How to Optimize C Code With memcpy
Written by Jeff Broth   
Tuesday, 09 May 2023

If you need to move data in C, you need memcopy. Here we explain what it is, what it does and how its optimization at the hardware level leads to its ability to copy large volumes of content between memory blocks faster and more efficiently than other methods.


As the name suggests, memcpy is a memory copy function, which enables the replication of memory from one location to another. It is a standard library function in the C programming language intended for the efficient copying of memory across different locations. It copies a specific number of bytes of memory and uses pointers to store the address of the source and the intended destination.

What makes memcpy special is that it is optimized for performance and the efficient copying of big volumes of memory. It can also harness hardware acceleration as it uses special instructions to expedite memory copying. Also, notably, memcpy supports the handling of any type of data stored in a device's memory, including arrays and structures. This makes it a useful tool in code optimization.

Memcpy: the basics

Before discussing code optimization with the memcpy C function, here’s a brief overview of how this standard library function works. The function’s syntax has three parameters, as shown below.

void *memcpy(void *dest, const void *src, size_t n);

Dest (destination) and src (source) refer to the void pointers that indicate the location of the intended destination and source of the memory to be copied. The size is the specific amount of memory to be copied.

“Void” in the syntax above refers to the void pointer, which is used to contain the address of the memory (copied or being copied to). It supports all known types of data. "Const," an abbreviation for "constant," indicates that a variable cannot be modified or reassigned if it already has an assigned value. Meanwhile, "size_t" indicates an unsigned integer or value that should not be a negative number, since this keyword is used for the size (in bytes) of the data being copied.

Memcpy is usually used in programming with C to copy the contents of an array to a different one. It is also used to copy structures that are not replicable through simple assignments. Memcpy is often regarded as the quicker and more efficient option when it comes to copying memory, as opposed to doing a loop-based approach. 

The efficiency and optimization potentials of memcpy

Memcpy gained the distinction of being an efficient memory-copying function mainly because of its optimization at the hardware level. It is designed to maximize the benefits brought about by the memory access patterns in newer CPUs. Since modern CPUs come with memory prefetching and parallel operation capabilities, they provide the advantage of memory access predictability. In contrast, loop-based approaches are characterized by random memory access, which can adversely impact performance and result in cache misses.

Another advantage of memcpy that creates enhanced efficiency lies in the ability of compilers to optimize memcpy calls according to target system architecture. For example, multiple copies can be produced simultaneously by utilizing SIMD instructions. Also, these instructions can be used to unroll a loop and boost instruction-level parallelism while decreasing the instances of branching. These approaches in harnessing the nature of memcpy allow developers to achieve considerable performance improvements in their code as opposed to relying on manually-created loops.

Additionally, since memcpy is implemented at the assembly code level, it can be optimized for certain memory layouts and data types. This is another way by which code can be optimized. Using memcpy to copy memory allows developers to produce a code that takes advantage of the features of the data being copied, particularly the data’s memory layout, alignment, and size.

Moreover, memcpy helps lower the overhead associated with function calls, especially with loop-based functions. Function calls inevitably use up overhead, but memcpy minimizes its overhead requirement because it does not necessitate more instructions to undertake loop setup, termination, and branch prediction, which are present in loop-based functions.

Optimization in different use cases

The efficiency and versatility of memcpy make it the preferred memory copy function in various development tasks. In particular, it is commonly used when working with strings, pointers, and arrays.

Working with strings

One of the most common use cases of memcpy is the linking or concatenation of substrings. This is usually done to build larger strings from smaller ones to format texts in a specific manner, generate a final output string, parse inputs, and build URLs.

It may be necessary to concatenate texts to link separate data like first and last names or dates and hours stored in different locations. Similarly, different data may have to be concatenated to produce a final output, like generating a report’s header and footer out of different bits of information from different locations. On the other hand, input parsing may be required to extract data from input strings that do not present data in a format ready for automated actions. Moreover, memory copying from between strings is useful when working with web apps to build URLs pointing to specific web servers or online resources.

Memcpy can also be used to copy a part of a source string into a dest buffer or bring multiple strings together to form a new buffer. This is done to manage memory allocations, manipulate data, generate outputs, and integrate APIs.

Working with arrays

Another use of memcpy is when copying the contents of a large array into another array to manipulate data, pass data between functions, prevent the existence of shared memory when working with multiple processes or threads, and boost performance. Copying the contents of large arrays is a time-consuming process when done with the loop-based approach. With memcpy, the task duration significantly decreases.

Working with pointers

Memcpy is also the memory copying method of choice when dealing with pointers. This is particularly true when working with complex data structures and copying data between memory spaces that are not adjacent to each other. Memcpy is usually the preferred option because it is capable of efficiently copying content from a memory location to any location, including non-contiguous memory blocks. The loop-based memory copy approach is also capable of replicating content to non-contiguous memory blocks, but not as speedily as memcpy does. 

Optimization through efficient memory copying

To be clear, the memcpy C function is not a method or tool that optimizes code. Instead, it is a function in the C programming language that provides efficiency benefits when it comes to copying memory across different locations. It can copy large volumes of content between memory blocks faster and more efficiently compared to other methods. As such, it becomes easier to work on optimizations involving strings, arrays, and pointers.



Last Updated ( Tuesday, 09 May 2023 )