Input validation

Buffering

In C input is read from a buffer
- When the buffer is empty, characters are moved into it
  - For terminal, buffer is usually filled with a line
  - For files, buffer is filled with a disk block
- When no more data is available, EOF is indicated
  - scanf returns the number of items actually read
  - getchar returns EOF
The operating system maintains its own buffers, unseen by your program

'E' 'C' 'E' ' ' '2' '0' '9' '\n'

Robust I/O programming

If input is not valid
- Terminate with message for file input
- Give another chance for interactive input
  - Be careful! It's hard to tell if the input is interactive.
  - Don't get into an infinite loop when no one is there.
Handle end-of-file gracefully
- With network outages, this can occur even on interactive input.
Handle missing files gracefully
- We'll get to this later...
Be kind.

Robust character input

On end-of-file, getchar() returns EOF
- EOF is not an ASCII value
  - usually it is -1
- Remember, getchar() returns an int

Skipping to the end of line

int inCh ;
while ((inCh=getchar()) && inCh != '\n' && inCh != EOF) {
}

Robust number input

Use gets() and sscanf()
- Allow the printing of nice error messages with line numbers.
- Covered in Chapter 12
scanf returns the numbers of items matched or assigned
- but can return EOF if end-of-file before a matching success or failure
For industrial strength programming, test the return code of scanf()
```
if (scanf("%d", &x) != 1) ....
```
- and skip to the next line for interactive input

Redirection

Unix, DOS, and Mac OS can redirect standard I/O to and from files
- Reading standard input from a file
```
myProg < testin.txt
```
- Writing standard output to a file
```
myProg > testout.txt
```
Very useful when testing assignments

End-of-file on a terminal

On Linux/Unix
- Type a ctrl+D
- The ASCII EOT, end of tranmission
On DOS/Windows
- Type a ctrl+Z
- Once stored as the last character of a file
The character is not read by the program