memmove() in C: Copying Overlapping Memory Regions Safely

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memmove() in C: Copying Overlapping Memory Regions Safely

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Lesson format11 sections1 video4 code blocks1 practice itemUpdated Aug 27, 2026

memmove copies n bytes from source to destination, correctly handling the case where the regions overlap. It does this by copying to a temporary buffer or by choosing the copy direction based on the relative addresses. The result is as if the source bytes were first saved and then written to the destination. memcpy makes no such guarantee. If you are not certain the regions are disjoint, use memmove. The performance difference is negligible on modern implementations.

memmove() Function

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

int main(void) {
    char buf1[] = "ABCDEFGHIJ";
    char buf2[] = "ABCDEFGHIJ";

    printf("Original:     %s\n", buf1);

    memmove(buf1 + 2, buf1, 5);
    printf("memmove +2:   %s\n", buf1);

    char buf3[] = "ABCDEFGHIJ";
    memmove(buf3, buf3 + 2, 5);
    printf("memmove -2:   %s\n", buf3);

    return 0;
}

Run memmove_demo

root@vulnarex:~#gcc -Wall -fsanitize=address memmove_demo.c -o memmove_demo && ./memmove_demo

Shifting right by 2: the first five bytes ABCDE are copied to positions 2-6. The overlap is handled correctly. Shifting left by 2: bytes starting at position 2 (CDEFG) are copied to positions 0-4. Both directions work. With memcpy, at least one of these would produce corrupted output because the copy would overwrite source bytes before they are read.

#Practical Use: Inserting into an Array#link

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

int main(void) {
    int arr[10] = {1, 2, 3, 4, 5, 0, 0, 0, 0, 0};
    int used = 5;

    int insert_pos = 2;
    int new_value = 99;

    memmove(&arr[insert_pos + 1], &arr[insert_pos],
            (used - insert_pos) * sizeof(int));
    arr[insert_pos] = new_value;
    used++;

    printf("After insert at index %d: ", insert_pos);
    for (int i = 0; i < used; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    return 0;
}

Run memmove_insert

root@vulnarex:~#gcc -Wall memmove_insert.c -o memmove_insert && ./memmove_insert

The elements from index 2 onward are shifted right by one position to make room for the new value. The regions overlap: the destination starts one element after the source. memmove handles this correctly. A manual loop would also work, but memmove is a single call and the compiler can optimize it to a block move instruction.

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quiz BLOCK (★ 30 XP)

When should you use memmove instead of memcpy?

Select your proof vectors above

memmove is the safe default for memory copies where you are not sure about overlap. The performance cost is minimal. The correctness guarantee is absolute. When in doubt, memmove. Next: sizeof, the compile-time operator that tells you how much memory a type or expression occupies.

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Practical Use: Inserting into an Array

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August 27, 2026

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Laboratory sanity code

Isolate active probes on matched virtual networks and keep execution streams sandboxed.

#include <stdio.h>

#include <string.h>

int main(void) {

char buf1[] = "ABCDEFGHIJ";

char buf2[] = "ABCDEFGHIJ";

printf("Original: %s\n", buf1);

memmove(buf1 + 2, buf1, 5);

printf("memmove +2: %s\n", buf1);

char buf3[] = "ABCDEFGHIJ";

memmove(buf3, buf3 + 2, 5);

printf("memmove -2: %s\n", buf3);

return 0;

}

#include <stdio.h>

#include <string.h>

int main(void) {

int arr[10] = {1, 2, 3, 4, 5, 0, 0, 0, 0, 0};

int used = 5;

int insert_pos = 2;

int new_value = 99;

memmove(&arr[insert_pos + 1], &arr[insert_pos],

(used - insert_pos) * sizeof(int));

arr[insert_pos] = new_value;

used++;

printf("After insert at index %d: ", insert_pos);

for (int i = 0; i < used; i++) {

printf("%d ", arr[i]);

}

printf("\n");

return 0;

}