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Files

Programs may directly read or write data to files in the system
So far we have been reading/writing data to command prompt – called standard input /output/error stream
To work with files we need file streams
Here is an example program to read from files

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Files

Data about file (ex text file) is information like the content of the file, how many characters it has, where does the file end, date of file creation etc
FILE is the structure used to store data about a file
It is defined in the header file stdio.h
Operations on files include opening/reading/writing-to/closing a file, moving to a particular location in file, creating a new file

Files

fopen is a library function that takes a file name and the mode of access (read/write etc) as input
Both the file name (input.txt) and the mode (r –reading mode) are specified as strings.
fopen

Files: fopen

In case fopen fails (for reasons like file not found, file not writable etc) it returns NULL
Always check if the returned file pointer is null or not before using

Files

fgetc is used to read a character from a file (whose pointer is given as input) similar to getchar for stdin
fgetc reads a character from the current “file position indicator”, advances the indicator to next character and returns the character read
A file implicitly ends with the EOF character (usually ASCII value -1) to signify the end of contents of the file

Files: fclose

fclose :

Files

The while loop reads the contents of the file “input.txt” character by character until the EOF of the file is encountered
The while loop also prints the characters read from file to the standard output stream stdout
fclose takes in a file pointer as input and closes the file corresponding to it.
The output of the program is

Files

Here is a program that reads from a file and writes to a file
The mode of accessing the file is “w” (write)
When a file is opened in the write mode, fopen creates a newfile with the name if the file doesn’t exist
fputc takes in a character, a file pointer as input and writes the input character to the file

Practice

Write a program that reads all characters of a file and writes these characters in reverse order to a new file.

File Modes

Mode	Operations
“r”	Reading from a file – sets up a pointer to the first character in it
“w”	Writing to a file. If the file already exists the file will be over-written, else a file with the name will be created
“a”	Opens a file for writing and sets up a pointer to the last character in it. If the file doesn’t exist a new file will be created. Used to append contents at the end of a file
“r+”	Reading existing content, writing new content, modifying existing content
“w+”	Writing new content, reading the new content and modifying existing contents of file
“a+”	Reading existing content, appending new content at the end of file – cannot modify existing content

Reference: Let us C

fopen returns a null pointer if the file cannot be opened (irrespective of the mode)