strncpy() Function in C

The strncpy() function copies at most a specified number of characters from one string to another. It is declared in the <string.h> header file.

• It copies up to n characters from the source string to the destination string.
• If the source string is shorter than n, the remaining characters in the destination are filled with '\0'.

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

int main() {
    char source[] = "GeeksforGeeks";
    char destination[20];

    strncpy(destination, source, 5);
    destination[5] = '\0';   // Null-terminate the copied string

    printf("Copied string: %s\n", destination);

    return 0;
}

Copied string: Geeks

Syntax 

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

Parameters

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

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

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

Example: Program to demonstrates the basic usage of the strncpy() function to copy a specified number of characters from one string to another.

#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

Example: Program to 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.

#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 

Example: Program to demonstrates how to use strncpy() to copy a fixed-length substring from the source string into the destination string.

#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

Points to Remember

strncpy() does not automatically append a null terminator if the source string length is greater than or equal to n. In such cases, you should manually add '\0' to the destination string.

• If the source string is shorter than n, strncpy() fills the remaining positions in the destination string with '\0'.
• Ensure the destination array has enough space to store the copied characters and the null terminator (if added manually).