C Programming Revision

1
C Programming Revision

• Simple C program layout
• C data types
• printf output
• C pointers and arrays
• Stack and Heap memory
• Global vs Local variables

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• Example output
• (terminated with D or Z)

• include ltstdio.hgt
• include ltmath.hgt
• int main()
• char ch / is really an
  int from -128 to 0 to 127 /
• unsigned char uch / is really an int from
  0 to 255 /
• short int si / can just say
  short si /
• int i / usually same
  as machine word ie 32 bits /
• long int li / could be 32
  but may be 64 bits /
• long long int ll / generally is 64
  bits on gcc systems /
• float f / IEEE format floating point,
  32 bits usually /
• double d / double precision - about 15
  significant figures /
• for(i0ilt5i) / loop from 0...4 ie do
  it 5 times /
• printf("d 2 d\n", i, i 2 ) /
  print a string with 2 ints /
• ch '\n' / the newline character
  /
• uch 255

0 2 0 1 2 2 2 2 4 3 2 6 4 2
8 li 2147483647 a a b b c c d d e e hello hello
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• include ltstdio.hgt
• void printArray( int iarr, int n ) /
  function to print out an int array /
• int i / print to stderr, could
  have used stdout instead /
• for(i0iltni)
• fprintf(stderr, "d\t", iarri ) /
  separate them with tabs /
• fprintf(stderr,"\n") /
  finish with a newline /
• int main()
• int i / an integer takes 32 bits
  of memory on the stack /
• int iarray4 / will take /
• int ipointer
• printf("size of int d\n", sizeof(i) )
• printf("size of iarray d\n", sizeof(iarray)
  )
• printf("size of ipointer d\n",
  sizeof(ipointer) )
• i 0
• iarrayi 42
• printArray( iarray, 4 )
• for(i0ilt4i)
• iarrayi i 10

• Example output
• size of int 4
• size of iarray 16
• size of ipointer 4
• 42 49 6 3
• 0 10 20 30
• ipointer 3221224552
• contents of ipointer 4
• 1 11 21 31

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Pointers

• Pointers in C and C are a special data type
  that is capable of pointing to the memory
  location of a normal data type.
• Pointers are typed - eg pointer to an int is
  not the same as pointer to a char
• Pointers let us do address manipulation and
  are a powerful but dangerous feature of the
  language

• In C and C arrays are implemented via pointer
  arithmetic
• int iarray10 tells the compiler you want the
  space for 10 integers set aside (not yet
  initialised)
• int iptr sets aside memory for a pointer but
  not for an actual integer!

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Working with pointers

• Address operator
• float x 2.71828
• x means address of x
• Declaring pointers
• float p x means give me a variable called p
  of type pointer to float and initialize it to
  the address of x
• float p does the same thing as float p
• De-reference (or indirection) operator
• p means give me the object at address p

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Stack Memory

• When you write and run a program each function
  can set up its own variable s on what is
  known as the stack - a part of the
  computers memory organised like a stack of
  cards.

• Variables set up on the stack typically last only
  for the duration of execution of the function
  that declares them.

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Stack example

• int myfunc() / prototype /
• int main()
• int i
• myfunc()
• return 0
• int myfunc()
• int j 10

• i is a stack variable set up by the function
  main()
• j is a stack variable set up by the function
  myfunc() and will last only for the duration of
  the call to myfunc()

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Global Variables - use with Caution

• int k 42
• int myfunc()
• int j 10
• k k j
• return k
• int main()
• int i 3
• int k 7
• i myfunc()
• printf(i d, k d\n, i, k )
• return 0

• k is a global variable
• It is accessible to both main() and myfunc()
• main() declares its own stack variable k which
  hides the global k
• Output would be
• 52, 7

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DO NOT USE GLOBAL VARIABLES
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Heap Memory

• The Heap is a pool of memory available to all
  functions which they must access using
  malloc() and related functions
• Mallocd memory stays allocated until you free
  it
• This must be used with great care (otherwise get
  Bus Error or Segmentation Fault)
• Remember to allocate memory before you try to
  use it
• Remember to free up memory once you are finished
  with it.
• C has neater ways of managing this

• int ia / a global pointer /
• int myfunc()
• ia (int )malloc( 10 sizeof(int) )
• ia2 7
• int main()
• myfunc()
• ia0 8
• ia9 87
• free( ia )
• return 0

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Arrays and pointers

• When we create an array, the compiler allocates
  as much memory as requested an remembers the
  address of the first element
• char s55
• Allocates 55 bytes starting at the address of
  s0
• s is shorthand for s0
• si is shorthand for (s i)

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Dynamic memory allocation

• Char s55 declares an array of fixed length, but
  memory can be allocated dynamically
• int nvalues 55
• char p (char )malloc( nvalues sizeof(char)
  )
• malloc(n) allocates a block of memory of n bytes
  and returns the address of the start of the block
• The return type type is void
• A raw pointer to a variable of unspecified type
• Needs to be recast to the appropriate pointer type

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Miscellaneous C Features

• if( ) else
• while( )
• do while( )
• Use of ints 0 or not zero in place of
  false/true
• Functions can call themselves (recursion)
• No string type in C so use arrays of char,
  terminated by \0 character

• Need include ltmaths.hgt
• to use the maths library functions eg
  sqrt()
• printf()/scanf() fprintf()/fscanf() I/O
  routines
• putc()/getc() for single character I/O

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C programming methodology

• A procedural oriented language
• Or an action oriented language
• Based on functions of variables which call one
  another
• OO goes beyond this

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Objects in C

• An object is a named area of memory that holds
  data
• Eg, consider the statement
• int num 1234
• The compiler generates a symbol table that
  associates the name of an object (ie variable)
  with its location (ie address) in computer memory

num
Location in memory (address)
1234
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Summary

• Think of pointers as memory addresses
• Memory your program uses can be stack or heap
  memory
• Stack variables last only as long as the called
  function does
• Heap variables last until you free them up

• Look at the C programming revision material on
  the web site.
• C is based around the idea of functions which
  call one another
• OO programming goes beyond this
• Next - C simple I/O