Implementation of StrCmp, strstr, memcpy, memmove

One 、strcmp

1、 Definition

int strcmp(const char* str1, const char* str2);

2、 Function parsing

This function compares the first character of two strings . If they are equal , Just continue to compare the next pair , Until the characters are different or encounter '\0' .str1 and str2 Are two strings to be compared , If the first mismatched character is str1 Small in the middle , Return to one <0 Value , If in str1 Hit in , Return to one >0 Value , If the contents of two strings are equal, it returns 0 .

3、 Function realization

4、 Code testing

Two 、strstr

1、 Definition

const char* strstr(const char* str1, const char* str2);

2、 Function parsing

This function will return a pointer , Point to str2 For the first time in str1 Where in , If str2 No str1 Part of , Just return a null pointer . The matching process will not include '\0' , But it will be there. '\0' Stop at .

3、 Function realization

#include <stdio.h>
#include <assert.h>
const char* MyStrstr(const char* str1, const char* str2)
{
	assert(str1 && str2);

	// If str1 End found , Just close the loop 
	while (*str1)
	{   // Find the first pair of matching characters 
		while (*str1 != *str2 && *str1 != '\0')
			str1++;
		
		int i = 0;
		// At this time, the first pair of equal characters appear , Continue to match 
		while (*(str1 + i))
		{
			// Because the first pair of characters have been matched equal , therefore i You can start with ++
			i++;
			// If str2 Yes '\0', prove str1 There is str2 The content of 
			if (*(str2 + i) == '\0')
				return str1;
			// If in '\0' Before matching to unequal , Just make the next match 
			if (*(str1 + i) != *(str2 + i))
				break;
		}
		// If str1 + i  yes '\0', signify str1 It is impossible to include str2 了 
		if (*(str1 + i))
			str1++;
		else
			return NULL;
	}
	if (*str2 == '\0')
		return str1;
	else
		return NULL;
}

4、 Code testing

3、 ... and 、memcpy

1、 Definition

void* memcpy(void* destination, const void* source, size_t num);

2、 Function parsing

This function will num A value of bytes from source Copy the location pointed to destination Point to memory block ;source Pointers and destination The underlying type of the object pointed to by the pointer is independent of the function itself , The result is a binary copy of the data ; The function will not be in source Detect any terminating null characters in , It will accurately copy num Bytes ; To avoid crossing the border ,destination and source The size of the array pointed to by the parameter , It should be at least num Bytes , And will not overlap .（ For overlapping blocks of memory ,memmove Is a safer function .）

destination Returned .

3、 Function realization

4、 Code testing

 

Four 、memmove

1、 Definition

void* memmove(void* destination, const void* source, size_t num);

2、 Function parsing

Before parsing the function of this function , Let's do one thing with the last function ;

  Obviously ,memcpy Function can't do , The reason is simple , When you want to ‘c’ When copying ,'c' The original content and by 'a' To cover , So it's impossible to copy . therefore , For overlapping blocks of memory ,memcpy Appear unsafe .

Now let's see memmove function , This function can realize the movement of memory blocks . from sourc Point to the location of the copy num A value of bytes to destination Point to memory block . It seems that an intermediate buffer is used for copying , allow destination and source overlap .

source Pointers and destination The underlying type of the object pointed to by the pointer is independent of the function itself , The result is a binary copy of the data ; The function will not be in source Detect any terminating null characters in , It will accurately copy num Bytes ; To avoid crossing the border ,destination and source The size of the array pointed to by the parameter , It should be at least num Bytes ;

destination Returned .

3、 Function realization

This function is implemented , It's very simple , Just open up a space and copy a copy of the original data . But we can try to open up as little space as possible .

When source The position pointed to is destination In front of , Let's start with the last data 5 Start copying , You can see that before copying , The data to be copied will not be overwritten

And when source The position pointed to is destination In the back of , From the first data 5 Start copying , This effect can also be achieved . 

 

4、 Code testing