Why is memmove faster than memcpy?

Both memmove and memcpy functions in the C standard library are used for memory copying, but they have different design purposes and application scenarios. Typically, memmove is not faster than memcpy; in fact, memcpy is generally faster in most scenarios. However, let's first understand their basic differences.

memcpy

• The memcpy function copies n bytes from a source memory address to a destination memory address. It assumes that these two memory regions do not overlap.
• Because it lacks additional logic for handling memory overlap, memcpy typically provides very high performance.

memmove

• The memmove function also copies n bytes from a source memory address to a destination memory address. However, memmove can correctly handle cases where the source and destination memory regions overlap.
• To handle overlapping cases, memmove may employ a temporary buffer or perform conditional checks to ensure that copied data is not lost due to overwriting, which typically makes memmove slower than memcpy.

Performance Comparison

• memcpy is typically faster than memmove because it lacks the additional overhead of handling memory overlap. When it is confirmed that the memory regions do not overlap, it is recommended to use memcpy for better performance.
• Although memmove may be slower than memcpy when handling non-overlapping memory, it is a safe choice, especially when it is uncertain whether the memory regions overlap.

Usage Scenario Example

• Suppose we have an array int arr[10], and we need to copy the first 5 elements to the middle of this array, specifically from arr[2] to arr[6]. In this case, using memcpy may result in the source data being overwritten during the copy, leading to incorrect results. On the other hand, memmove can safely handle this memory overlap, ensuring correct data copying.

In summary, memmove is not faster than memcpy; instead, it is typically slower because it needs to handle more scenarios, such as memory overlap. However, it is necessary when handling memory overlap and provides a safe memory copy guarantee.