Homework Exam

1
Homework / Exam

• HW7 due class 25
• Exam 3 - class 26
• Open Book, Open Notes
• Covers up through end of KR 7
• and Appendix B Standard Library
• Plus UNIX Shells / Processes / Shell Scripts

2
Line Input and Output, KR 7.7

• The standard C library equivalents to getline and
  putline fgets and fputs
• Function fgets like getline() from a file
• char fgets(char line, int maxline, FILE fp)
• Reads the next input line (including '\n' at the
  end) from file fp into the char array line
• at most maxline-1 chars will be read
• then a terminal '\0' will be added.
• Returns ptr to line or NULL (means EOF)

3
Line Input and Output

• / sample code to understand fgets similar to
  getline /
• char fgets(char s, int n, FILE iop)
• register int c
• register char cs
• cs s
• while (--n gt0 (c getc(iop)) ! EOF))
• if ((cs c) '\n')
• break
• cs '\0''
• return (c EOF cs s) ? NULL s

4
Line Input and Output

• Function fputs writes line to fp
• int fputs(char line, FILE fp)
• It returns EOF if error occurs (disk fills up?)
  otherwise it returns zero
• Can use perror to print out exact error cause (to
  stderr, not user screen)

5
Miscellaneous Functions, KR 7.8

• Look at pg. 251-253 where they are covered
• 7.8.1 String Operations (string.h)
• Used many of these already
• Examples
• size_t strlen(const char s)
• int strcmp(const char s, const char t)
• char strcpy(char s, const char t)
• char strncpy(char s, const char t, size_t n)

6
Miscellaneous functions

• 7.8.2 Character Processing Functions (ctype.h)
• Used at least one of these already
• Examples
• int isalnum(int c)
• int isalpha (int c)
• int isdigit (int c)
• int isxdigit (int c)
• int isspace (int c)
• int tolower (int c)
• int toupper (int c)

7
Miscellaneous functions

• 7.8.3 Ungetc (stdio.h)
• Had something like this in homework, ungetch()
• Only have guarantee that you can push one char
  back, but that is usually enough in practice
• Example
• int c getc (stdin) / expecting an alpha
  character /
• if (!isalpha(c))
• ungetc (c, stdin) / if not alpha -
  return it /

8
Miscellaneous functions

• 7.8 4 Command Execution (stdlib.h)
• The function system() See pg 253
• int system(const char s)
• The string s contains a UNIX system command
• system(pwd)
• system(ls gtfname)
• system(prog)
• Return value depends on the command executed
• Learn that from man page for the command

9
Miscellaneous functions

• In program compiled as prog, determine values for
  a and b through argc and argv
• int a, b char commandMAXCMD
• sprintf(command, "prog d d gt prog.out", a, b)
• system(command)
• System call doesn't return data from the command,
  but by writing gt prog.out it creates an output
  file
• Calling program can then open/read prog.out file

10
Miscellaneous Functions

• 7.8.5 Storage Management (stdlib.h)
• malloc() andfree()
• Review Why is this incorrect?
• for (p head p ! NULL p p-gtnext)
• free(p)
• Review Why must it be written like this?
• for (p head p ! NULL p q)
• q p-gtnext
• free(p)

11
Miscellaneous Functions

• 7.8.6 Math functions (math.h)
• Need to use include ltmath.hgt in source code
• And use math library flag in execution of gcc
• gcc -lm source.c
• See list in appendix, mostly familiar!
• Examples
• double pow (double x, double y) / Not xy
  /

12
Miscellaneous Functions

• 7.8.7 Random number functions (stdlib.h)
• Two functions
• rand (void)
• srand (unsigned int seed) / default value is 1
  /

13
qsort

• Library function qsort prototype (stdlib.h)
• void qsort(void base, size_t n, size_t size,
• int (cmp) (const void , const
  void ))
• It sorts an array of data using quick sort
  algorithm
• base a pointer to the table (an array of unknown
  type data)
• n the number of elements in the table
• size the size of each element
• Whats the last argument?
• A pointer to a compare function for specific data
  type
• NOT the same function as qsort in KR, pg 120

14
bsearch

• Library function bsearch prototype (stdlib.h)
• void bsearch(const void key, const void base,
• size_t n, size_t size,
• int (cmp) (const void , const void ))
• It searches for element containing key in an
  array of data of unknown that is already sorted
  in ascending order
• Returns
• pointer to element (if found) or
• NULL (if not found)
• Last argument is a pointer to the compare
  function
• Should use same compare function as was used for
  sorting

15
qsort and bsearch

• qsort and bsearch dont understand the data type
  for the elements of the table that it sorts or
  searches
• How can we tell that?
• key (for bsearch only) is a type void
• base (the pointer to the table for both) is a
  type void
• size (the size of each element for both) is
  provided
• Last argument is a pointer to the correct compare
  function with a cast of the argument list
  variables to void

16
Example qsort and bsearch

• / compare function for integers lowest to
  highest, we pass a pointer to this function to
  qsort/bsearch /
• int intcompare(int i, int j)
• return (i - j)

17
Example qsort and bsearch

• main ()
• int i, ip, kp, a 10 8, 2, 9, 6, 5, 1, 3,
  7, 4, 0
• qsort ( (void ) a, sizeof a/sizeof (int),
  sizeof(int),
• (int () (void , void )) intcompare)
• for (i 0 i lt 10 i)
• printf(d, , ai)
• / prints 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
  /

18
Example qsort and bsearch

• kp a3
• ip (int ) bsearch ( (void ) kp, (void )
  a, sizeof a/sizeof (int), sizeof(int),
• (int () (void , void )) intcompare)
• if (ip ! NULL) / ip will be a3 /
• printf(found d\n, ip)
• / prints found 3 /

19
UNIX Time Representation

• UNIX time is kept as a signed int representing
  the number of seconds from Jan 1, 1970 UTC
• That date is referred to as the UNIX epoch
• Earliest representable date 12/13/1901
• Latest representable date 01/18/2038
• UNIX will have a Y2.038K problem!

20
UNIX Time Representation

• clock_t / elapsed processor time /
• time_t / calendar time /
• struct tm / calendar time components /
• int tm_sec
• int tm_min
• int tm_hour
• int tm_mday
• int tm_mon
• int tm_year
• . . .

21
UNIX Time Representation

• Time functions (time.h)
• clock_t clock(void) / returns processor
  time /
• time_t time (time_t tp) / returns current time
  /
• double diff_time(time_t time2, time_t time1)
• / returns time difference in seconds /
• struct tm gmtime(const time_t tp)
• / returns tp as UTC struct tm /
• struct tm localtime(const time_t tp)
• / returns tp as local time struct tm /
• char asctime(const struct tm tp)
• / returns a string representing tp, e.g.
• Sun Jan 3 130842 1988\n\0 /