C File Handling


If you’ve written the C helloworld program before, you’ve already done file INPUT/OUTPUT (generally referred to as IO) in C! Congratulations!

// A simple hello world in C #include <stdlib.h> // Import IO functions #include <stdio.h> int main() { // This printf is where all the file IO magic happens! // How exciting! printf("Hello, world!\n"); return EXIT_SUCCESS; }

File Handling is one of the most important part of a programming . In C language we use a FILE type structure pointer to declare a file.

FILE *fp; ..// declaration of file name as fp

C provides a number of build-in functions to perform basic file operation

FunctionOperation
fopen()creates a new file or open an existing file
fclose()closes a file
getc()reads a character from a file
putc()writes a character to a file
fscanf()reads a set of data from a file
fprintf()writes a set of data to a file
getw()reads an integer from a file
putw()writes an integer to a file
fseek()sets the position to desired point
ftell()gives current position in the file
rewind()sets the position to the starting point

fopen() creates a new file or open an existing file

fclose() closes a file
//Most important part is to close the file to restrict any further changes.

getc() reads a character from a file

putc() writes a character to a file
//getc and putc() both method are are used for string or char arrays.

fscanf() reads a set of data from a file

fprintf() writes a set of data to a file

getw() reads an integer from a file

putw() writes an integer to a file

fseek() sets the position to desired point

ftell() gives current position in the file

rewind() sets the position to the starting point

Opening a file

The fopen() creates a file or opens an existing file

fp = fopen(const char filename,const char mode);

In C there are many mode for opening a file

r open a file in reading mode
..//Provide access only to read a file but not to write it.

w opens or create a text file in writing mode
..//Provides access only to write on file not to read it.

a opens a file in append mode
..//Provides acces to append more words in file.

r+ opens a file in both reading and writing mode

a+ opens a file in both reading and writing mode

w+ opens a file in both reading and writing mode

b opens a file in binary mode

Here’s an example of reading and writing data to a file

#include<stdio.h> #include<conio.h> main() { FILE *fp; char ch; fp = fopen("hello.txt", "w"); printf("Enter data"); while( (ch = getchar()) != EOF) { putc(ch,fp); } fclose(fp); fp = fopen("hello.txt", "r"); while( (ch = getc(fp)! = EOF) printf("%c",ch); fclose(fp); }

Now you might be thinking, “This just prints text to my screen. How is this file IO?”
The answer isn’t obvious at first, and needs some understanding about the UNIX system.
Under a UNIX system, everything is treated as a file, meaning you can read and write from it.
This means that your printer can be abstracted as a file since all you do with a printer is write with it.
It is also useful to think of these files as streams, since as you’ll see later, you can redirect them with the shell.

So how does this relate to helloworld and file IO?

When you call printf, you are really just writing to a special file called stdout, short for standard output.

stdout represents the standard output as decided by your shell, which is usually the terminal.
This explains why it printed to your screen.

There are two other streams (i.e. files) that are easily available, stdin and stderr.
stdin represents the standard input, which your shell usually attaches to the keyboard.
stderr represents the standard error output, which your shell usually attaches to the terminal.

Rudimentary File IO, or How I Learnt to Lay Pipes

Enough theory, let’s get down to business by writing some code!
The easiest way to write to a file is to redirect the output stream using the output redirect tool, >.
If you want to append, you can use >>. N.b. these redirection operators are in bash and similar shells.

# This will output to the screen... ./helloworld # ...but this will write to a file! ./helloworld > hello.txt

The contents of hello.txt will, unsurprisingly, be

Hello, world!

Say we have another program called greet, similar to helloworld, that greets you given your name.

#include <stdio.h> #include <stdlib.h> int main() { // Initialize an array to hold the name. char name[20]; // Read a string and save it to name. scanf("%s", name); // Print the greeting. printf("Hello, %s!", name); return EXIT_SUCCESS; }

Instead of reading from the keyboard, we can redirect stdin to read from a file using the < tool.

# Write a file containing a name. echo Kamala > name.txt # This will read the name from the file and print out the greeting to the screen. ./greet < name.txt # ==> Hello, Kamala! # If you wanted to also write the greeting to a file, you could do so using ">".

The Real Deal

The above methods only worked for the most basic of cases. If you wanted to do bigger and better things, you will probably want to work with files from within C instead of through the shell.
To accomplish this, you will use a function called fopen. This function takes two string parameters, the first being the file name and the second being the mode.
Mode is basically permissions, so r for read, w for write, a for append. You can also combine them, so rw would mean you could read and write to the file. There are more modes, but these are the most used.

There are a number of modes that can be used for file handling.

“r” Open file for reading

rb Open file for reading in binary mode

w Open file for writing

wb Open file for writing in binary mode

a Open file for appending

ab Open file for appending in binary mode

r+ Open file for both reading and writing

rb+ Open file for both reading and writing in binary mode

w+ Open file for both writing and reading

wb+ Open file for both writing and reading in binary mode

a+ Open file for both reading and appending

ab+ Open file for both reading and appending in binary mode

After you have a FILE pointer, you can use basically the same IO commands you would’ve used, except that you have to prefix them with f and the first argument will be the file pointer.
For example, printf‘s file version is fprintf.

Here’s a program called greetings that reads a from a file containing a list of names and writes to another file the greetings.

#include <stdio.h> #include <stdlib.h> int main() { // Create file pointers. FILE *names = fopen("names.txt", "r"); FILE *greet = fopen("greet.txt", "w"); // Check that everything is OK. if (!names || !greet) { fprintf(stderr, "File opening failed!\n"); return EXIT_FAILURE; } // Greetings time! char name[20]; // Basically keep on reading until there's nothing left. while (fscanf(names, "%s\n", name) > 0) { fprintf(greet, "Hello, %s!\n", name); } // When reached the end, print a message to the terminal to inform the user. if (feof(names)) { printf("Greetings are done!\n"); } fclose(names); fclose(greet); return EXIT_SUCCESS; }

Suppose names.txt contains the following:

Kamala Logan Carol

Then after running greetings the file greet.txt will contain:

Hello, Kamala! Hello, Logan! Hello, Carol!

Super awesome, right!

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