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The strncpy() is a string function defined in the string.h header file, which is used to copy the first n number of characters of a string to another string.

Signature of strcpy() function

char * strncpy ( char * destination, const char * source, size_t num )

• This function copies the first num, number of characters of source string to another destination string.
• The function returns a char[] array pointer, pointing to the destination string containing the just copied string value.

Example of strncpy() function

In the upcoming code, we are calling strncpy() function, to copy the first 2 characters of one string named arr2 to another string named arr1.

/*  Program to append n number of characters of first string at the end of another string. */

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

int main()
{
char arr1[15] = "Keep Smiling";
char arr2[15] = "Always";

printf("The content of first string is : %s", arr1);
printf("\n");

printf("The content of second string is : %s", arr2);
printf("\n");

strncat(arr1,arr2,3); /* only first three characters from arr2 will be appended to the end of arr1 */

printf("The addition of two string is : %s", arr1);
printf("\n");
printf("The content of second string : %s", arr2);

return 0;
}

Output is -

The content of first string is : Keep Smiling
The content of second string is : Always
The addition of two string is : Keep SmilingAlw
The content of second string : Always

• The first char[] array i.e. arr1 is initialized with a string value Keep Smiling and the second char[] array i.e. arr2 is initialized with the value Always.
• The function strncpy() copies the first 3 characters of string value in the arr2 char[] array to the end of first char[] array i.e. arr1.

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Performing strncpy without using strncpy() function

Let's create a program copies the first n number of characters of one string to another string without using strncpy() function.

/* Program to copy n, number of characters of one string to another string without strncpy() function */


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

int main()
{
char dest[20] = "Hello";
char src[10] = "World";

printf("The value in dest string : %s", dest);
printf("\n");

printf("The second in src string : %s", src);
printf("\n");

int len1 = strlen(dest);
int len2 = strlen(src);

printf("Length of characters in dest string : %d \n", len1);

int num; 
printf("How many characters of the first string you to copy from str string : ");
scanf("%d", &num);

for(int i=0;i<num; i++)
{
	dest[i] = src[i];
	len1 = len1 + 1 ;
}
dest[len1]='\0';

printf("The new value in dest string : %s ", dest);

return 0;
}

Output is :

The value in dest string : Hello
The second in src string : World
Length of characters in dest string : 5
How many characters of the first string you to copy from str string: 2
The new value in dest string : Wollo

• The first char[] array i.e. dest is initialized with a string value Hello and the second char[] array i.e. dest is initialized with the value World.
• Next, we have calculated the total length of characters actually present in the first char[] array, by using strlen() function.
• Next, we ask the user to enter the number of characters of the first string, which we have manually copied to dest char[], replacing its content from Hello to Wollo.

Note: - The function strlen returns the total number of characters actually present in the char[] and not the total number of character it can hold.

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