strncat() function in C/C++

The strncat() function appends a specified number of characters from one string to the end of another. It is declared in the <string.h> header file.

It appends at most n characters from the source string and adds a null terminator ('\0').
The destination array must have enough space to hold the concatenated string; otherwise, the behavior is undefined.

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

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

    strncat(dest, src, 3);

    printf("Result: %s\n", dest);

    return 0;
}

Output

Result: Hello Wor

Syntax

char *strncat(char *dest, const char *src, size_t n)

Parameters

This method accepts the following parameters:

dest: the string where we want to append.
src: the string from which 'n' characters are going to append.
n: represents a maximum number of characters to be appended. size_t is an unsigned integral type.

Return Value

The strncat() function shall return the pointer to the string(dest).

Application

Given two strings src and dest in C++, we need to append 'n' character from src to dest, let's say n=5.

Examples

Input: src = "world"
dest = "Hello "
Output: "Hello world"
Input: src = "efghijkl"
dest = "abcd"
Output: "abcdefghi"

Program

C++

#include <cstring>
#include <iostream>
using namespace std;

int main()
{
  
   // Take any two strings
   char src[50] = "efghijkl";
   char dest[50]= "abcd";
 
   // Appends 5 character from src to dest
   strncat(dest, src, 5);
    
   // Prints the string
   cout <<"Source string : "<< src << endl;
   cout <<"Destination string : "<< dest;
   
   return 0;
}

// C,C++ program demonstrate functionality of strncat()
#include <stdio.h>
#include <string.h>

int main()
{
  
   // Take any two strings
   char src[50] = "efghijkl";
   char dest[50]= "abcd";
 
   // Appends 5 character from src to dest
   strncat(dest, src, 5);
    
   // Prints the string
   printf("Source string : %s\n", src);
   printf("Destination string : %s", dest);
   
   return 0;
}

Output:

Source string : efghijkl
Destination string : abcdefghi

Comparison of strncat() and strcat()

Both strcat() and strncat() are used to concatenate strings, but strncat() provides better control over the number of characters appended.

strcat() appends the entire source string to the destination string until it encounters the null terminator ('\0').
strncat() appends only up to n characters from the source string, helping reduce the risk of buffer overflow.

C++

#include <cstring>
#include <iostream>
using namespace std;

int main()
{
  
   // Take any two strings
   char src[50] = "forgeeks";
   char dest1[50] = "geeks";
   char dest2[50] = "geeks";
 
   cout << "Before strcat() function execution, ";
   cout << "destination string : "<< dest1 << endl;
   
   // Appends the entire string of src to dest1
   strcat(dest1, src);
 
   // Prints the string
   cout << "After strcat() function execution, ";
   cout << "destination string : "<< dest1 << endl;
 
   cout << "Before strncat() function execution, ";
   cout << "destination string : "<< dest2 << endl;
   
   // Appends 3 characters from src to dest2
   strncat(dest2, src, 3);
    
   // Prints the string
   cout << "After strncat() function execution, ";
   cout << "destination string : "<< dest2 << endl;
   
   return 0;
}

// C,C++ program demonstrate difference between 
// strncat() and strcat()
#include <stdio.h>
#include <string.h>

int main()
{
  
   // Take any two strings
   char src[50] = "forgeeks";
   char dest1[50] = "geeks";
   char dest2[50] = "geeks";
 
   printf("Before strcat() function execution, ");
   printf("destination string : %s\n", dest1);
   
   // Appends the entire string of src to dest1
   strcat(dest1, src);
 
   // Prints the string
   printf("After strcat() function execution, ");
   printf("destination string : %s\n", dest1);
 
   printf("Before strncat() function execution, ");
   printf("destination string : %s\n", dest2);
   
   // Appends 3 characters from src to dest2
   strncat(dest2, src, 3);
    
   // Prints the string
   printf("After strncat() function execution, ");
   printf("destination string : %s\n", dest2);
   
   return 0;
}