Simulation Implementation of string function (Part 1)

strlen function

size_t strlen ( const char * str );

strlen The return value of the function is size_t

What is? size_t ?

size_t： typedef unsigned int size_t ( stay 32 Bit operating system )

That is to say unsigned int

strlen Use of functions

int main()
{
    
    char arr1[] = "hello world";
    int len = strlen(arr1);
    printf("%d\n", len);
    return 0;
}

Simulation Implementation strlen

// The method of counting 
int my_strlen(char* str)
{
    
	int count = 0;
	while (*str != '\0')
	{
    
		count++;
		str++;
	}
	return count;
}

// Method of subtracting pointer from pointer 
int my_strlen(char* str)
{
    
	char* tmp = str;
	while (*tmp != '\0')
	{
    
		tmp++;
	}
	return tmp - str;
}

// Recursive method 
int my_strlen(char* str)
{
    
	// Termination conditions 
	if (*str == '\0')
	{
    
		return 0;
	}
	return 1 + my_strlen(str + 1);
}

1、strlen The function returns a string with ‘\0’ The number of characters that appear before , barring ‘\0’

But in sizeof In keywords Will be able to ‘\0’ As the size of the occupied space sizeof Just look at the size of the space

2、（ It's easy to get wrong ） The return value of the function is an unsigned integer

3、 If the string does not start with ‘\0’ At the end of the day ,strlen The value returned by the function is a random value

strcpy function

char * strcpy ( char * destination, const char * source );

from source Copy the source address to destination In the address

1、 The target space has to be large enough , It can accommodate the source string

2、 The target space has to be variable

3、 copy to ‘\0’ When , Copy Stop

xxxxxxxx zhang \0 san

At the time of copying , Memory will store zhang \0 xxx

This proves the above view

4、 The source data should be guaranteed to have ‘\0’

strcpy Use of functions

int main()
{
    
    char arr1[] = "hello world";
    char arr2[20] = {
     0 };
    strcpy(arr2, arr1);
    puts(arr2);
    
    return 0;
}

Simulation Implementation strcpy

#include<assert.h>
char* my_strcpy(char* str1,char* str2)
{
    
	assert(str1 && str2);
	while (*str1++ = *str2++)
	{
    
		;
	}
	return str1;
}
int main()
{
    
	char arr1[20] = {
     0 };
	char arr2[] = "abcdef";
	my_strcpy(arr1, arr2);
	puts(arr1);
	return 0;
}

strcat function

char * strcat ( char * destination, const char * source );

1、 The target space has to be large enough , It can accommodate the contents of the source string

2、 The target space must be modifiable

3、 The source string must be ‘\0’ end

strcat Use of functions

Simulation Implementation strcat

#include<assert.h>
char* my_strcat(char* str1, char* str2)
{
    
    assert(str1 && str2);
	while (*str1 != '\0')
	{
    
		str1++;
	}
	while (*str1++ = *str2++)
	{
    
		;
	}
	return str1;
}
int main()
{
    
	char arr1[30] ="hello world";
	char arr2[] = "abcdefghi";
	my_strcat(arr1, arr2);
	puts(arr1);
	return 0;
}

strcmp function

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

To compare two strings strcmp function

The first string is larger than the second string , Return greater than 0 The number of

The first string is equal to the second string , Then return to 0

The first string, little fish, the second string , Then return less than 0 The number of

strcmp Use of functions

int main()
{
    
    char arr1[] = "abcdef";
    char arr2[] = "abcq";
    int ret = strcmp(arr1, arr2);
    if (ret > 0)
    {
    
        printf(">\n");
    }
    else if (ret == 0)
    {
    
        printf("==\n");
    }
    else
        printf("<\n");
       
    return 0;
}

Simulation Implementation strcmp

#include<assert.h>
int my_strcmp(const char* str1, const char* str2)
{
    
    assert(str1 && str2);
    while (*str1 == *str2)
    {
    
        if (*str1 == '\0')
        {
    
            return 0;
        }
        str1++;
        str2++;
    }
    return *str1 - *str2;
}

int main()
{
    
    char arr1[] = "abc";
    char arr2[] = "abc";
    int ret = my_strcmp(arr1, arr2);
    if (ret > 0)
    {
    
        printf(">\n");
    }
    else if (ret == 0)
    {
    
        printf("==\n");
    }
    else
        printf("<\n");
       
    return 0;
}

memcpy function

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

1、memcpy Function encountered ‘\0’ When It won't stop

2、 Function from source The position of begins to be copied back num Bytes of data to destnation The location of

memcpy Use of functions

#include<stdio.h>
#include<string.h>
int main()
{
    
    int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
    int arr2[20] = {
     0 };
    memcpy(arr2, arr1, 40);
    int i = 0;
    for (i = 0; i < 10; i++)
    {
    
        printf("%d ", arr2[i]);
    }
    return 0;
}

Simulation Implementation memcpy

1、 When simulating the implementation , Parameter type void* , Because I don't know the specific data type , It is more reasonable to design generic pointers

2、 Return the starting address of the target space

3、num Determines the number of bytes copied

#include<stdio.h>
#include<string.h>
#include<assert.h>
void* my_memcpy(void* dest, const void* src, size_t num)
{
    
    assert(dest && src);
    //  because dest To change the   Save the starting address to return 
    void* ret = dest;
    while (num--)
    {
    
        *(char*)dest = *(char*)src;
        dest = (char*)dest + 1;
        src = (char*)src + 1;
    }
    return ret;
}
int main()
{
    
    int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
    int arr2[20] = {
     0 };
    my_memcpy(arr2, arr1, 40);
    int i = 0;
    for (i = 0; i < 10; i++)
    {
    
        printf("%d ", arr2[i]);
    }
    return 0;
}

memmove function

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

memmove Functions and memcpy The parameters of the function are exactly the same

memmove Functions and memcpy Function comparison

If there is memory overlap, you need to use memmove memcpy Although functions can also be implemented memmove The function of , however C The language standard stipulates that when encountering memory overlap memmove function

memmove Use of functions

// hold arr1 in   from 1 At the beginning 20 The contents of bytes are copied to from 3 Start on memory 
int main()
{
    
    int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
    memmove(arr1+2, arr1, 20);
    int i = 0;
    for (i = 0; i < 10; i++)
    {
    
        printf("%d ", arr1[i]);
    }
    return 0;
}

Simulation Implementation memmove

When dest stay src In front of ： There is a situation

When dest stay src In the back ： There are two situations

In the array , The address changes from low to high

When src The address of is less than dest The address of , That is to say src stay dest In front of , Copy from back to front

When src The address of is greater than dest The address of , That is to say src stay dest At the back of , Copy from front to back

void* my_memmove(void* dest, const void* src, size_t num)
{
    
    assert(dest && src);
    //  because dest To change the   Save the starting address to return 
    void* ret = dest;
    if (dest < src)
    {
    
        while (num--)
        {
    
            *(char*)dest = *(char*)src;
            dest = (char*)dest + 1;
            src = (char*)src + 1;
        }

    }
    else
    {
    
        while (num--)
        {
    
            *((char*)dest + num) = *((char*)src + num);
        }
    }
    
    return ret;
}
// hold arr1 in   from 1 At the beginning 20 The contents of bytes are copied to from 3 Start on memory 
int main()
{
    
    int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
    memmove(arr1+2, arr1, 20);
    int i = 0;
    for (i = 0; i < 10; i++)
    {
    
        printf("%d ", arr1[i]);
    }
    return 0;
}

memcmp function

int memcmp ( const void * ptr1, const void * ptr2, size_t num 

Compare from ptr1 and ptr2 The pointer starts with num Bytes

memcmp Use of functions

#include<stdio.h>
#include<string.h>
int main()
{
    
    char arr1[] = "hello";
    char arr2[] = "abcde";
    int ret = memcmp(arr1, arr2, 20);
    if (ret > 0)
    {
    
        printf(">\n");
    }
    else if (ret == 0)
    {
    
        printf("==\n");
    }
    else
        printf("<\n");

    return 0;
}