Read data from a file using read() in C

C is a high-level general-use programming language for making software and system programming.

The read() function

The read() function is a low-level file manipulation function used to perform read operations on a file. It can be accessed by using the unistd.h library provided by C.

Syntax

Below, we can see the syntax for the read() function.

It takes in three arguments which are described below:

1. fileDescriptor: We need to provide the function with an integer file descriptor for the opened file, which the open() function returns when opening a file.

2. buffer: This pointer points to a buffer where data that is read will be stored.

3. bytesToRead: Here, we provide an unsigned integer variable that specifies the maximum number of bytes we want to read from the file.

The function, when executed, returns an integer value that refers to the number of bytes read.

Note: To avoid buffer overflow, the max value of the bytesToRead variable should not exceed the size of the buffer variable.

Program to read a file

Below, we can see a C program that uses the read() function to read the contents of the file hello.txt.

Code explanation

Below, we can see a line-by-line breakdown of the above code.

• Lines 1–4: We import the required libraries.

• Line 8: A char pointer is created that stores the file name to be opened.

• Line 10: We call the open() function to open the file. We pass two arguments, fileName and O_RDWR, to open the file in read and write mode. It will return an integer variable representing the file descriptor which we will store in a variable named fd.

• Lines 12–18: Here, we check if the file opened correctly. If it does not, we prompt the user and exit the program with an exit status of 1. Otherwise, we print a success message and continue.

• Line 20: We create a char array of size 1024 bytes called buffer that will store the file's contents.

• Line 22: We call the read() function to read the file's content. We pass three arguments: the file descriptor fd, the char array buffer, and the maximum number of bytes to read sizeof(buf). The function will return the number of bytes it read from the file, which we will store in an integer variable named bytesRead.

• Lines 24–25: Now, we will print the number of bytes that were read and the file's content by displaying the values inside the buffer and the bytesRead variables.

Coding exercise

Now we understand how to read a file's data using the read() function. We will now try to complete the coding challenge below.

Challenge

We have a text file named sample.txt with the content "The harder you work, the better you get."

We want to retrieve the first 15 characters from the files and display them in the console.

Complete the read() function below to display "The harder you" onto the console, which are the first 15 bytes of the file's content.