C中模拟实现各种常用字符串函数

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本文链接： https://blog.csdn.net/ZhangaZhaoLong/article/details/83963641

1. #define _CRT_SECURE_NO_WARNINGS 1

   #include <stdio.h>
   #include <assert.h>
   
   size_t my_strlen(const char *arr)
   {
   	assert(arr != NULL);
       size_t ret = 0;
   
   	while (*arr++ != '\0')
   	{
   		ret++;
   	}
   	return ret;
   }
   
   char *my_strcpy(char *arr2, const char *arr)
   {
   	assert(arr2 != NULL);
   	assert(arr != NULL);
   	char *ret = arr2;
   
   	while (*arr2++ = *arr++)
   	{
   		;
   	}
   	return ret;
   }
   
   char *my_strcat(char *arr, const char *arr3)
   {
   	assert(arr != NULL);
   	assert(arr3 != NULL);
   	char *ret = arr;
   
   	while (*arr != '\0')
   	{
   		arr++;
   	}
   	while (*arr++ = *arr3++)
   	{
   		;
   	}
   	return ret;
   }
   
   int my_strcmp(const char *arr2, const char *arr3)
   {
   	assert(arr2 != NULL);
   	assert(arr3 != NULL);
   
   	while (*arr2 != '\0' && *arr3 != '\0')
   	{
   		if (*arr2 < *arr3)
   		{
   			return -1;
   		}
   		else if (*arr2 > *arr3)
   		{
   			return 1;
   		}
   	}
   
   	if (*arr2 != '\0')
   	{
   		return -1;
   	}
   	else if (*arr3 != '\0')
   	{
   		return 1;
   	}
   	else
   	{
   		return 0;
   	}
   }
   
   char *my_strncpy(char *arr, const char *arr2, size_t n)
   {
   	assert(arr != NULL);
   	assert(arr2 != NULL);
   	size_t count = 0;
   	char *ret = arr;
   
   	while (count < n)
   	{
   		*arr = *arr2;
   		arr++;
   		arr2++;
   		count++;
   	}
   	return ret;
   }
   
   char *my_strncat(char *arr4, const char *arr3, size_t n)
   {
   	assert(arr4 != NULL);
   	assert(arr3 != NULL);
   	size_t count = 0;
   	char *ret = arr4;
   
   	while (*arr4 != '\0')
   	{
   		arr4++;
   	}
   
   	while (count < n)
   	{
   		*arr4 = *arr3;
   		arr4++;
   		arr3++;
   		count++;
   	}
   	return ret;
   }
   
   int my_strncmp(const char *arr4, const char *arr3, size_t n)
   {
   	assert(arr4 != NULL);
   	assert(arr3 != NULL);
   	size_t count = 0;
   
   	while (count < n)
   	{
   		if (*arr4 < *arr3)
   		{
   			return -1;
   		}
   		else if (*arr4 > *arr3)
   		{
   			return 1;
   		}
   		count++;
   	}
   	return 0;
   }
   
   char *my_strstr(const char *arr, const char *arr3)
   {
   	assert(arr != NULL);
   	assert(arr3 != NULL);
   	char *a = (char *)arr;
   	char *b = (char *)arr3;
   
   	for (; *a != '\0'; a++)
   	{
   		if (0 == strcmp(a,arr3))
   		{
   			return a;
   		}
   	}
   	return NULL;
   }
   
   void *my_memcpy(void *arr4, const void *arr, size_t num)
   {
   	assert(arr4 != NULL);
   	assert(arr != NULL);
   	void *ret = arr4;
   
   	while (num--)
   	{
   		*(char *)arr4 = *(char *)arr;
   		arr4 = (char*)arr4 + 1;
   		arr = (char*)arr + 1;
   	}
   	return ret;
   }
   
   void *my_memmove(void *arr4, const char *arr, size_t num)
   {
   	assert(arr4 != NULL);
   	assert(arr != NULL);
   	void *ret = arr4;
   
   	if (arr4 <= arr || (char*)arr4 >= ((char*)arr + num))
   	{
   		while (num--)
   		{
   			*(char*)arr4 = *(char*)arr;
   			arr4 = (char*)arr4 + 1;
   			arr = (char*)arr + 1;
   		}
   	}
   	else
   	{
   		arr4 = (char*)arr4 + num - 1;
   		arr = (char*)arr + num - 1;
   
   		while (num--)
   		{
   			*(char*)arr4 = *(char*)arr;
   			arr4 = (char*)arr4 - 1;
   			arr = (char*)arr - 1;
   		}
   	}
   	return ret;
   }
   
   int main()
   {
   	char arr[100] = "abcdefi";
   	char arr2[100] = { 0 };
   	char arr3[100] = "fi";
   	char arr4[100] = { 0 };
   
   	printf("%lu\n", my_strlen(arr));
   	printf("%s\n", my_strcpy(arr2, arr));
   	printf("%s\n", my_strcat(arr, arr3));
   	printf("%d\n", my_strcmp(arr2, arr3));
   	printf("%s\n", my_strncpy(arr4, arr2, 3));
   	printf("%s\n", my_strncat(arr4, arr3, 1));
   	printf("%d\n", my_strncmp(arr4, arr3, 2));
   	printf("%s\n", my_strstr(arr, arr3));
   	printf("%s\n", my_memcpy(arr4, arr, 6));
   	printf("%s\n", my_memmove(arr4, arr, 6));
   	
   	system("pause");
   	return 0;
   }