C strncpy() Function

The strncpy() function in C is a standard library function that copies a specified number of characters from one string to another. It is part of the C standard library (string.h). This function is useful for copying strings safely, as it allows you to specify the maximum number of characters to copy, thus preventing buffer overflows.

Table of Contents

1. Introduction
2. strncpy() Function Syntax
3. Understanding strncpy() Function
4. Examples
   • Copying a Simple String
   • Copying a String into a Struct
5. Real-World Use Case
6. Conclusion

Introduction

The strncpy() function copies up to n characters from the source string to the destination string. If the source string is shorter than n characters, the destination string is padded with null characters. If the source string is longer than n characters, the destination string will not be null-terminated.

strncpy() Function Syntax

The syntax for the strncpy() function is as follows:

char *strncpy(char *dest, const char *src, size_t n);

Parameters:

• dest: A pointer to the destination array where the content is to be copied.
• src: A pointer to the source null-terminated string to be copied.
• n: The maximum number of characters to copy.

Returns:

• The function returns a pointer to the destination string (dest).

Understanding strncpy() Function

The strncpy() function copies up to n characters from the source string to the destination string. If the source string is shorter than n characters, the destination string is padded with null characters until a total of n characters have been written. This ensures that the destination string is always null-terminated if n is greater than or equal to the length of the source string.

If the source string is longer than n characters, the destination string will not be null-terminated, so you may need to manually add a null terminator.

Examples

Copying a Simple String

To demonstrate how to use strncpy() to copy a simple string, we will write a simple program.

Example

#include <stdio.h>
#include <string.h>

int main() {
    char src[] = "Hello, World!";
    char dest[50];  // Ensure the destination is large enough

    // Copy the string using strncpy
    strncpy(dest, src, sizeof(dest) - 1);
    dest[sizeof(dest) - 1] = '\0';  // Ensure null termination

    // Print the destination string
    printf("Destination string: %s\n", dest);

    return 0;
}

Output:

Destination string: Hello, World!

Copying a String into a Struct

This example shows how to use strncpy() to copy a string into a struct.

Example

#include <stdio.h>
#include <string.h>

typedef struct {
    int id;
    char first_name[20];
    char last_name[20];
} Person;

int main() {
    Person p;
    p.id = 1;
    strncpy(p.first_name, "Ramesh", sizeof(p.first_name) - 1);
    p.first_name[sizeof(p.first_name) - 1] = '\0';  // Ensure null termination
    strncpy(p.last_name, "Fadatare", sizeof(p.last_name) - 1);
    p.last_name[sizeof(p.last_name) - 1] = '\0';  // Ensure null termination

    // Print the struct
    printf("Person ID: %d, First Name: %s, Last Name: %s\n", p.id, p.first_name, p.last_name);

    return 0;
}

Output:

Person ID: 1, First Name: Ramesh, Last Name: Fadatare

Real-World Use Case

Safely Copying User Input

In real-world applications, the strncpy() function can be used to safely copy user input strings into fixed-size buffers, ensuring that buffer overflows do not occur.

Example: Safely Copying User Input

#include <stdio.h>
#include <string.h>

int main() {
    char user_input[100];
    char copy[50];

    printf("Enter a string: ");
    fgets(user_input, sizeof(user_input), stdin);

    // Remove the newline character if present
    user_input[strcspn(user_input, "\n")] = '\0';

    // Copy the user input using strncpy
    strncpy(copy, user_input, sizeof(copy) - 1);
    copy[sizeof(copy) - 1] = '\0';  // Ensure null termination

    // Print the copied string
    printf("Copied string: %s\n", copy);

    return 0;
}

Output (example user input "Hello, Ramesh!"):

Enter a string: Hello, Ramesh!
Copied string: Hello, Ramesh!

Conclusion

The strncpy() function is used for copying strings safely in C. By understanding and using this function correctly, you can prevent buffer overflows and ensure that your strings are properly null-terminated. Always remember to manually add a null terminator if the source string is longer than the specified number of characters to copy.