CSC195, Class 27: Files in C

Overview:
* What is a file?
* C's FILE type
* Opening files
* Writing to files
* Reading from files

Notes:
* No Lindsey show.  Sorry.
* Exam 1 ready.  Yay!
* No Sam on Friday.  Daren and Mr. Gum teach.  Yay!
* Questions on HW5?  Devin: You need to learn to sort
  eventually, even if it takes you redoing the same
  assignment in fifteen different languages.

Questions on sorting:
* Do you really have to deal with all types, including
  user-defined?  Of course.
* Why void **?
  void qsort(void **array, int len, int (*lessThan)(void *, void*))

We're sorting arrays of pointers to values, since that's
really the only way to be general in C.
* Yes, that's okay for the homework.

**array == void *array[]

(*lessThan)(array[0], array[1])

To sort the number 10, 12, 4, 23 6

int iLessThan(int *ip, int *jp) {
  return *ip < *jp;
}

int values[] = { 10, 12, 4, 23, 6 };
int *pointers[] = { &values[0], &values[1], ... }

qsort((void **) pointers, 5, iLessThan);

page 119

 (int(*)(void *, void *))(numeric ? numcmp : strcmp)

----------------------------------------

What's a file?
* A named persistent collection of bits

Many files can be treated as sequences of bytes rather than
bits (and perhaps even sequences of characters)

Most C I/O is based on bytes.

In most languages, you don't deal directly with files, you
deal with an abstraction
* How does the abstraction help us? It keeps track of the
  position within the file.
* In C, the name of the typical abstraction used is FILE *
* To open a file
FILE *myfile = fopen("filename", "mode")
  "r" - read
  "w" - write
  "a" - append
  "r*" - read and write, don't change contents
  "w*" - read and write, zap first
90+% of the time, you'll use one of the first three.

Key non-file output operations?
* putchar, putc
* printf
Key file output operations
* fputc(FILE *, char)
* fprintf(FILE *, str, val1, ... valn)

Key non-file input operations? 
* getc
* scanf
* gets

Key file input operations
* fgetc(FILE *)
* fscanf(FILE *, char *, val1 ... valn)
* fgets(char *, int, FILE *)

"We clean up after ourselves"
* fclose(FILE *)

----------------------------------------

How can you tell that the author of readLine was not thinking
like a C programmer when (s)he wrote it?