Do you want to know the difference between memcpy and memmove?
We’ve got you covered with this guide.
Memcpy and memmove are built-in C language functions that work similarly—copying memory blocks from one memory address to another within C.
However, the two are not the same, and they have varying, specific functions.
It’s crucial to know the difference between the two to understand which function to leverage in specific use cases.
After all, you need to determine the best tool and method to use, such as when deciding when to pick Crystal programming language to develop software.
This guide covers the main differences between memcpy and memmove to help you understand how each works and when to use them best.
Let’s go.
Memcpy and memmove: An overview
Memcpy is a C function used for copying a specific number of bytes or data (n characters) from a single location or array (source object) to another (destination object).
The function helps you copy or move data across memory locations.
Memcpy is crucial since memory, which are electronic components, can’t store infinite data, making it necessary to move your data to other locations to free up some space.
Consider the following when using memcpy:
- Ensure you copy data to and from objects that don’t overlap, or the behavior will show as undefined.
- Include a <string.h> header file before using the function.
- Specify the number of bytes before using the memcpy function to copy the memory areas.
- Copy memory areas or regions that are the same data type.
The best way to understand the function is to look at memcpy examples and their use cases.
Memcpy syntax:
void *memcpy(void * restrict dst ,const void * src ,size_t n);
Where:
- src is the pointer to the source object
- dst refers to the pointer to the destination object
- n is the number of bytes you wish to copy.
The memmove function also copies data from one location to another.
However, memmove copies the data from the source object first and moves it to the temporary array.
Then, the function copies the data from the temporary array to the destination object. It allows you to copy between objects that can overlap.
The function prevents an undefined behavior when your source and destination objects overlap.
Memmove Syntax:
void *memmove(void * restrict dst, const void *src, size_t n);
Memmove’s basic parameters are the same as memcpy.
Implementing memcpy and memmove in C
Generally, it’s best not to create your own memcpy since your standard or compiler library likely has a tailored and efficient memcpy implementation.
However, some situations call for creating your own memcpy function to work around certain limitations.
You can create a memcpy function with a simple memory-transfer algorithm that reads one byte individually, writing the byte before reading the next.
While this algorithm is pretty easy to deploy, it doesn’t always offer optimal performance, especially with memory buses that are wider than 8 bits.
As you can see in the code below, lines seven and eight handle the scenario so that the source and destination memory is not NULL.
Lines 10 to 15 contain a while loop that copies the data from the source to the destination one at a time and increments the source and destination pointer by one.
Similar to memcpy, your standard or compiler library will likely have an efficient memmove function implementation.
That being said, unless there are special use cases, avoid creating your own memmove function.
Implementing memmove is similar to memcpy.
The difference is that it can handle an overlapping scenario through a temporary array.
The code below copies all the n characters to a temporary array first, then copies the temporary array data to the destination.
Key differences between memcpy and memmove
Below are the main differences between the memcpy and memmove functions:
- Memcpy returns undefined behavior if the memory location that the source and destination pointers point to overlap.
On the other hand, memmove has a defined behavior that can handle overlapping scenarios. If you’re unsure if there is overlapping, your safest bet is to use memmove.
- It’s best to use memcpy when forwarding or duplicating copies and memmove when there is overlapping.
- Memmove is generally slower than memcpy because the memmove function uses an extra temporary array to copy data before copying the stored data to the destination.
Important considerations when using memcpy and memmove
While the memcpy and memmove functions work well, it’s best to know their limitations before using them, including the following:
- The memcpy and memmove functions can show undefined behaviors if you access the source and destination buffer beyond their lengths.
- Memcpy and memmove don’t check the destination and source buffer’s validity.
- The memcpy and memmove functions don’t check the terminal null character.
- Memcpy has limitations. For instance, the memcpy function can fail if the source parameter that is pointed to is in a shared block memory (between processes).
- Memmove allows memory regions to overlap but can require additional overhead than memcpy, including extra memory, resources, etc.
Quick summary: memcpy vs memmove
Check out the quick memcpy and memmove comparison below.
| memcpy | memmove | |
| Purpose | Copies data directly from the source to the destination. | Copies data to the temporary buffer or array before copying it to the destination. |
| Overlap | Shows a wrong output (undefined behavior) when the source and destination overlap. | Can handle overlaps by copying to a temporary array first. |
| Performance | Faster than memmove | Two times slower than memmove. |
| Best to use | In general use cases | When the source and destination overlap. |
Memcpy vs memmove: Which is better?
Using memcpy or memmove depends on your data and whether the source and destination overlap.
While the memcpy and memmove functions give the same results, it’s best to know how each works to ensure you use them correctly and effectively.
Both C programming language functions make it easier to move your data but ensure you understand their limitations before you start copying and moving.
Doing so helps you minimize potential data loss and avoid using an inefficient method to move large amounts of data.