sizeof

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sizeof

• A unary operator computes the size, in bytes,
  of any object or type
• Usage sizeof object or sizeof(type)
• If x is an int, sizeof x sizeof(int) is true
• Works for arrays too total bytes in whole array
• Sometimes can use to find an arrays length
• int size sizeof x / sizeof xi
• Actual type of result is size_t
• An unsigned integer defined in
• Similarly, diff_t is result type of pointer
  subtraction
• Especially useful to find the sizes of structures

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C structures

• Structures are variables with multiple data
  fields
• e.g., define structure to hold an int and a
  double
• struct example
• int x
• double d
• Create a structure, and assign a pointer to it
• struct example e, ep e
• Now can access fields by e or by ep
• e.d 2.5 / use name and the dot . operator
  /
• ep-x 7 / or use pointer-to-structure-field
  - operator /
• Second way is short-cut version of (ep).x 7
• Note sizeof e sizeof(int)sizeof(double)

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typedef and macros

• Can precede any declaration with typedef
• Defines a name for the given type
• typedef struct example ExampleType
• ExampleType e, ep / e, ep same as prior
  slide /
• Very handy for pointer types too
• typedef ExampleType ETPointer
• ETPointer ep / ep same as above /
• Macros can simplify code too
• define X(p) (p)-x
• X(ep) 8 / preprocessor substitutes
  correct code /

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Unions

• Can hold different data types/sizes (at different
  times)
• e.g., define union to hold an int or a double
• union myValue
• int x
• double d
• u, up / u is a union, up can point to one /
• Access x or d by u. or up- just like structures
• sizeof u is size of largest field in union
• Equals sizeof(double) in this case
• Often store inside a structure, with a key to
  identify type

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C function memory notes

• Parameters and local variables are automatic
• i.e., they exist only while the function executes
• So should never return a pointer to an automatic
  variable
• Dynamic memory allocation is different (will
  discuss)
• Variables always passed to functions by value
• i.e., the value is copied, so functions operate
  on a copy
• One issue is inefficient to pass structures
  pointers better
• Another issue functions need pointers to change
  values
• change(x) / xs value unchanged when function
  returns/
• change(x) / function may have changed xs
  value /
• Return values are copies too so similar issues

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A parameter passing example

• void triple1(int x) x x 3
• void triple2(int x) x x 3
• int a 10, 7
• void main(void)
• triple1(a0) / What is being passed? /
• printf("d\n", a0) / What is printed? /
• triple2(a) / What is being passed? /
• printf("d\n", a0) / What is printed? /
• Be sure to understand why these results occur.
• Hint draw the memory storage including storage
  duration

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Analogous example, re pointers

• First, recall that pointers are variables too
  then
• void repoint1(int p) p p 1
• void repoint2(int p) p p 1
• int a 10, 7
• int ap a
• void main(void)
• repoint1(ap) / What is being passed? /
• printf("d\n", ap) / What is printed? /
• repoint2(ap) / What is being passed? /
• printf("d\n", ap) / What is printed? /

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2 ways to allocate memory

• Static memory allocation done at compile-time
• int x double a5 / space for 1 int, 5
  doubles /
• Both size and type are clearly specified ahead of
  time x can only hold int values, a only doubles
• Dynamic memory allocation during execution
• Must use library methods like malloc
• int ip malloc(sizeof(int))
• Actually returns void or NULL if memory not
  available
• Must free the memory when done with it free(ip)

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Returning pointers from functions

• Okay if points to dynamically allocated (or
  external) storage
• int goodPtr(void)
• int p (int )malloc(sizeof(int))
• p 4
• return p
• Big mistake if points to local storage (inc.
  parameter values)
• int danglingPtr(void)
• int x 8
• int p x
• return p
• p is a dangling pointer as memory for x is
  erased and/or reused

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Self-Quiz Pointers memory

• Say int a, b
• a (int )malloc(sizeof(int)) b (int
  )malloc(sizeof(int))
• a 5 b 17
• What does this mean?
• What are (all) the results of a b in this
  case?
• What code would swap the values stored at a and
  b?
• What would happen if we tried this b 17 ?

Pointer fun
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int scanf(char fmt, a1, a2, )

• Like printf, but inputs from stdin
• For all except c skips white space
• Arguments corresponding to conversion characters
  must be pointers
• int x
• char word20
• scanf(d s, x, word)
• Note word is already a pointer, so no
• Another note word array must be large enough
• Also sscanf, and sprintf corresponding
  functions to get from, or put to a string instead

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File input/output

• FILE fp / declare a file pointer /
• fp fopen(filename, mode)
• / associate a file with the pointer /
• mode is char either r, w, or a
• Input or output using the file pointer
• getc(fp) / returns next int from file /
• putc(intValue, fp) / outputs value to file /
• fscanf(fp, format, ) / input from file /
• fprintf(fp, format, ) / output to file /

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Error handling basics

• Do NOT print errors to stdout
• Print error messages to stderr instead
  fprintf(stderr, message, args)
• Often need to terminate execution due to errors
• In main return EXIT_FAILURE / or any non-zero
  /
• In other functions exit(EXIT_FAILURE)
• Sometimes want to check error status of file (fp)
• General error ferror(fp) / returns 0 if no
  errors /
• End-of-file feof(fp) / returns non-0 if end
  of file /

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Line input and output

• Note KR getline is non-standard better to use
  fgets from
• char fgets(char line, int max, FILE fp)
• Reads at most max 1 characters, including \n
• The array, line, must be able to hold max chars
• fputs alternative to fprintf to output lines
• int fputs(char line, FILE fp) / returns EOF
  if error /
• Or just use puts() for stdout
• But do not use gets() its dangerous

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More library functions

• Become familiar with KR appendix B!
• to deal with char data
• to handle individual chars
• trig functions, logs, many more
• Note usually must link to libm.a use -lm
• various utilities
• Inc. atoi, qsort, rand, malloc, exit, system,
• one cool macro assert(int)
• , , check them out!

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Linked data structures

• Made up of nodes and links between nodes
• As purpose is data storage/retrieval, also
  contains information field(s) inside nodes
• Simplest is a linear linked list with single
  links
• Define node structure to hold info and a link
• typedef char AirportCode4 / e.g., LAX /
• typedef struct NodeTag
• AirportCode airport
• struct NodeTag link
• NodeType, NodePointer
• By convention, link NULL if last node in list

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So what is a linked list, really?

• Answer a sequence of zero or more nodes, with
  each node pointing to the next one
• Need a pointer to the first node
• Often referred to as the list
• Might be NULL means it is an empty list
• So define EMPTY(list) (list)NULL

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