File Management in C

• Unit 4 is about file management in C

Table of Contents

• The unit covers file definitions, structures, and record concepts.
• It includes file access modes like sequential, random, and binary.
• Creating, opening, reading, writing, moving within, and closing files are covered.
• Error handling using I/O operations is to be discussed.
• It also includes command line arguments and how to use them.

Introduction to File Management

• File management in C involves creating, reading, writing, and closing files
• The C programming language has functions to read and write to files in a structured manner
• Functions such as feof() to check the end-of-file and ferror() to check for errors are used in file management

Why Use Files?

• Data is lost when a program terminates unless you store it in a file
• Files allow preservation of data
• It is easy to access the file contents using C commands, if you have a file containing all the data
• Data can be moved easily between computers

File I/O

• Storing data for later retrieval is necessary using a "file" on disk
• Different functions exist to handle this
• fscanf() is for formatted input, while fprintf() is for formatted output
• getc(), getw(), and fread() are for unformatted input, with putc(), putw(), and fwrite() for unformatted output
• Each function has its own syntax and meaning

File Streams

• A stream is either reading or writing data and allowing user access to files. Stream is a file or physical device like key board, printer, or monitor
• When a C program starts, the OS opens three streams: stdin for standard input, stdout for standard output, and stderr for standard error
• Stdin connects to the keyboard, while stdout and stderr link to the monitor

Files

• Data is stored as a collection of bytes in files on secondary storage devices like hard disks
• Files store a sequence of bytes
• stdio.h header files are used in file operations

File Structure in C

• Text files and binary files are the 2 types of files to be known when working with files
• Text files are normal .txt files that can be easily created and edited using simple text editors like Notepad
• Binary files store data in binary
• Binary files can store higher amounts of data, is not readable easily and provides more security than text files

Concept of a Record in C

• A record in C is structured with different data types, often in a "struct"
• structs group related data under one name

File Management using Records in C

• Records are often read from and written to files
• fwrite() writes a record to a binary file
• fread() reads a record from a binary file

Writing a Record to a File

• First, a file is opened in binary mode
• The record is then written to the file using fwrite()

Reading a Record from a File

• The file will be opened in reading and binary mode
• The fread() will read the record from the binary file

File Access Modes

• Access modes define how a file is opened and used, with three common types: sequential, random, and binary

Sequential Access

• Data is read or written in order from the start to the end of the file
• Used when reading or writing from start to finish
• For example, reading from a log file or writing to a CSV file

Random Access

• Data can read or write to a file at any location without going through the data sequentially, there is a specific pointer to move to any location within the file
• Use cases include updating specific records in databases and binary files without having to read or write the whole file
• For example, database files where records can be updated

Binary Access

• For reading/writing data in binary format, it is faster and compact
• Used for non-text formats like images, audio, or executables
• You can read image files and can write to files

File Operations

• File operations are actions on computer files like reading, modifying, creating, deleting, and querying

File Operations Steps

• Declaration of the file pointer, opening with fopen(), processing with suitable file functions, and then closing with fclose()
• Example syntax to declare a file: FILE *fp;

File Opening Modes

• "r" opens a text file in read mode
• "w" opens a text file in write mode
• "a" opens a text file in append mode
• "r+" opens a text file in read and write mode
• "w+" opens a text file in read and write mode
• "a+" opens a text file in read and write mode
• "rb" opens a binary file in read mode
• "wb" opens a binary file in write mode
• "ab" opens a binary file in append mode
• "rb+" opens a binary file in read and write mode
• "wb+" opens a binary file in read and write mode
• "ab+" opens a binary file in read and write mode

Append vs Write Mode

• Write ("w") and Append ("a") are used to write in a file
• In both modes, new file is created if the file doesn't exists already, if it does, in write mode, the file is reset and data is deleted
• Append mode adds data to the existing data of the file if it exists

File Operations Continued

• The fopen() function makes a new file if a file doesn't exist

Steps to create a new file in C

• Include the stdio.h library
• Create a new file using fopen()
• Write to the file using fprintf()
• Close the file with fclose()

Opening a File in C

• Files can be opened for use in different modes
• Use "r" to read files

Create a new file using fopen()

• The fopen() will create a file depending on which mode is given
• "w" opens the file for writing, its content will be truncated if it exists
• "a" opens the file for appending, it will add new content to the end of the file if it exists

Reading from a File

• Use the standard library (stdio.h) to read from a file in C
• Common functions includes fgetc() that reads each characters, fgets() to read whole line and fread() to read a block of data
• Files can be read line by line
• Can also be read with fscanf()
• Binary files can also be read

Writing to a file

• Files can be opened using different mode such as fprintf() and puts()

Function fseek()

• The file pointer will move to a location within the file using the fseek() function
• The arguments are stream which means pointer to the file, offset number of bytes to move and whence which is the direction to move the pointer
• SEEK_SET moves to beginning
• SEEK_CUR moves from current position
• SEEK_END moves from the end of the file

File Position can be read using code

• Code is used for specific positioning and reading, by moving to the end to get file size, for moving backwards to the end to get file and for overwriting specific positions

Closing a File

• Closes the system resources that are used
• Can also close multiple files using code

Error Handling in File Operations

• Essential to have good code

Error Handling using I/O operations

• Avoid crashing program with File handling
• stdio.h has error functions such as fopen(), fread(), fwrite() and fseek()
• Check if the fopen() is NULL to indicate if there is an error

Check File Read Errors

• Use ferror() to make sure that there were no reading while the code is reading

Check the File Write Errors

• A check is needed if fwrite() writes fewer elements, check for errors

Command Line Arguments

• Input values can be passed from the terminal when the program runs
• Makes programs more flexible
• Basic syntax is int main(int argc, char *argv[])
• Arguments include number of commands that are being passed, also an array function to hold the arguments as strings