C语言字符串函数：教科书不敢告诉你的那些隐藏用法和逆天技巧

	int ret = islower('A');

	int tolower(int c); //	将参数传进去的大写字⺟转小写
	int toupper(int c); //	将参数传进去的小写字⺟转大写

#include<ctype.h>
#include<stdio.h>
int main()
{
	char arr[] = "i AM stUdent";
	int i = 0;
	while (arr[i] != '\0')
	{
		if (islower(arr[i]))
		{
			arr[i] = toupper(arr[i]);
		}
		i++;
	}
	printf("%s", arr);
	return 0;
}

int main()
{
	if ((strlen("abc") - strlen("abcdef"))>0)
	{
		printf("haha");
	}
	else
	{
		printf("hehe");
	}
	return 0;
}

int main()
{
	char arr[] = "abcdef";
	int count = 0;
	char* start = arr;
	while (*start != '\0')
	{
		start++;
		count++;
	}
	printf("%d", count);
	return 0;
}

int main()
{
	char arr[] = "abcdef";
	char* start = arr;
	while (*start != '\0')
	{
		start++;
	}
	printf("%d", start-arr);
	return 0;
}

size_t my_strlen(const char * p)
{
	if (*p == '\0')
	{
		return 0;
	}
	else
	{
		return 1 + my_strlen(p + 1);
	}
}
int main()
{
	char arr[] = "abcdefg";
	size_t ret = my_strlen(arr);
	printf("%d", ret);
	return 0;
}

void my_strcpy(char *p2,char *p1)
{
	while (*p1 != '\0')
	{
		*p2 = *p1;
		p1++;
		p2++;
	}
	*p2 = *p1;
}
int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "***********";
	my_strcpy(arr2, arr1);
	printf("%s", arr2);
	return 0;
}

char* my_strcpy(char* p2, const char* p1)
{
	char* ret = p2;
	while (*p2++ = *p1++)
	{
		;
	}
	return ret;
}
int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "***********";
	char* ret = my_strcpy(arr2, arr1);
	printf("%s", ret);
	return 0;
}

#include<string.h>
#include<stdio.h>
#include<assert.h>
char* my_strcat(char* p1, const char* p2)
{
	assert(p1 && p2);
	char* ret = p1;
	while (*p1 != '\0')
	{
		p1++;
	}
	while (*p1++ = *p2++);
	return ret;
}
int main()
{
	char arr1[20] = "abcdef";
	char arr2[] = "******";
	char* ret = my_strcat(arr1, arr2);
	printf("%s", ret);
	return 0;
}
}

int my_strcmp(const char* p1, const char* p2)
{
	assert(p1 && p2);
	while (*p1 == *p2)
	{
		if (*p1 == '\0')
		{
			return 0;
		}
		p1++;
		p2++;
	}
	return *p1 - *p2;
}
int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "abcaqw";
	int ret =my_strcmp(arr1, arr2);
	printf("%d", ret);
	return 0;
}

int main()
{
	char arr[] = "this is bit";
	char* p = "Is";
	char * ret = strstr(arr, p);
	if (ret != NULL)
	{
		printf("%s", ret);
	}
	else
	{
		printf("找不到\n");
	}
	return 0;
}

char* Fuc(const char* p1, const char* p2)
{
	const char* s1 = NULL;
	const char* s2 = NULL;
	char* s3 = p1;
	if (p2 == NULL)
	{
		return (char*)p1;
	}
	while (*s3 != '\0')
	{
		s1 = s3;
		s2 = p2;
		while (*s1!='\0'&&*s2!='\0'&& * s1 == *s2)
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
			return (char*)s3;
		}
		s3++;
	}
	return NULL;
}
int main()
{
	char arr[] = "this is dert";
	char* p = "is";
	char *ret = Fuc(arr, p);
	printf("%s", ret);
	return 0;
}

#include<stdio.h>
#include<string.h>
int main()
{
	char arr[] = "3456@qq.com";
	char* p = "@.";
	char arr1[20] = { 0 };
	strcpy(arr1, arr);
	char* ret = NULL;
	for (ret = strtok(arr, p); ret != NULL; ret = strtok(NULL, p))
	{
		printf("%s\n", ret);
	}
	return 0;
}

int main()
{
	for (int i = 0; i <= 10; i++)
	{
		printf("%d = %s\n",i, strerror(i));
	}
	return 0;
}

int main()
{
	FILE* ret =fopen("test.txt", "r");
	if (ret == NULL)
	{
		printf("%s", strerror(errno));
	}
	return 0;
}

#include<stdio.h>
#include<string.h>
int main()
{
	int arr[] = { 1,2,3,4,5,6,7 };
	int arr1[20] = { 0 };
	memcpy(arr1, arr, 16);
	for (int i = 0; i < 20; i++)
	{
		printf("%d ", arr1[i]);
	}
	return 0;
}

#include<string.h>
#include<stdio.h>
#include<assert.h>
void* my_memcpy(void* p1, void* p2, int n)
{
	void* ret = p1;
	assert(p1 && p2);
	while (n--)
	{
		*(char*)p1 = *(char*)p2;
		p1 = (char*)p1+1;
		p2 = (char*)p2+1;
	}
	return ret;
}
int main()
{
	int arr[] = { 1,2,3,4,5,6 };
	int arr1[20] = { 0 };
	my_memcpy(arr1, arr, 16);
	for (int i = 0; i < 20; i++)
	{
		printf("%d ", arr1[i]);
	}
	return 0;
}

int main()
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,10 };
	memmove(arr + 2, arr, 5 * sizeof(int));
	for (int i = 0; i < 10; i++)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}

void* my_memmove(void* p1, void* p2, size_t num)
{
	if (p2 > p1)
	{
		while (num--)
		{
			*(char*)p1 = *(char*)p2;
			p1 = (char*)p1 + 1;
			p2 = (char*)p2 + 1;
		}

	}
	else
	{
		while (num--)
		{
			*((char*)p1 + num) = *((char*)p2 + num);
		}
	}
}
int main()
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,10 };
	my_memmove(arr, arr+3, 5 * sizeof(int));
	for (int i = 0; i<10; i++)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}

int main()
{
	char arr[] = "abcdef";
	memset(arr, 'x', 3);
	printf("%s", arr);
	return 0;
}