---

strlen

strlen函数的作用就是求字符串的首元素开始往后计算字符串的长度，直到’\0‘，但不会把\0的长度计算进去

#include<stdio.h>

size_t Strlen(const char* src)
{
	size_t count = 0;
	while (*src != '\0')
	{
		src++;
		count++;
	}
	return count;
}

int main()
{
	char arr[] = "abcdef";
	printf("%zd", Strlen(arr));
	return 0;
}

---

strncpy

strncpy的作用就是从source指向的字符串复制num个字节的内容到destination指向的字符串去
如果source的长度小于num，则填充0
注意source和destination的内容在空间上不要重叠，如要进行重叠空间内容的拷贝，则参考后续的memmove

#include<stdio.h>


size_t Strlen(const char* src)
{
	size_t count = 0;
	while (*src != '\0')
	{
		src++;
		count++;
	}
	return count;
}


char* Strncpy(char* dest, const char* src, size_t num)
{
	char* ret = dest;
	size_t len = Strlen(src);


	if (len >= num)
	{
		while (num--)
		{
			*dest++ = *src++;
		}
	}
	else
	{
		int tmp = num - len;
		while (len--)
		{
			*dest++ = *src++;
		}
		while (--tmp > 0)
		{
			*dest++ = '\0';	
		}
	}
	*dest = '\0';
	return ret;
}

---

strcmp

strcmp函数的作用是比较两个字符串，str1小于str2返回小于0的数字，相等返回0.大于则返回大于0的数字
vs中一般是返回1、0、-1

#include<stdio.h>
#include<assert.h>


//c++可以用函数重载把strcmp和strncmp实现在一起
int Strcmp(const char* arr1,const char* arr2)
{
	assert(arr1 && arr2);
	while (*arr1 != '\0' && *arr2 != '\0' && *arr1 == *arr2)
	{
		arr1++;
		arr2++;
	}
	if (*arr1 != '\0' && *arr2 != '\0' && *arr1 > *arr2)
	{
		return 1;
	}
	else if (*arr1 != '\0' && *arr2 != '\0' && *arr1 < *arr2)
	{
		return -1;
	}
	else if (*arr1 == '\0' && *arr2 == '\0')
	{
		return 0;
	}
	else
	{
		if (*arr1 != '\0' && *arr2 == '\0')
		{
			return 1;
		}
		else
		{
			return -1;
		}
	}
	//	assert(str1 && str2);
	//
	//	while (*str1 == *str2)
	//	{
	//		if (*str1 == '\0')
	//			return 0;
	//
	//		str1++;
	//		str2++;
	//	}
	//
	//	return (*str1 - *str2);
}




int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "abc";
	char arr3[] = "abcdefg";
	char arr4[] = "abd";
	char arr5[] = "abb";
	printf("%d\n", Strcmp(arr1, arr2));
	printf("%d\n", Strcmp(arr1, arr3));
	printf("%d\n", Strcmp(arr1, arr3));
	printf("%d\n", Strcmp(arr1, arr4));
	printf("%d\n", Strcmp(arr1, arr5));
	return 0;
}

---

strcat

strcat函数的作用就是在destination指向的字符串后面追加上sourse指向的字符串
source和destination指向的字符串也不能重叠

#include<stdio.h>
#include<assert.h>


char* Strcat(char* arr1,const char* arr2)
{
	assert(arr1 && arr2);


	char* ret = arr1;
	while (*arr1 != '\0')
	{
		arr1++;
	}
	while (*arr1++ = *arr2++)
	{
		;
	}
	return ret;
}


int main()
{
	//如果这里我不提前给arr1一个20的初始空间的话
	//编译器自动赋值成6，追加之后arr1长度会超出范围
	char arr1[20] = "abcdef";
	char arr2[] = "ghi";
	printf("%s", Strcat(arr1, arr2));
	return 0;
}

---

strstr

strstr函数的作用就是在str1指向的字符串中找到str2指向的字符串第一次在str1指向的字符串中出现的位置

#include<stdio.h>
#include<assert.h>


size_t Strlen(const char* src)
{
	size_t count = 0;
	while (*src != '\0')
	{
		src++;
		count++;
	}
	return count;
}


char* strstr(const char* arr1, const char* arr2)
{
	assert(arr1 && arr2);


	char* ret = arr1;
	char* p1 = ret;
	char* p2 = arr2;
	size_t len1 = Strlen(arr1);
	size_t len2 = Strlen(arr2);


	while (*(ret + len2) != '\0')
	{
		if (*ret != *p2)
		{
			ret++;
			len1--;
		}
		if (len1 > 0)
		{
			p1 = ret;
		}
		else
		{
			return NULL;
		}


		while (*p1 == *p2)
		{
			p1++;
			p2++;
		}
		if (*p2 == '\0')
		{
			return ret;
		}
		ret++;
		p1 = ret;
		p2 = arr2;
	}
}


int main()
{
	char arr1[] = "abbbcbcf";
	char arr2[] = "bbc";
	char arr3[] = "bbbb";
	char arr4[] = "xyz";
	printf("%s\n", strstr(arr1, arr2));
	printf("%s\n", strstr(arr1, arr3));
	printf("%s\n", strstr(arr1, arr4));
	return 0;
}

---

memcpy

memcpy是内存块拷贝，是更精确的拷贝函数，并且可以针对各种类型的变量（strcpy只针对字符串）
source和destination指向的空间不应该重叠（重叠的考虑memmove）

#include<stdio.h>
#include<assert.h>


void* Memcpy(void* arr1,const void* arr2, int num)
{
	void* ret = arr1;
	assert(arr1 && arr2);
	while (num--)
	{
		*((char*)arr1+num) = *((char*)arr2 + num);
	}
}


int main()
{
	int arr1[20] = { 0 };
	int arr2[] = { 1,2,3,4,5,6,7,8,9 };
	Memcpy(arr1, arr2, 4 * sizeof(arr2[0]));
	return 0;
}

---

memmove

加强版memcpy，可以拷贝空间上重叠的内容

#include<stdio.h>
#include<assert.h>


void* Memmove(void* arr1,const void* arr2, int num)
{
	void* ret = arr1;
	assert(arr1 && arr2);


	if (arr1 < arr2)
	{
		while (num--)
		{
			*((char*)arr1) = *((char*)arr2);
			++(char*)arr1;
			++(char*)arr2;
		}
	}
	else
	{
		while (num--)
		{
			*((char*)arr1 + num) = *((char*)arr2 + num);
		}
	}
}


int main()
{
	int arr1[] = { 1,2,3,4,5,6,7,8,9 };
	Memmove(arr1+4, arr1, 4 * sizeof(arr1[0]));
	return 0;
}