strncpy() Function in C

The strncpy() function in C is a predefined function in the string.h library used to copy a specified number of characters from one string to another. To use this function, we must include the <string.h> header file in our program. It copies up to n characters from the source string to the destination string and works with char data types.

Syntax of strncpy() in C

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

Parameters of strncpy()

The strncpy(dest, src, n) function takes three parameters:

• dest: A pointer to the destination array where the content is to be copied.
• src: A pointer to the source string to be copied.
• n: The number of characters to be copied from the source string.

Return Value of strncpy()

The strncpy(dest, src, n) function returns a pointer to the destination string dest. If the length of src is less than n, the remainder of dest will be padded with null characters.

Examples of strncpy() in C

Input:
char src[] = "Hello, World!";
char dest[20];

Output:
Destination string: Hello     //Copied first 5 characters from src to dest

The below examples demonstrates the usage of strncpy() in different scenarios in C language.

Example 1

The below program demonstrates the basic usage of the strncpy() function to copy a specified number of characters from one string to another.

// C program to demonstrate the basic usage of the strncpy() function

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

int main()
{
    // Source string
    char src[] = "Hello, World!";
    // Destination string
    char dest[20];

    // Copy the first 5 characters from src to dest
    strncpy(dest, src, 5);

    // Null-terminate the destination string
    dest[5] = '\0';

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

    return 0;
}

Destination string: Hello

Time Complexity: O(n), where n is the number of characters to be copied.
Auxiliary Space: O(1)

Example 2

The below example demonstrates how we can use strncpy() to copy a fixed length of characters from a source string to a destination string, ensuring the destination string is properly null-terminated.

// C program to demonstrate how we can use strncpy() to copy a fixed length of characters

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

void customStrncpy(char *dest, const char *src, size_t n)
{
    strncpy(dest, src, n);
    // Ensure the destination string is null-terminated
    dest[n] = '\0';
}

int main()
{
    // Source string
    char src[] = "Programming in C is fun!";
    // Destination string
    char dest[20];

    // Custom string copy with fixed length
    customStrncpy(dest, src, 15);

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

    return 0;
}

Custom copied string: Programming in 

Time Complexity: O(n), where n is the number of characters to be copied.
Auxiliary Space: O(1)

Example 3

The below example demonstrates how to use strncpy() to copy a fixed-length substring from the source string into the destination string.

// C program to demonstrate how to use strncpy() to copy a fixed-length substring

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

int main()
{
    // Source string
    char src[] = "Boyer-Moore Algorithm";
    // Destination string
    char dest[20];

    // Copy a fixed-length substring from src to dest
    strncpy(dest, src + 6, 5);

    // Null-terminate the destination string
    dest[5] = '\0';

    // Print the destination string
    printf("Fixed-length substring: %s\n", dest);

    return 0;
}

Fixed-length substring: Moore

Time Complexity: O(n), where n is the number of characters to be copied.
Auxiliary Space: O(1)

Points to Remember

• The strncpy() function does not null-terminate the destination string if the length of src is greater than or equal to n. Therefore, it is important to manually null-terminate the destination string.
• If src is shorter than n, the remainder of dest will be padded with null characters, ensuring the destination string is properly null-terminated.

pankajbind

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Article Tags :

• C Programs
• C Language
• C-String
• C-Functions