CS 241 Section Week

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CS 241 Section Week 4(09/18/08)
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Topics This Section

• C Review (regarding HW 1)
• Disk I/O in C
• Function pointers in C
• MP 1
• MP 2

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C Review
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There is no bool in C

• In HW1, a number of people replaced a void
  function with bool.
• Theres no bool in C.
• Correct Response int
• General Convention (but not always)
• 0 false
• (positive) true
• (negative) error

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strcat() v. strcpy()

• In HW1, we asked
• char str8strcat(str, abcdefgh)
• There were two problems
• Not enough memory to store an 8-length string.
• strcat() concatenates strings together, but str
  may not be a 0-length string (it is initialized
  to undefined values).

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Return Paths

• In HW1, we asked
• int main(a) if (a gt 5) return (1)
• There was only one problem
• Not all paths returned a value from main().
• The declaration of main() wasnt actually wrong,
  its completely valid!

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argc, argv

• In HW1, we asked
• int main(int argv, char argv)
  print(argv1)
• There were two problems
• printf() requires a format string.
• Example printf(s, str)
• argv1 may be NULL if (argc 1). Always need
  to change the correct number of command line
  arguments are passed!

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Other minor problems

• Other less common problems
• Knowing (p) from (p).
• Knowing define macros.
• Freeing allocated memory.

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I/O in C
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I/O in C

• MP2 requires you to read and write simple file in
  C.
• File I/O was discussed briefly in Lecture 5.
• Two primary means of doing I/O in C
• Through lightly-wrapped system calls
• open(), close(), read(), write(), etc
• Through C-language standards
• fopen(), fclose(), fread(), fwrite(), etc
  

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I/O in C

• Opening a file (Method 1)
• fopen(const char filename, const char mode)
• filename path to file to open
• mode what do you wish to do with the file?
• r read only
• r read and write (file must already exist)
• w write (or overwrite) a file
• w write (or overwrite) a file and allow for
  reading
• a append to the end of the file (works for new
  files, too)
• a appends to end of file and allows for
  reading anywhere in the file however, writing
  will always occur as an append

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I/O in C

• Opening a file (Method 2)
• open(const char filename, int flags, int mode)
• filename path to file to open
• flags what do you wish to do with the file?
• One of the following is required
• O_RDONLY, O_WRONLY, O_RDWR
• And any number of these flags
• O_APPEND Similar to a in fopen()
• O_CREAT Allows creation of a file if it doesnt
  exist
• O_SYNC Allows for synchronous I/O
  (thread-safeness)
• To add these flags, simply binary-OR them
  together
• (O_WRONLY O_APPEND O_CREAT)
• mode what permissions should the new file have?
• (S_IRUSR S_IWUSR) creates a user read-write
  file.

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Opening Files in C

• Return value of opening a file
• Having called open() or fopen(), they will both
  create an entry in the OSs file descriptor
  table.
• Specifics of a file descriptor table will be
  covered in-depth in the second-half of CS 241.
• Both open() and fopen() returns information about
  its file descriptor
• open() Returns an int.
• fopen() Returns a (FILE ).

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Reading Files in C

• Two ways to read files in C
• fread(void ptr, size_t size, size_t count, FILE
  s)
• ptr Where should the data be read into?
  C-string?
• size What is the size of each piece of data?
  sizeof(char)?
• count How many pieces?
  strlen(str)?
• s What (FILE ) do we read from?
  ptrFile?
• read(int fd, void buf, size_t count)
• fd What file do we read from?
• buf Where should the data be read into?
• count How many bytes should be read?

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Reading Files in C

• Two basic ways to read files in C
• fread(void ptr, size_t size, size_t count, FILE
  s)
• ptr Where should the data be read into?
  C-string?
• size What is the size of each piece of data?
  sizeof(char)?
• count How many pieces?
  strlen(str)?
• s What (FILE ) do we read from?
  ptrFile?
• read(int fd, void buf, size_t count)
• fd What file do we read from?
• buf Where should the data be read into?
• count How many bytes should be read?

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Reading Files in C

• Reading more advancely
• fscanf(FILE stream, const char format, )
• Saw this on HW 1.
• Allows for reading at a semantic-level (eg ints,
  doubles, etc) rather than a byte-level.
• The format string (format) is of the same format
  as printf().
• No equivalent command for open().

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Writing Files in C

• Writing is a lot like reading
• fwrite(void ptr, size_t size, size_t count, FILE
  s)
• Writing of bytes with (FILE ).
• write(int fd, void buf, size_t count)
• Writing of bytes with a file descriptor (int).
• fprintf(FILE stream, const char format, )
• Formatted writiing to files (works like
  printf()).

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Closing Files in C

• Always close your files!
• fclose(FILE stream)
• close(int fd)
• Failure to close a file is much like a memory
  leak, but may corrupt files rather than memory.
• If a file is written to, the program ends, and
  the file is never closed the write never may
  never be written!
• Reason write(), and especially
  fwrite()/fprintf(), may be buffered before being
  written out to disk.
• On close()/fclose(), all buffers are cleared and
  the file is properly closed.

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Function Pointers in C
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Passing Functions in C

• In lecture, you saw pthreads
• When calling pthread_create(), you must pass a
  function
• pthread_create(, , void (start_routine)(void)
  , )
• What is void (start_routine)(void)?
• A parameter that requires a void ___(void ___)
  function!
• Ex void myfunc(void vptr)
  printf("I'm a thread!")

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Passing Functions in C

• In this MP, you must use qsort()
• The qsort() function definition looks like
• void qsort (void base, size_t num, size_t size,
  int (comparator)(const void , const
  void ))
• What is int (comparator)(const void , const
  void )?
• A parameter that requires a function of the
  following format int ___(const void ___,
  const void ___)
• That function should return (negative) if
  the (first param) lt (second param) 0
  if the (first param) (second param)
  (positive) if the (first param) gt (second param)

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Passing Functions in C

• Comparing Strings
• A function that could be accepted by qsort() to
  compare strings is int mystrcmp(const void
  s1, const void s2) return strcmp((char
  )s1, (char )s2)
• In MP 2, you will write a function for integers.

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Multithreaded I/O
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Reading/Writing across Threads

• Thread safe vs. expected output
• The C documentation states that fopen(), fread(),
  fwrite(), and fclose() are thread-safe.
• What does thread-safe mean?
• Short Answer Concurrent calls wont cause a
  race-condition.
• Results
• If (t1) calls fread() before (t2), (t1) will
  receive data before (t2).
• However, from execution to execution, the value
  returned to (t1) and (t2) will be different
  because the calls are made in a different order.

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MP2 Overview
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MP2 Overview

• Assignment
• Create a C program to read in N files
• Launch N threads to process each file
  concurrently
• Have each thread sort the specific file assigned
  to it
• Write the sorted data to N different files
• and count the total lines written as youre
  writing them.
• and print that output to the console.

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MP2 Overview

• Grading
• 80 60 Execution 20 Question
• 15 Memory cleaned up? (Includes files!)
• 5 Comments/readability
• This MP is harder than MP1!
• Dont wait until Monday to start on it!
• Requires a good understanding of pointers,
  threads, and files all in one MP.

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MP1 Initial Results
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MP1 Initial Results

• Autograder
• Maximum Possible 80 of the 100 total points
• Max 80/80
• Min 12/80
• Average 67.696/80 (aka 84.62)
• Std. Dev. 18.34
• Expect full MP1 grades next week.