Lecture 8a: File I/O

1. Lecture 8a: File I/O BJ Furman 21MAR2011

2. Learning Objectives • Explain what is meant by a data stream • Explain the concept of a ‘file’ • Open and close files for reading and writing

3. 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

4. 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())

5. 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

6. Opening a File - 2 Source: http://www.cppreference.com/wiki/c/io/fopen • 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; }

7. 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

8. 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

9. Review

10. 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.