C memmove() function

C memmove() function - copy bytes in memory with overlapping areas

The memmove() function is used to copy n bytes of src to dest.

Use memmove() function when:

• You need to copy data within the same memory block, particularly if there’s a possibility of overlapping regions, such as shifting data in an array.
• Making in-place modifications to an array or buffer, allowing part of the array to overwrite another part without losing data.
• Adjusting sections within a structure or array where safe handling of overlapping regions is essential.

Syntax:

void *memmove(void *src, const void *dest, size_t n)

Parameters:

Return value from memviod()

The memmove() function shall return dest. No return value is reserved to indicate an error.

Examples: memmove () function

Example 1: Copy a specific part of a string within the same memory block

Following memmove() example demonstrates how to copy a specific part of a string within the same memory block. It sets up a pointer to the 8th position in the target string, where the word "brown" begins, and another pointer to the 2nd position. Using memmove(), it copies five characters from position 2 to position 8, effectively replacing "brown" with "e qui". This example highlights memmove()'s ability to safely handle overlapping memory areas without corrupting data, making it useful for in-place modifications.

Code:

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

#define SIZE  45 
char target[SIZE] = "The quick brown fox jumps over the lazy dog."; 
int main( void )
{
  char * p = target + 8;  /* p points at the starting character
                          of the word we want to replace */
  char * source = target + 2;   
  printf( "Before memmove, target is \"%s\"\n", target );
  memmove( p, source, 5 );
  printf( "After memmove, target becomes \"%s\"\n", target );
}

Output:

Before memmove, target is "The quick brown fox jumps over the lazy dog."
After memmove, target becomes "The quice quiwn fox jumps over the lazy dog."

Example 2: Copy data from one location to another safely

This example demonstrates the use of the memmove() function to copy data from one location to another safely, even if the source and target memory regions overlap. Here, memmove() copies the contents of str1 to str2.

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

#define MAX_LEN 80
 
char str1[ MAX_LEN ] = "This is the string1";
char str2[ MAX_LEN ] = "This is the string2";
 
int main(void)
{
  printf("Before memmove, strings are:");
  printf("\n%s",str1);
  printf("\n%s",str2);  
  memmove(str2, str1, sizeof(str1));
  printf( "\n\nAfter memmove, strings are:");
  printf("\n%s",str1);
  printf("\n%s",str2);  
}

Output:

Before memmove, strings are:
This is the string1
This is the string2

After memmove, strings are:
This is the string1
This is the string1

Example 3: Shifting elements in an array

This example demonstrates using memmove() to shift elements in an integer array by one position.

Code:

#include <stdio.h>

#include <string.h>
int main() {
    // Define an array of integers
    int arr[] = {1, 2, 3, 4, 5, 6};
    int size = sizeof(arr) / sizeof(arr[0]);

    // Shift elements in the array one position to the right
    memmove(&arr[1], &arr[0], (size - 1) * sizeof(int));

    // Update the first element
    arr[0] = 0;

    // Display the modified array
    printf("Shifted array: ");
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output:

Shifted array: 0 1 2 3 4 5

Explanation:

• An integer array is defined and initialized.
• memmove() shifts elements one position to the right by moving data from arr[0] to arr[1].
• The first element is set to 0, and the modified array is printed, showing the shift.

Example 4: Overwriting Part of a String

This example demonstrates replacing a portion of a string using memmove().

Code:

#include <stdio.h>
#include <string.h>
int main() {
    // Define a string
    char text[] = "Hello, wonderful world!";
    char *src = text + 7; // Points to "wonderful"
    char *dest = text + 0; // Points to the beginning of the string

    // Copy "wonderful" to the beginning of the string
    memmove(dest, src, 9);

    // Display the modified string
    printf("Modified text: %s\n", text);

    return 0;
}

Output:

Modified text: wonderfulnderful world!

Explanation:

• A string text is defined with the phrase "Hello, wonderful world!".
• Pointers are set to copy "wonderful" over "Hello," at the beginning of text.
• memmove() safely copies "wonderful" to the beginning, resulting in "wonderful world!".

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