C语言字符串函数

1.模拟实现strlen

#include
#include
size_t my_strlen(const char * str)
{
	int count = 0;
	while (*str)
	{
		count++;
		str++;
	}
	return count;
}
void main()
{
	char *str = "Hello Panana";
	printf("%dn", strlen(str));
	printf("%dn", my_strlen(str));
}

2.模拟实现strcpy

#include
#include
#include
char *my_strcpy(char *dest, const char*src)
{
	assert(dest != NULL&&src != NULL);
	char *pdest = dest;
	const char *psrc = src;

	while (*psrc != '')
	{
		*pdest++ = *psrc++;
	}
	*pdest = '';
	return dest;
}

void main()
{
	char str1[20] = "hello Panana";
	char *str2 = "monday";

	printf("str1 = %sn", str1);
	char *ret = my_strcpy(str1, str2);
	printf("str1 = %sn", ret);
}

3.模拟实现strcmp

#include
#include
#include
int my_strcmp(const char *string1, const char *string2)
{
	assert(string1 != NULL && string2 != NULL);

	while (*string1 != '' && *string2 != '')
	{
		if (*string1 > *string2)
			return 1;
		else if (*string1 < *string2)
			return -1;

		string1++;
		string2++;
	}

	if (*string1 != '')
		return 1;

	if (*string2 != '')
		return -1;

	return 0;
}
void main()
{
	char *str1 = "helloPanana";
	char *str2 = "hello";

	int res =my_strcmp(str1, str2);
	printf("str1 = %dn", res);
}

4.模拟实现strcat

#include
#include
#include
char* my_strcat(char *Dest, const char *Sour)
{
	assert(Dest != NULL && Sour != NULL);
	char *pDest = Dest;
	const char *pSour = Sour;
	while (*pDest != '')
	{
		pDest++;

	}
	while (*pSour != '')
	{
		*pDest++ = *pSour++;

	}
	*pDest = *pSour;
	return Dest;
}
void main()
{
	char str1[20] = "helloPanana";
	char *str2 = "hello";
	printf("str1 = %sn", str1);
	char* res = my_strcat(str1, str2);
	printf("str1 = %sn", res);
}

5.模拟实现strstr

#include
#include
#include
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 == '')
		return NULL;

	while (*cp)
	{
		s1 = cp;
		substr = str2;
		while (*s1 && *substr && (*s1 == *substr))
		{
			s1++;
			substr++;
		}
		if (*substr == '')
			return cp;
		cp++;
	}
}
int main()
{
	char *str = "This is a simple string ABC DEF XYZ";

	char *pch = strstr(str, "string");

	printf("pch = %sn", pch);
	return 0;
}

6.模拟实现memcpy

#include
#include
#include
void* my_memcpy(void *dest, const void *src, size_t count)
{
	assert(dest != NULL && src != NULL);
	char *pdest = (char *)dest;
	const char *psrc = (const char *)src;

	if (psrc >= pdest || psrc + count <= pdest)
	{
		while (count-- != 0)
		{
			*pdest++ = *psrc++;
		}
	}
	else
	{
		pdest = pdest + count - 1;
		psrc = psrc + count - 1;
		while (count-- != 0)
		{
			*pdest-- = *psrc--;
		}
	}
	return dest;
}
void main()
{
	char str[] = "1234567890";
	my_memcpy(str + 2, str, 4);
	printf("str = %sn", str);
}

7.模拟实现memmove

#include
#include
#include
void * my_memmove(void * dst, const void * src, size_t count)
{
	void * ret = dst;
	if (dst <= src || (char *)dst >= ((char *)src + count)) {
	
		while (count--) {
			*(char *)dst = *(char *)src;
			dst = (char *)dst + 1;
			src = (char *)src + 1;
		}
	}
	else {
		dst = (char *)dst + count - 1;
		src = (char *)src + count - 1;
		while (count--) {
			*(char *)dst = *(char *)src;
			dst = (char *)dst - 1;
			src = (char *)src - 1;
		}
	}
	return(ret);
}
void main()
{
	char str[] = "1234567890";
	my_memmove(str + 2, str, 4);
	printf("str = %sn", str);
}