一、memmove

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

 memmove函数将count字节的字符从src复制到dest。

如果源区域和目标区域的某些区域重叠，memmove将确保在覆盖之前复制重叠区域中的原始源字节。

#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 );

memcpy函数将src的count字节复制到dest。

如果源和目标重叠，此函数不能确保在覆盖之前复制重叠区域中的原始源字节。使用memmove处理重叠区域。

#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 );

在一个字符串中找子字符串

#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);elseprintf("找不到字符串\n");return 0;
}

 运行结果：

四、strcat 

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

追加字符串

#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);

 字符串的比较

#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 );

把含有’\0’结束符的字符串复制到另一个字符串中

#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 );

计算指定字符串的长度

#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：