Strange behavior when using `strncpy` with overlapping memory regions in C
I'm working with an scenario where I'm using `strncpy` to copy strings within the same buffer, but the result isn't what I expect. I have a character array where I want to copy part of it to another location within the same buffer, but it seems like the operation is corrupting the data. Here’s a simplified version of what I’m doing: ```c #include <stdio.h> #include <string.h> int main() { char buffer[20] = "abcdefghij"; printf("Before: %s\n", buffer); strncpy(buffer + 5, buffer, 5); printf("After: %s\n", buffer); return 0; } ``` I expect the output to show the first five characters of `buffer` copied to the position starting at index 5, but instead, I get unexpected results. The output I see is: ``` Before: abcdefghij After: abcdeabcde ``` This seems to indicate that the `strncpy` is reading data from the original buffer while simultaneously writing to it, leading to this overlapping behavior. I've read that `strncpy` is not safe for overlapping memory regions, but I didn't expect such a blatant corruption of data. I've tried using `memmove` instead, which I believe is the safer option for handling overlapping regions: ```c #include <stdio.h> #include <string.h> int main() { char buffer[20] = "abcdefghij"; printf("Before: %s\n", buffer); memmove(buffer + 5, buffer, 5); printf("After: %s\n", buffer); return 0; } ``` Using `memmove` gives me the expected output of `abcdeabcde`. However, I’d like to understand why `strncpy` fails in this scenario and what the underlying issues are. Is my understanding correct that `strncpy` is not designed for this kind of operation with overlapping memory, and are there any specific guidelines I should follow when dealing with string manipulation in C?