C language memcpy library functions and the role of memmove

1. Let's start with memcpy Return value and parameters of library function , Let's be more familiar with this library function

As you can see in the picture above memcpy The return value is （void*） Pointer to type

The first parameter dest It refers to that the target spatial location type of the copied content is strongly changed to void*.

The second parameter src The data source type to be copied is const Embellished void*, Why do we have to change to void* Because of this memcpy Library functions are not limited by type , Can accept many types .

The third parameter is the number of bytes to be copied .

The following is the simulation memcpy function

---

#include<stdio.h>
#include<assert.h>
void* My_memcpy(void* s1, const void* s2, size_t count)        //s1 Namely p2  Is the target space to be copied 
{                                                              //s2 Namely p1  It is the data source that needs to be copied 
	assert(s1 && s2);
	void* flag = s1;
	while (count--)
	{
		*(char*)s1 = *(char*)s2;
		s1 = (char*)s1+1;
		s2 = (char*)s2+1;
	}
	return flag;
}
int main()
{
	int p1[10] = { 1,2,3,4,5,6,7,8,9,10 };
	int p2[5] = { 0 };
	My_memcpy(p2, p1, 20);
	int i = 0;
	int sz = sizeof(p2) / sizeof(p2[0]);        // Print p2 Contents of array 
	for (i = 0; i < sz; i++)
	{
		printf("%d ", p2[i]);
	}
	return 0;
}

2. Let's see next memmove Return values and parameters of library functions . 

 

  We can see memmove The return value and parameters of this library function are the same as memcpy It's exactly the same , that memmove What is the function of ？

memcpy When operating in the same array ,dest The starting position of is the same as src When the starting positions of overlap, the effect of coverage will be poor , As a result, the program does not achieve the desired effect , The following figure explains in detail .

 

 

When we want to modify in the same array, we can use memmove This library function , It perfectly fills memcpy Disadvantages of library functions , So as to achieve the effect we want .

The following is a simulated implementation memmove function

#include<stdio.h>
#include<assert.h>
#include<string.h>
void* My_memmove(void* s1, const void* s2, size_t count)             
{
	assert(s1 && s2);
	void* flag = s1;
	if (s1 < s2)
	{
		while (count--)                                     // From front to back 
		{
			*(char*)s1 = *(char*)s2;
			s1 = (char*)+1;
			s2 = (char*)+1;
		}
	}
	else
	{
		while (count--)
		{
			*((char*)s1 + count) = *((char*)s2 + count);    // Copy from back to front 
		}
	}
	return flag;
}
int main()
{
	int p1[10] = { 1,2,3,4,5,6,7,8,9,10 };
	My_memmove(p1+2, p1, 20);
	int i = 0;
	int sz = sizeof(p1) / sizeof(p1[0]);        
	for (i = 0; i < sz; i++)
	{
		printf("%d ", p1[i]);
	}
	return 0;
}

  Detailed illustration ：