Understanding gets() Function in C: Explained

Introduction

The gets() function in C is used to read a line of text from the standard input (stdin) and store it in a character array. This function is part of the standard C library and is commonly used in C programs.

How does gets() Function Work?

The gets() function reads a line of text from the standard input and stores it in the character array passed as an argument. It reads characters until it encounters a newline character or the end-of-file (EOF) character. The newline character is also stored in the array.

Here is an example of how to use the gets() function:

#include <stdio.h>

int main() {
   char str[50];

   printf("Enter a string: ");
   gets(str);

   printf("You entered: %sn", str);

   return 0;
}

In this example, the gets() function is used to read a line of text from the user and store it in the str character array. The printf() function is then used to print the input back to the user.

Why is gets() Function Dangerous?

The gets() function is considered dangerous because it does not perform any bounds checking on the input. This means that if the input is longer than the size of the array, it can overwrite the memory beyond the array and cause undefined behavior.

For example, consider the following code:

#include <stdio.h>

int main() {
   char str[5];

   printf("Enter a string: ");
   gets(str);

   printf("You entered: %sn", str);

   return 0;
}

In this code, the str array has a size of 5, but the gets() function can read input longer than 5 characters. If the user enters a string longer than 5 characters, the gets() function will overwrite memory beyond the str array and cause undefined behavior.

What is the Alternative to gets() Function?

To avoid the security issues caused by the gets() function, the fgets() function can be used instead. The fgets() function is similar to the gets() function, but it takes an additional argument to specify the maximum size of the input.

Here is an example of how to use the fgets() function:

#include <stdio.h>

int main() {
   char str[50];

   printf("Enter a string: ");
   fgets(str, 50, stdin);

   printf("You entered: %sn", str);

   return 0;
}

In this example, the fgets() function is used to read a line of text from the user and store it in the str character array. The second argument specifies the maximum size of the input, which is 50 in this case.

Conclusion

The gets() function in C is commonly used to read a line of text from the standard input and store it in a character array. However, it is considered dangerous because it does not perform any bounds checking on the input. To avoid security issues, the fgets() function can be used instead, which takes an additional argument to specify the maximum size of the input.