C：字符串库函数的模拟实现（memmove、memcpy、strstr、strcat、strcmp、strcpy、strlen等）

一、memmove

void *memmove( void *dest, const void *src, size_t count );

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

void* my_memmove(void* dst, const void* src, size_t count)
{
    void* ret = dst;
    assert(dst);
    assert(src);
    //dst <= src表示，如果dst在src的前面，从前往后复制不会覆盖src中还没有复制的内容
    if (dst <= src || (char*)dst >= ((char*)src + count))
    {
        //从前往后复制，则不会出现覆盖src中没有复制的内容
        while (count--)
        {
            *(char*)dst = *(char*)src;
            dst = (char*)dst + 1;
            src = (char*)src + 1;
        }
    }
    else
    {
        //从后往前复制，则不会出现覆盖src中没有复制的内容
        dst = (char*)dst + count - 1;//移动到末尾
        src = (char*)src + count - 1;
        while (count--)
        {
            *(char*)dst = *(char*)src;
            dst = (char*)dst - 1;
            src = (char*)src - 1;
        }
    }
    return ret;
}
int main()
{
    char arr[10] = "abcdefg";
    char arr0[10] = "abcdefg";
    char arr1[10] = { 0 };
    my_memmove(arr + 2, arr, 4);
    my_memmove(arr1, arr0, 4);
    printf("内存覆盖情况：%s\n", arr + 2);
    printf("正常情况：%s\n", arr1);
    return 0;
}

运行结果：

 二、memcpy

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

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

void* my_memcpy(void* dst, const void* src, size_t count)
{
    void* ret = dst;
    assert(dst);
    assert(src);
    while(count--)
    {
        *(char*)dst = *(char*)src;
        dst = (char*)dst + 1;
        src = (char*)src + 1;
    }
    return ret;
}
int main()
{
    char dst[] = "abcdefghigk";
    char src[] = "hello world";
    void* ret = my_memcpy(dst, src, 8);
    printf("%s\n", (char*)ret);
    return 0;
}

 运行结果：

三、 strstr

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

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

char* my_strstr(const char* str1, const char* str2)
{
    assert(str1);
    assert(str2);

    char* cp = (char*)str1;
    char* substr = (char*)str2;
    char* s1 = NULL;

    if (*str2 == '\0')
        return NULL;

    while (*cp)
    {
        s1 = cp;
        substr = str2;
        while (*s1 && *substr && (*s1 == *substr))
        {
            s1++;
            substr++;
        }
        if (*substr == '\0')
            return cp;
        cp++;
    }
}
int main()
{
    char str1[] = "abcdefghi";
    char str2[] = "bcd";
    char* ret = my_strstr(str1, str2);//在arr1中找arr2字符串第一次出现的位置
    if (ret != NULL)
        printf("%s\n", ret);
    else
        printf("找不到字符串\n");
    return 0;
}

 运行结果：

四、strcat 

char* strcat( char* dest, const char* src );

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

char* my_strcat(char* dest, const char* src)
{
    assert(dest);
    assert(src);
    char* ret = dest;
    while (*dest)
    {
        dest++;
    }
    while (*dest++ = *src++)
    {
        ;
    }
    return ret;
}
int main()
{
    char dest[20] = "hello ";
    char src[] = "world";
    strcat(dest, src);
    printf("%s\n", dest);
    return 0;
}

 运行结果：

五、strcmp 

int strcmp (const char* src, const char* dest);

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

int my_strcmp(const char* src, const char* dest)
{
    assert(src);
    assert(dest);
    int ret = 0;
    while (!(ret = *(unsigned char*)src - *(unsigned char*)dest) && *dest)
        ++src, ++dest;
    if (ret < 0)
        ret = -1;
    else if (ret > 0)
        ret = 1;
    return ret;
}
int main()
{
    char src[] = "hello";
    char dest[] = "world";
    int ret = my_strcmp(src, dest);
    printf("%d\n", ret);
    return 0;
}

运行结果：

 六、strcpy

char* strcpy( char* Destination, const char* Source );

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

char* my_strcpy(char* dest, const char* src)
{
    assert(dest);
    assert(src);
    char* ret = dest;
    while (*dest++ == *src++)
    {
        ;
    }
    return ret;
}
int main()
{
    char arr1[20] = "****************";
    char arr2[] = "hello";
    char* ret = my_strcpy(arr1, arr2);
    printf("%s\n", ret);
    return 0;
}

 运行结果：

 七、strlen

size_t strlen ( const char* str );

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>

int my_strlen(const char* str)
{
    assert(str);
    int count = 0;
    while (*str != '\0')
    {
        count++;
        str++;
    }
    return count;
}
int main()
{
    char* str = "abcdef";
    printf("%d\n", my_strlen(str));
    return 0;
}

 运行结果：