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:

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:

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, 3 rd ed., Springer, New York, p Visited 23OCT Visited 23OCT