File Handling in C — How to Open, Close, and Write to Files (2024)

If you’ve written the C helloworld program before, you already know basic file I/O in C:

/* 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 parts of programming. In C, we use a structure pointer of a file type to declare a file:

FILE *fp;

C provides a number of build-in function to perform basic file operations:

• fopen() - create a new file or open a existing file
• fclose() - close 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 a integer from a file
• putw() - writes a integer to a file
• fseek() - set the position to desire point
• ftell() - gives current position in the file
• rewind() - set the position to the beginning point

Opening a file

The fopen() function is used to create a file or open an existing file:

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

There are many modes for opening a file:

• r - open a file in read mode
• w - opens or create a text file in write mode
• a - opens a file in append mode
• r+ - opens a file in both read and write mode
• a+ - opens a file in both read and write mode
• w+ - opens a file in both read and write mode

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

#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 the screen. How is this file IO?”

The answer isn’t obvious at first, and needs some understanding about the UNIX system. In a UNIX system, everything is treated as a file, meaning you can read from and write to 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 available to you with effort, 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 Learned 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 >>:

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

The contents of hello.txt will, not surprisingly, be

Hello, world!

Say we have another program called greet, similar to helloworld, that greets you with a given 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 ">".

Note: these redirection operators are in bash and similar shells.

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.

The mode are 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 commonly used.

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 write the greetings to another file:

#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 untill 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"); } return EXIT_SUCCESS;}

Suppose names.txt contains the following:

KamalaLoganCarol

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

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