Lecture 8a: File I/O

BJ Furman

21MAR2011

Learning Objectives

Explain what is meant by a data stream

Explain the concept of a ‘file’

Open and close files for reading and writing

Data Streams Data ‘stream’

“an ordered series of bytes” (Darnell and Margolis, 1996)

Like a 1D array of characters that can flow from your program to a device or file or vice-versa

IO involves reading data from or writing data to a stream

Prior to UNIX Programmers had to handle all the intricacies and complexities of

interfacing to input and output devices, such as card readers, printers, terminals, etc.

UNIX Abstracted away the details of IO to the concept of the data stream Established standard data streams:

stdin – data coming into your program (usually from the keyboard) stdout – data going out of your program (usually to the display) stderr – for error information going out of your program

File IO Need to first associate a stream with a file or

device Three streams are automatically opened and

associated with your program: stdin, stdout, stderror Ex. printf() defaults to printing to the display

To read from or write to another file stream, you need to declare a pointer to a data structure called FILE This pointer is used to read from, write to, or close the

stream Use IO functions for file operations (like fprintf())

Opening a File - 1 Key steps:

Declare a pointer to FILE FILE *fp;

Provides the means to associate a file with a data stream Will be used by other functions such as fprintf()

Use fopen() function with a path to the file and a file mode as arguments

Ex. Open file_name.txt to be able to read from it fp = fopen(“file_name.txt”, “r”);

fopen() returns a pointer to the file fp stores the pointer to the file, file_name.txt “r” opens the file for reading from

Can also open a file to write to it or append to it

See reference: http://www.cppreference.com/wiki/c/io/fopen

Opening a File - 2 Other modes

Good idea to test that the file was opened without error fopen() will return NULL if there is an error opening the file

fp = fopen(“file_name.txt”, “r”); if(fp == NULL) { printf("Error: can't open file to read\n"); return 1;}

Source: http://www.cppreference.com/wiki/c/io/fopen

Closing a File Function header:

int fclose(FILE *fp); Good idea to test the file was closed without

error Test the return value of fclose

fclose() will return EOF if there is an error closing the file

if(fclose(fp) == EOF) { printf("Error closing file\n"); return 1;} It is best practice to close all files that you opened,

somewhere in your program

Arrays and File I/O

Arrays are often used with data consisting of many elements Often too tedious to handle I/O by keyboard

and monitor File I/O is used instead

File I/O and array example file_I_O.c

Review

References Darnell, P. A. & Margolis, P. E. (1996) C, a

software engineering approach, 3rd ed., Springer, New York, p. 327.

http://www.cppreference.com/wiki/c/io/fopen, Visited 23OCT2010.

http://en.wikipedia.org/wiki/Standard_streams, Visited 23OCT2010.