Understanding and Solving the Issue with gets() after scanf() in C

In C programming, the use of the gets() function can lead to significant problems, especially when it is called after a scanf() call. This article delves into the root causes of these issues and provides solutions to ensure your C programs behave as expected.

The Problem with gets() after scanf()

The main issue arises from the way scanf() and gets() interact with the input buffer, leading to unexpected behavior with gets(). Here's a detailed explanation of why this problem occurs:

Buffering and Newline Character

When a user inputs data using scanf(), it reads the input from standard input stream, stdin. However, scanf() does not consume the newline character ( ) that the user presses to submit the input. For example, in the code snippet below:

int num; scanf("%d", num);

After entering a number and pressing Enter, the newline character ( ) remains in the input buffer.

The Behavior of gets() After scanf()

When you call gets() after scanf(), it reads inputs from the input buffer. Since the newline character is still present, gets() reads this newline character and immediately terminates the string read, resulting in an empty string being stored. This behavior is illustrated in the following code:

#include int main() { int num; char str[100]; printf("Enter an integer: "); scanf("%d", num); // The newline character remains in the buffer printf("Enter a string: "); gets(str); // This will read the newline and store an empty string printf("You entered: %s ", str); return 0; }

Recommended Solutions

To avoid the problems associated with this behavior, you can:

Use getchar() to Consume the Leftover Newline After scanf()

A common solution is to use getchar() to consume the leftover newline character after scanf(). Here's how you can modify the code:

int num; scanf("%d", num); getchar(); // Consume the newline character char str[100]; gets(str);

This way, the next call to gets() does not read the newline character left in the buffer, and the string is correctly read.

Prefer Using fgets() Instead of gets()

fgets() is generally considered a safer alternative to gets(). It allows you to specify the size of the buffer, and it does not provide the option to overflow the buffer, which can lead to buffer overflow vulnerabilities. Here's an example of how to use fgets() instead:

char str[100]; fgets(str, sizeof(str), stdin);

Conclusion

The use of gets() is strongly discouraged due to its risks, including buffer overflow and the behavior after scanf(). It is better to use fgets() for reading strings safely in C. Ensuring your code handles input buffers correctly can prevent many common pitfalls in C programming.