Write a program that uses function strncmp to compare two strings input by the user. The program should input the number of characters to compare. The program should state whether the first string is less than, equal to or greater than the second string.

String comparison in programming is essential for various tasks like sorting, searching, and validation. In C++, comparing strings character by character is a common operation that can be performed using the strncmp function. This function takes two strings and an integer indicating how many characters to compare as its arguments. It then proceeds to compare both strings up to the specified number of characters.

For example, if you wanted to compare the first n characters of 'apple' and 'apricot', strncmp would go character by character, evaluating 'a' with 'a', 'p' with 'p', and so on, until it has examined the requested number of characters, or a difference is found. The function returns an integer indicating the result: 0 if the strings are equal, a negative value if the first string is lexicographically less than the second, or a positive value if it is greater. This allows the programmer to not only check for equality but also determine the lexicographical order of the strings.

Manipulating strings is a frequent requirement in many applications. C++ offers multiple ways to edit and handle strings through its standard library. Functions such as strcpy, strcat, strlen, and strncmp, all of which are part of the cstring header (also known as string.h in C), are used for different string operations.

For instance, the strncpy function is used to copy a certain number of characters from one string to another, ensuring that you don't exceed the destination string's bounds. It's important to be aware of buffer overruns and to ensure that strings are properly null-terminated after manipulation. Always take into account the size of the receiving buffer to prevent overflows and potential vulnerabilities.

The C++ Standard Library provides a collection of predefined functions for performing input/output operations, mathematical computations, time management, and string manipulation, among other tasks. These functions are encapsulated in various headers, such as iostream, cmath, ctime, and cstring.

Understanding these functions is crucial for efficient coding. In the context of the strcmp function, it's part of the cstring header and is a C-style string manipulation tool. While C++ also offers string class as part of the Standard Template Library (STL) that allows for easier and safer string operations, understanding the C-style functions is still valuable, especially for dealing with legacy code or interfacing with APIs expecting C-style strings.