Standard I/O Library

1
Standard I/O Library

• ANSI C standard specification
• handles details of I/O operations
• buffering
• block sizing
• uses calls to lower level functions

2
Streams and File Objects

• streams associated with files via file object
• FP pointer to a file object used for stream
  management
• object black box to programmer
• contains relevant info for file
• FPs passed to I/O functions for operations

3
stdin, stdout, stderr

• three file pointers that reference the same files
  as file descriptors STDIN_FILENO, STDOUT_FILENO,
  STDERR_FILENO
• use FPs stdin, stdout, stderr from stdio.h

4
Buffering

• used to decrease read/write call overhead
• intended to be as automatic as possible
• three types of buffering
• fully buffered
• line buffered
• unbuffered
• buffer obtained at first I/O function using open
  stream
• background operation
• buffering can be specified with setbuf, setvbuf

5
Fully Buffered I/O

• actual I/O takes place when buffer is full
• buffer is flushed when full, or when manually
  flushed via fflush.
• ANSI C required IF not interactive

6
Line Buffered I/O

• actual I/O takes place when newline character is
  encountered, or buffer is full, or input is
  requested via an interactive device
• line buffering typically used on terminals
• since I/O takes place when full, does not
  necessarily imply line ouput

7
Unbuffered I/O

• actual I/O takes place immediately, without
  buffering
• standard error normally unbuffered
• ANSI C requirement

8
Opening a Stream

• fopen, freopen, fdopen functions for opening
  streams
• All return FP on success, NULL on error
• All require type specification

9
fopen

• opens a specified file

10
freopen

• opens file on specified stream
• closes stream first if already open
• typically used to open file on standard stream

11
fdopen

• associates existing FD with stream (FP)
• uses previously obtained FD from low-level call
  (open,fcntl, etc.)
• typically used with network com channels

12
Stream opening types

• read
• write
• append
• read and write
• Each type has several extended capabilities
• binary option possible on all, but meaningless
  under UNIX

13
Closing a stream

• Streams are closed using fclose
• flushes output buffer and discards input buffer
  before close
• normal program termination will flush and close
  streams

14
Reading and Writing a Stream

• Three types of unformatted I/O
• character
• one character at a time
• line
• one line at a time
• direct (object)
• one object at a time
• sometimes called binary, structure or record I/O

15
Input Functions

• getc (macro)
• takes FP
• fgetc (true function)
• takes FP
• getchar
• getc(stdin)
• all used for single character input

16
Handling EOF

• functions return same value for EOF or error
• ferror, feof, clearerr used to handle EOF
• take FP, return TRUE/FALSE
• clearerr clears EOF and error flags for FP

17
Push back

• ungetc used to push characters back
• takes of chars, FP
• assume only one char possible
• once pushed back, character will be read on next
  read call
• used for peeking at data

18
Output Functions

• putc (macro)
• fputc (true function)
• putchar
• putc(c, stdout)
• all used for single character output

19
I/O by lines

• use fgets, gets for input
• gets deprecated (DO NOT USE)
• no buffer size specification
• drops newline
• use fputs, puts for output
• puts add newline
• use for line (string) input, output
• buffers are automatically terminated with NULL

20
Binary I/O

• use fread, fwrite
• used for reading/writing structures
• replace getc, putc using loops
• line I/O not usable for embedded newlines, NULLS

21
Positioning a Stream

• use functions ftell, fseek, fgetpos, fsetpos
• ftell fseek UNIX specific (long integer)
• fgetpos, fsetpos portable
• use fpos_t for position

22
Formatted Output

• use printf, fprintf, sprintf
• printf writes to stdout
• fprintf writes to FP
• sprintf fills an array
• appends NULL
• vprint... functions identical, but used with
  variable argument lists (see stdarg)

23
Formatted Input

• use scanf, fscanf, sscanf
• scanf reads from stdin
• fscanf reads from FP
• sscanf reads from string
• vscanf... functions identical but take variable
  argument list (see stdarg)

24
Implementation

• standard lib functions call low level I/O
  routines
• FD assigned to each FP
• obtain with fileno function
• stdio.h has all basic definititions for given
  system

25
Temporary Files

• use tmpnam, tmpfile
• used to create temporary files
• tmpnam creates (cross your fingers) unique
  pathname
• corresponding file must be created, or static
  buffer used
• mkstemp preferred under Linux
• tempnam same as tmpname but allows prefix
  directory and prefix specification
• tmpfile creates binary file