The Dangers of Using `gets` in C/C : Why `fgets` is the Safer Alternative

Introduction

The function `gets` in C/C has been a part of the language for decades, but its use is now generally considered unsafe. This article explores why `gets` is dangerous and why you should use the safer alternative, `fgets`. The focus will be on the risks associated with `gets`, why it was deprecated, and how to effectively use `fgets` to prevent buffer overflows and ensure program stability.

Why `gets` is Unsafe

The `gets` function reads input from the standard input until a newline character is encountered. One of the primary reasons `gets` is considered unsafe is that it does not perform any bounds checking on the input buffer. This can lead to buffer overflow vulnerabilities.

No Length Limitation

`gets` reads until it encounters a newline or the end of the file, which means that if the user enters more characters than the buffer can hold, it will overflow. This can overwrite adjacent memory, leading to undefined behavior, crashes, or even security vulnerabilities. Attackers can exploit this behavior to execute arbitrary code or corrupt data.

Buffer Overflow

A buffer overflow occurs when a function writes more data to a buffer than it has space for. In the case of `gets`, if the user inputs more characters than the buffer size, the excess data overflows into adjacent memory locations. This behavior can cause the program to crash or behave unpredictably.

No Error Handling

`gets` does not provide any error handling mechanisms for reaching the end of the file or encountering errors during input. This can lead to unpredictable program behavior, making it difficult to diagnose and fix issues.

`gets` Deprecation

Due to its inherent risks, `gets` has been removed from the C11 standard. Instead, safer alternatives like `fgets` should be used. C11 formally marked `gets` as obsolescent and deprecated in ISO/IEC 9899:1999/Cor.3:2007 — Technical Corrigendum 3 for C99. Safe alternatives like `fgets` are preferred because they allow you to specify the size of the buffer and limit the amount of input that can be read.

usage of `fgets`

The `fgets` function allows you to specify the size of the buffer and reads a specified number of characters, which helps prevent buffer overflows. Here is an example of how to use `fgets`:

c            char buffer[100];            if (fgets(buffer, sizeof(buffer), stdin) ! NULL) {                // Process input safely            }        

This code reads input into the `buffer` until it reaches the specified size or encounters a newline character. By specifying the buffer size, `fgets` ensures that the buffer does not overflow.

Historical Context

The first internet worm, the Morris Internet Worm, used `gets` and a buffer overflow to propagate from system to system. The basic problem with `gets` is that it doesn't know how big the buffer is and continues reading until it finds a newline or encounters EOF, potentially overwriting the bounds of the buffer it was given. This historical example underscores the danger of using `gets` in practice.

Given the risks associated with `gets`, many modern development environments issue warnings when you use this function. For example, in modern versions of the Linux compilation system, using `gets` will generate warnings. This underscores the importance of using safer alternatives like `fgets` to prevent security vulnerabilities.

Conclusion

Using `gets` in C/C is inherently dangerous due to its lack of bounds checking, leading to potential buffer overflows, undefined behavior, and security vulnerabilities. To ensure your code is safe and secure, it is crucial to use safer alternatives like `fgets`. By following best practices and utilizing safer functions, you can mitigate risks and ensure the stability and security of your programs.