C语言头文件＜string.h＞函数详解

#include <stdio.h>
#include <string.h>
 
int main()
{
	char str1[] = "hello world";
	char* str2 = "hello world";
 
	int len1 = strlen(str1);//数组名表首元素地址
	int len2 = strlen(str2);//str2 指针变量存放的也是首元素地址
 
	printf("%d\n", len1);
	printf("%d\n", len2);
 
	//这种写法也可以输出长度
	//printf("%d\n", strlen(str1));
	//printf("%d\n", strlen(str2));
 
	return 0;
}

#include <stdio.h>
#include <assert.h>
unsigned int my_strlen(const char* str)//我们不改变字符串的内容，所以用 const 来进行修饰
{
	assert(str);//避免是一个空指针
	unsigned int count = 0;
	while (*str)
	{
		count++;
		str++;
	}
	return count;
}
int main()
{
	char str1[] = "hello world";
	char* str2 = "hello world";
 
	int ret1 = my_strlen(str1);
	int ret2 = my_strlen(str2);
 
	printf("%d\n", ret1);
	printf("%d\n", ret2);
 
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char dest[20] = "row your boat";
	char src[] = "hello world";
	strcpy(dest, src);
	printf("%s\n", dest);
	return 0;
}

#include <stdio.h>
#include <assert.h>
 
char* my_strcpy(char* dest, const char* src)//dest 指向的数组空间是要被改变的，但是 src 指向的字符串不需要改变
{
	assert(dest && src);//防止其中某个是无效指针
	char* ret = dest;
	while (*dest++ = *src++)
		;	//注意 while 循环执行了空语句
 
	return ret;
}
int main()
{
	char dest[30] = "gently down the stream";
	char src[] = "life is but a dream";
 
	my_strcpy(dest, src);
	printf("%s\n", dest);
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char* str1 = "abbbcd";
	char* str2 = "abbbdd";
	int ret = strcmp(str1, str2);
	if (ret > 0)
		printf("str1 > str2\n");
	else if (ret < 0)
		printf("str1 < str2\n");
	else
		printf("str1 == str2\n");
	return 0;
}

#include <stdio.h>
#include <assert.h>
 
int my_strcmp(const char* str1, const char* str2)//两个字符串的内容都不需要修改，用 const 修饰
{
	assert(str1 && str2);//防止是无效指针
 
	while (*str1 == *str2)//如果相等则进入循环
	{
		if (*str1 == '\0')//*str1 == '\0' 了并且进入循环了，说明两个字符串比较完成了，没有不相等的字符
			return 0;
		str1++;
		str2++;
	}
	return *str1 - *str2;//两个字符的差作为返回值
}
int main()
{
	char* str1 = "abbbcd";
	char* str2 = "abbbdd";
	int ret = my_strcmp(str1, str2);
	if (ret > 0)
		printf("str1 > str2\n");
	else if (ret < 0)
		printf("str1 < str2\n");
	else
		printf("str1 == str2\n");
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char str1[50] = "row row row your boat,";
	char* str2 = "gently down the stream";
	strcat(str1, str2);
	printf("%s\n", str1);
	return 0;
}

#include <stdio.h>
#include <assert.h>
 
char* my_strcat(char* dest, const char* src)//src 指向的字符串是不改变内容的，所以用 const 修饰
{
	assert(dest && src);//确保两个指针有效
	char* ret = dest;
	while (*dest)
		dest++;//先找到 dest 指向的数组的第一个 '\0' 的位置
	while (*dest++ = *src++)//拷贝
		;
	return ret;
}
int main()
{
	char str1[50] = "row row row your boat,";
	char* str2 = "gently down the stream";
	my_strcat(str1, str2);
	printf("%s\n", str1);
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char str1[20] = "row your boat";
	char str2[] = "hello world";
	strncpy(str1, str2, 3);//我们从 str2 中拷贝三个字节的内容到 str1 去
	printf("%s\n", str1);
	return 0;
}
 

#include <stdio.h>
#include <assert.h>
 
char* my_strncpy(char* dest, const char* src, unsigned int num)
{
	assert(dest && src);//确保两个指针有效
	char* ret = dest;
 
	while (num--)//拷贝几个字节
	{
		*dest = *src;
		dest++;
		src++;
	}
 
	return ret;
}
int main()
{
	char str1[20] = "row your boat";
	char str2[] = "hello world";
	my_strncpy(str1, str2, 3);
	printf("%s\n", str1);
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char* str1 = "abbbcd";
	char* str2 = "abbbdd";
	int ret=strncmp(str1, str2,3);//我们只想比较字符串的前三个字节
	if (ret > 0)
		printf("str1 > str2\n");
	else if (ret < 0)
		printf("str1 < str2\n");
	else
		printf("str1 == str2\n");
	return 0;
}

#include <stdio.h>
#include <assert.h>
 
int my_strncmp(const char* str1, const char* str2, unsigned int num)//两个字符串的内容都不需要变，所以用 const 修饰
{
	assert(str1 && str2);
	while (num-- && *str1 == *str2)
	{
		if (*str1 == '\0')
			return 0;
		str1++;
		str2++;
	}
	return *str1 - *str2;
}
int main()
{
	char* str1 = "abbbcd";
	char* str2 = "abbbdd";
	int ret = my_strncmp(str1, str2, 3);//我们只想比较字符串的前三个字节
	if (ret > 0)
		printf("str1 > str2\n");
	else if (ret < 0)
		printf("str1 < str2\n");
	else
		printf("str1 == str2\n");
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char str1[50] = "row row row your boat,";
	char str2[] = "gently down the stream";
	strncat(str1, str2, 6);//我们只想追加 6 个字节的字符到 str1 中
	printf("%s\n", str1);
	return 0;
}

#include <stdio.h>
#include <assert.h>
 
char* my_strncat(char* dest, const char* src, unsigned int num)//str2 中的字符串不需要被修改，所以用 const 修饰
{
	assert(dest && src);//确保两个指针是有效指针
	char* ret = dest;//记录返回值
 
	while (*dest)//先找到 dest 指向的 '\0' 的位置
		dest++;
	while (num-- && (*dest++ = *src++) )//拷贝限制的字节数
		;
	*dest = '\0';
 
	return ret;
 
}
int main()
{
	char str1[50] = "row row row your boat,";
	char str2[] = "gently down the stream";
	my_strncat(str1, str2, 8);
	printf("%s\n", str1);
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char* str1 = "row row row your boat,gently down the stream";
	char* str2 = "row your boat";
	char* ret = strstr(str1, str2);//在 str1 中查找 str2 
	printf("%s\n", ret);
	return 0;
}

#include <stdio.h>
#include <assert.h>
 
char* my_strstr(const char* str1, const char* str2)//不需要改变其内容，用 const 修饰
{
	assert(str1 && str2);//避免是无效指针
	const char* s1 = str1;
	const char* s2 = str2;
	const char* cp = str1;//这个指针变量是至关重要的
	while (*cp)
	{
		s1 = cp;
		s2 = str2;
 
		//在 str1 中查找 str2 的核心循环
		while (*s1 && *s2 && *s1 == *s2)
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')//查找完成
			return (char*)cp;
 
		cp++;
	}
	return NULL;
}
int main()
{
	char* str1 = "row row row your boat,gently down the stream";
	char* str2 = "row your boat";
	char* ret = my_strstr(str1, str2);
	printf("%s\n", ret);
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	char* str1 = "row@row~row%your^boat,gently@down~the^stream";
	char* str2 = "@~%^";
 
	//将 str1 拷贝至 tmp 数组，这样不会丢失 str1 的原始数据
	char tmp[50] = { 0 };
	strcpy(tmp, str1);
 
	char* ret = NULL;
	for (ret = strtok(tmp, str2); ret != NULL; ret = strtok(NULL, str2))
	{
		printf("%s ", ret);
	}
	return 0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
	FILE* p;
	p = fopen("test.txt", "r");//在我们的工程目录下并没有 test.txt 这个文件
	if (p == NULL)//那么打不开 p 就是一个空指针
	{
		printf("%s\n", strerror(errno));//这里就会解释为什么是空指针的原因
	}
	return 0;
}