C语言字符串函数与内存函数精讲

//size_t strlen(const char *string);
#include<stdio.h>
#include<assert.h>
size_t my_strlen(const char* str)
{
	assert(str != NULL);
	int count = 0;
	while (*str)
	{
		count++;
		str++;
	}
	return count;
}
int main()
{
	int len = my_strlen("abcedfg");
	printf("%d\n", len);
	return 0;
}

#include<stdio.h>
#include<assert.h>
char* my_strcpy(char* dest, const char* src)
{
	assert(dest && src);
	char* ret = dest;
	//将src指向的字符串拷贝到dest指向的空间中，包括'\0'
	while (*dest++ = *src++)
		;
	//返回目的地空间的起始地址
	return ret;
}
int main()
{
	//char* arr1 = "abcdefghi";//err  目标空间必须可变
	char arr1[] = "abcdefghi";
	//char arr2[] = { 'h','e','l','l','o' };//err  源字符串必须以'\0'结尾
	char arr2[] = "hello";
	my_strcpy(arr1, arr2);
	printf(arr1);
	return 0;
}

#include<stdio.h>
#include<assert.h>
char* my_strcat(char* dest, char* src)
{
	char* ret = dest;
	assert(dest && src);
	//1.找到目的字符串中的'\0'
	while (*dest)
		dest++;
	//2.追加
	while (*dest++ = *src++)
		;
	return ret;
}
int main()
{
	char arr1[37] = "hello";
	char arr2[] = "world";
	my_strcat(arr1, arr2);//在arr1后面追加arr2
	printf("%s\n", arr1);
	return 0;
}

#include<stdio.h>
#include<assert.h>
int my_strcmp(const char* str1, const char* str2)
{
	assert(str1 && str2);
	while (*str1 == *str2)
	{
		if (*str1 == '\0')
		{
			return 0;//相等
		}
		str1++;
		str2++;
	}
	return *str1 - *str2;
}
int main()
{
	char* p1 = "abcdef";
	char* p2 = "abqjf";
	int ret = my_strcmp(p1, p2);
	printf("%d\n", ret);
	return 0;
}

char* my_strncpy(char* dest, const char* src, size_t count)
{
	char* ret = dest;
	while (count && (*dest++ = *src++))
	{
		count--;
	}
	if (count)
	{
		while (--count)
		{
			*dest++ = '\0';
		}
	}
	return ret;
}
int main()
{
	char arr1[20] = { 0 };
	char arr2[] = "hey";
	my_strncpy(arr1, arr2, 6);
	printf(arr1);
	return 0;
}

char* my_strncat(char* dest, const char* src, size_t count)
{
	char* ret = dest;
	while (*dest++)
		;
	dest--;
	while (count--)
	{
		if (!(*dest++ = *src++))
		{
			return ret;
		}
	}
	*dest = '\0';
	return ret;
}
int main()
{
	char arr1[37] = "hello";
	char arr2[] = "world";
	my_strncat(arr1, arr2, 7);
	puts(arr1);
	return 0;
}

#include<stdio.h>
#include<assert.h>
char* my_strstr(const char* str1, const char* str2)
{
	assert(str1 && str2);
	char* cur = (char*)str1;
	char* s1, * s2;
	if (!*str2)
		return ((char*)str1);
	while (*cur)
	{
		s1 = cur;
		s2 = (char*)str2;
		while (*s1 && *s2 && !(*s1 - *s2))
		{
			s1++;
			s2++;
		}
		if (!*s2)
			return cur;
		cur++;
	}
	return NULL;
}
int main()
{
	char* p1 = "aqqqcdef";
	char* p2 = "qqc";
	char* ret = my_strstr(p1, p2);
	if (ret == NULL)
		printf("字串不存在\n");
	else
		printf("%s\n", ret);
	return 0;
}

int main()
{
	char arr[] = "hyr@baidu.com";
	char* p = "@.";
	char buf[1024] = { 0 };
	strcpy(buf, arr);
	char* ret = NULL;
	for (ret = strtok(arr, p);ret != NULL;ret = strtok(NULL, p))
	{
		printf("%s\n", ret);
	}
	return 0;
}

#include<stdio.h>
#include<string.h>
#include<errno.h>//必须包含的头文件
int main()
{
	FILE* pf = fopen("unexist.ent", "r");
	if (pf == NULL)
	{
		printf("Error opening file unexist.ent:%s\n", strerror(errno));
		//errno: Last error number
		return 0;
	}
	//....
	return 0;
}

#include<stdio.h>
#include<assert.h>
struct Stu
{
	char name[20];
	int age;
};
void* my_memcpy(void* dest, const void* src, size_t num)
{
	assert(dest && src);
	void* ret = dest;
	while (num--)
	{
		*((char*)dest) = *((char*)src);
		++(char*)dest;
		++(char*)src;
	}
	return ret;
}
int main()
{
	struct Stu arr1[] = { {"zhangsan",13},{"lisi",17} };
	struct Stu arr2[3] = { 0 };
	my_memcpy(arr2, arr1, sizeof(arr1));
	return 0;
}

#include<stdio.h>
#include<assert.h>
void* my_memmove(void* dest, const void* src, size_t count)
{
	assert(dest && src);
	void* ret = dest;
	if (dest < src)
	{
		while (count--)
		{
			*(char*)dest = *(char*)src;
			++(char*)dest;
			++(char*)src;
		}
	}
	else
	{
		while (count--)
		{
			*((char*)dest + count) = *((char*)src + count);
		}
	}
	return ret;
}
int main()
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,10 };
	//将1，2，3，4，5数据拷贝到3，4，5，6，7空间上去
	my_memmove(arr + 2, arr, 20);
	for (int i = 0;i < 10;i++)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}