C - strncpy() function

C strncpy() function - copy fixed length string, returning a pointer to the array end

The strncpy() function is used to copy n characters of string2 to string1. If n is less than or equal to the length of string2, a null character (\0) is not appended to the copied string. If n is greater than the length of string2, the string1 result is padded with null characters (\0) up to length n.

Why and when to use strncpy()

• Copying Part of a String: strncpy() is useful for copying a specific number of characters from one string to another, allowing partial copying.
• Size-Limited Copying: Helps avoid buffer overflow by limiting the number of copied characters, making it safer when dealing with strings of unknown sizes.
• String Initialization: Commonly used for initializing strings with a fixed number of characters without exceeding array bounds.
• Padding with Null Characters: If the specified number of characters exceeds the source string length, strncpy() pads the destination with null characters.

Syntax:

char *strncpy(char *string1, const char *string2, size_t n);

Parameters:

Return value from strncpy()

• The strncpy() function returns a pointer to string1.

Examples: strncpy() function

Example 1: Copy a specific number of characters from one string to another

This example shows how strncpy() can be used to copy a specific number of characters from one string to another. First, only 2 characters from string2 are copied into string1, partially replacing it. In the second example, 5 characters from string4 are copied into string3, demonstrating partial copying and string padding when the character count exceeds the destination’s current length.

Code:

#include <stdio.h>
#include <string.h>
 
#define SIZE 40
 
int main(void)
{
  char string1[ SIZE ] = "123456789";
  char string2[ SIZE ] = "abcdefg";
  char string3[ SIZE ] = "123456789";  
  char string4[ SIZE ] = "abcdefg";
  char * return_string;
  int  n = 2; 
  printf("Original strings:");
  printf("\nString1: %s",string1);
  printf("\nString2: %s",string2);
  printf("\nn = %d",n);
  return_string = strncpy( string1, string2, n);
  printf("\nAfter strncpy(string1, string2, n):");
  printf("\nString1: %s",string1);
  printf("\nString2: %s",string2);
  n = 5; 
  printf("\n\nOriginal strings:");
  printf("\nString3: %s",string3);
  printf("\nString4: %s",string4);
  printf("\nn = %d",n);
  return_string = strncpy(string3, string4, n);
  printf("\nAfter strncpy(string3, string4, n):");
  printf("\nString3: %s",string3);
  printf("\nString4: %s",string4);
}
 

Output:

Original strings:
String1: 123456789
String2: abcdefg
n = 2
After strncpy(string1, string2, n):
String1: ab3456789
String2: abcdefg

Original strings:
String3: 123456789
String4: abcdefg
n = 5
After strncpy(string3, string4, n):
String3: abcde6789
String4: abcdefg

Example 2: Copying a partial string to initialize a file extension code:

Code:

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


int main() {
    // Source file name with extension
    char filename[] = "example.txt";
    // Array to store file extension
    char extension[5];

    // Copy only the last 4 characters to capture the file extension
    strncpy(extension, filename + 8, 4);
    // Ensure extension string is null-terminated
    extension[4] = '\0';

    // Print the extracted file extension
    printf("File extension: %s\n", extension);
    return 0;
}

Output:

File extension: txt

Explanation:

• Defines filename with "example.txt".
• Copies the last 4 characters of filename to extension, capturing ".txt".
• Manually null-terminates extension to ensure it functions as a string.
• Displays the extracted file extension.

Example 3: Copying a Prefix from a String

Code:

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

int main() {
    // Define the full name
    char fullName[] = "Bethari Cunmin";
    // Define an array to store the first name
    char firstName[8];

    // Copy the first 7 characters into firstName
    strncpy(firstName, fullName, 7);
    // Null-terminate firstName
    firstName[7] = '\0';

    // Print the first name
    printf("First name: %s\n", firstName);
    return 0;
}

Output:

First name: Bethari

Explanation:

• Defines fullName with "Jonathan Smith".
• Copies the first 7 characters into firstName.
• Adds a null terminator to ensure firstName is valid.
• Outputs "Jonathan" as the extracted first name.

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