• which C will attempt to interpret as an
  address
• and try to store string at that location

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Pointers and Strings

• This can result in
• Trying to access protected memory segmentation
  fault
• Trying to access unaddressable memory, e.g. into
  the middle of a byte or byte 0 bus error
• Appearing to be successful, but then another
  process legally overwrites the data

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Initialising pointers

• Always initialise a pointer
• If no initial value, then use NULL
• char cptr NULL
• And then always test for NULL pointer before
  retrieving values via the pointer!
• if (!cptr)
• printf(Error occurred)

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Pointer Arithmetic

• char c hello
• char cptr c
• These statements are equivalent
• c cptr c0
• c3 cptr3 (cptr3) (c3)
• C treats c as a pointer to an array!
• You cannot pass by value an array to a function
• You cannot return an array from a function!

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Passing arrays to functions

• char c hello
• printstring(c)
• void printstring(char c)
• \\ OR
• void printstring(char c)
• Some caveats for using char see Love, 10

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Pointers-to-pointers-to

• A character string is really a pointer to an
  array of characters
• But what if we want a two dimensional array?
• an array of strings, for instance
• char c OR char c

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Differences

• Can initialise char c dog, cat,
  horse, bat
• Cannot initialise char c in the same way!
• Why?

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Next week

• Lab sessions for exercise 2 in Love
• Attempt Exercises 11.1 to 11.5 at home
• First part of lab session will cover any problems
  with these, and then attempt 11.6 to 11.8 in lab
• After Lab session, read Love, 12 on your own